Article about cardiac arrhythmias - technical
The term cardiac arrhythmia (or dysrhythmia) is used to describe an abnormality of cardiac rhythm of any type. The spectrum of cardiac arrhythmias ranges from innocent extrasystoles to immediately life-threatening conditions such as asystole or ventricular fibrillation.
The key to the successful diagnosis of cardiac arrhythmias is the systematic analysis of an electrocardiogram ECG (EKG -US) of optimal quality obtained during the arrhythmia.
Continuous monitoring is necessary for identification when arrhythmias are intermittent. Ambulatory electrocardiographic recordings are of most value when they provide correlation between the patient’s symptoms and the cardiac rhythm at that moment. Alternative strategies for the detection of infrequent arrhythmias include the use of a patient-activated recorder, which is applied and activated during symptoms, or an external or implanted loop recorder.
More detailed investigation of cardiac arrhythmias is undertaken by invasive cardiac electrophysiological testing. Multipolar electrodes are inserted transvenously to record electrograms from the atrium, ventricle, His bundle, and commonly from the coronary sinus. Electrophysiological mapping is an essential part of radiofrequency ablation.
Bradycardia is defined as a ventricular rate of less than 60 beats/min. The principal indications for active intervention in bradycardia are symptomatic (disturbances of consciousness, fatigue, lethargy, dyspnoea, or bradycardia-induced tachyarrhythmias) or prognostic (prevention of sudden cardiac death).
In the presence of haemodynamic compromise, immediate attempts to increase heart rate should be employed, using atropine, isoproterenol (isoprenaline), and/or temporary cardiac pacing (transvenous or transcutaneous). Following stabilization, factors causing or contributing to the presentation should be sought and corrected—especially, acute ischaemia and infarction, concomitant drug therapy, or electrolyte disorders.
Specific disorders causing bradycardia include (1) sinoatrial disease (‘sick sinus syndrome’); (2) neurocardiogenic syncope (e.g. carotid sinus hypersensitivity); and (3) atrioventricular (AV) conduction disorders (‘heart block’).
AV block—the commonest cause of AV block is idiopathic fibrosis of the His–Purkinje system, and the severity (degree) of block can be classified as (1) first-degree—defined as a PR interval greater than 0.2 s, which produces no symptoms and does not require treatment; (2) second-degree—when there is intermittent failure of conduction from atrium to ventricle, either with (Mobitz type I, Wenckebach) or without (Mobitz type II) a characteristic pattern of increasing PR-interval duration preceding the nonconducted P wave; pacemaker implantation is not necessary for type I in most cases, but is required for type II; (3) third-degree (complete) AV block—when there is complete dissociation between atrial and ventricular activity, which is an indication for permanent pacemaker implantation, except in the context of an acutely reversible condition.
The principal mechanisms responsible for tachyarrhythmias are (1) abnormal automaticity; (2) triggered activity; or (3) re-entry. Most clinically important sustained tachycardias appear to arise on the basis of re-entry, which requires the presence of a potential circuit comprising two limbs with different refractoriness and conduction properties.
The first and most important step in the diagnosis and management of tachycardias is to determine whether the arrhythmia arises within the atria and/or AV junction, or from the ventricles, which can often be achieved by careful analysis of a 12-lead ECG.
Diagnosis—it is safe to assume that virtually all narrow-complex tachycardias have a supraventricular origin, but wide-complex tachycardias (QRS duration ≥0.12 s) may arise either from the ventricle or from supraventricular mechanisms, and few areas in cardiology cause more difficulty—or result in more mismanagement—than the diagnosis of wide-complex tachycardias. Careful scrutiny of the 12-lead ECG may reveal diagnostic features, but the commonest reason for error is that the clinical context is not considered, or erroneous conclusions are drawn from it: key issues to recognize are (1) elderly patients or those with a history of ischaemic heart disease are most likely to have ventricular arrhythmia; (2) the patient’s haemodynamic status is a poor predictor of the type of tachycardia; (3) ventricular tachycardia can present with a history of paroxysmal self-terminating episodes.
Treatment—R-wave synchronized, direct-current (DC) cardioversion under general anaesthesia or deep sedation is the most effective and immediate means of terminating sustained tachycardias, and should be employed when tachycardia is associated with haemodynamic compromise. In patients with tachycardia who are haemodynamically stable, manoeuvres that produce transient vagal stimulation, such as the Valsalva manoeuvre or carotid sinus massage, may be employed. The response to intravenous adenosine, which will often terminate arrhythmias dependent on the AV node, may be of therapeutic or diagnostic value, and should be considered in all patients with tolerated regular tachycardia. In the long term, tachycardias can be treated with antiarrhythmic drugs (usefully categorized by the Vaughan Williams classification), implantable cardioverter–defibrillators (ICDs), radiofrequency ablation, or arrhythmia surgery.
Cardioversion—if it is clinically appropriate to attempt cardioversion, the drugs of choice are the class Ic agents (e.g. flecainide) for patients without significant underlying heart disease; class III drugs are somewhat less effective but are safer in the presence of left ventricular dysfunction or ischaemic heart disease (e.g. sotalol or amiodarone). Normally, only one drug should be tried in any individual patient: if drug therapy fails, DC cardioversion is commonly effective.
Risk of thromboembolism—because atrial fibrillation is a risk factor for the development of intracardiac thrombus formation, cardioversion—by chemical or electrical means—should not be attempted if arrhythmia has been present for longer than 48 h. Anticoagulation plus rate control with a β-blocker, calcium-channel blocker, or digoxin should be considered in these circumstances. Prophylaxis against thromboembolism should be considered in all patients with atrial fibrillation.
Paroxysmal atrial fibrillation—drug therapy may not be necessary for patients with infrequent paroxysms, or a ‘pill-in-the-pocket’ approach can be used in those without structural heart disease, whereby they take a dose of an antiarrhythmic drug after the onset of arrhythmia. No drug is entirely satisfactory for recurrent paroxysmal atrial fibrillation: a β-blocker is often prescribed as first-line therapy.
Persistent atrial fibrillation—usually requires electrical cardioversion to achieve sinus rhythm and has a high recurrence rate even after successful cardioversion. The key decision is whether to employ a rhythm or rate-control strategy. In general, a rate-control strategy (AV nodal blocking drug, e.g. β-blocker, calcium channel blocker, or digoxin) should be employed in patients with few or minor symptoms, elderly patients, and those with contraindications to antiarrhythmic therapy or cardioversion. A rhythm-control strategy (elective cardioversion) may be best in more severely symptomatic or younger patients, or in those with atrial fibrillation due to a treated precipitant.
It is important to attempt to terminate atrial flutter since the ventricular rate is often poorly controlled by AV nodal blocking drugs: this may be achieved by chemical or electrical cardioversion. Prophylaxis against thromboembolism should be given as for atrial fibrillation.
The term supraventricular tachycardia (also called junctional re-entry tachycardia) is commonly reserved for those in which the AV node is an obligate part of a re-entry circuit—AV nodal re-entrant tachycardia (AVNRT) or AV re-entry tachycardia (AVRT).
Termination of an attack of AV nodal re-entrant tachyxardia is achieved by producing transient AV nodal block by vagotonic manoeuvres, adenosine or by verapamil. Drug prophylaxis is undertaken with β-blockers, a combined β-blocker/class III agent such as sotalol, or AV nodal blocking drugs such as verapamil or digoxin. Curative treatment is by radiofrequency ablation.
Attacks of AV re-entry tachycardia are treated in the same way as is AV nodal re-entrant tachycardia. Antiarrhythmic prophylaxis may be effective, but radiofrequency ablation offers high success rates with low incidence of complications and should be considered early.
The term ‘pre-excitation’ refers to the premature activation of the ventricle via one or more accessory pathways that bypass the normal AV node and His–Purkinje system. The commonest type is Wolff–Parkinson–White (WPW) syndrome, where a δ-wave is characteristically seen on the ECG, and the main prognostic concern is pre-excited atrial fibrillation, which can be very rapid and degenerate into ventricular fibrillation. Patients with symptomatic WPW syndrome should be offered radiofrequency ablation as first-line therapy.
Ventricular tachycardia normally occurs in individuals with overt heart disease, but is also seen in young and apparently healthy subjects, when occult cardiac disease or cardiac genetic syndromes should be considered.
Sustained ventricular tachycardia is a medical emergency. Immediate DC cardioversion is necessary if the patient is hypotensive; haemodynamically tolerated VT may be terminated pharmacologically, with intravenous lidocaine (lignocaine) or sotalol being the usual first-choice options. Unless there is a clear precipitating factor, the risk of sudden death is high and patients should be considered for an implantable cardioverter-defibrillator.
Polymorphic ventricular tachycardia, of which torsades de pointes is a well-recognized type associated with acquired or congenital prolongation of the QT interval, is an unstable rhythm with varying QRS morphology that undergoes spontaneous termination or degenerates into ventricular fibrillation. In patients with this condition, it is essential to discontinue predisposing drugs or other agents and to avoid empirical antiarrhythmic drug therapy. Intravenous magnesium sulphate is a safe and effective emergency measure.
Patients who survive an episode should be assessed carefully to determine the risk of recurrence and may require an implantable cardioverter-defibrillator or antiarrhythmic therapy as for patients with ventricular tachycardia.
The congenital long-QT syndromes are inherited conditions due to mutations in genes encoding ion-channel proteins: their prognosis, if untreated, is poor. β-Blockers are highly effective in the commonest form of the condition.
The Brugada syndrome is an autosomal dominant condition where there is an unusual pattern of variable ST-segment elevation and partial right bundle branch block in the right precordial leads, associated with a risk of polymorphic ventricular tachycardia and sudden death. Patients with congenital long QT or Brugada syndrome considered at high risk of sudden death should be considered for an implantable cardioverter-defibrillator.
The term cardiac arrhythmia (or dysrhythmia) is used to describe an abnormality of cardiac rhythm of any type. The spectrum of cardiac arrhythmias ranges from innocent extrasystoles to immediately life-threatening conditions such as asystole or ventricular fibrillation. Arrhythmias may occur in the absence of cardiac disease, but are more commonly associated with structural heart disease or external provocative factors.
Symptoms of cardiac arrhythmias
The symptoms produced by bradyarrhythmias depend on the extent of cardiac slowing. They may include sudden death, syncope (Stokes–Adams attacks), or dizziness (presyncope). Continuous bradycardia without asystolic pauses may produce symptoms of fatigue, lethargy, dyspnoea, or mental impairment.
The symptoms caused by tachyarrhythmias depend on a variety of factors including the heart rate, the difference between the rate during the arrhythmia and the preceding heart rate, the degree of irregularity of the rhythm, and the presence or absence of underlying cardiac disease. Symptoms of tachycardia include a feeling of rapid palpitation, angina or dyspnoea, syncope or sudden death.
Investigation of arrhythmias
History taking must include a detailed description of the symptoms associated with the arrhythmia. Evidence should be sought for factors that may precipitate the arrhythmia (e.g. exercise, alcohol) and for the presence of underlying cardiac disease, in particular valvular heart disease, myocardial ischaemia/infarction, or congestive heart failure. Examination of the pulse may be unremarkable if the arrhythmia is intermittent. Physical examination for evidence of structural heart disease is essential. Further investigations to establish the presence of structural heart disease and to determine ventricular function may include 12-lead ECG, chest radiography, echocardiography, exercise stress testing, coronary arteriography, or MRI.
The key to the successful diagnosis of cardiac arrhythmias is the systematic analysis of ECG (EKG) of optimal quality obtained during the arrhythmia (Table 1. Ideally, this should comprise all 12 leads recorded on a multichannel recorder, which can allow the identification of P-waves in one lead while they may be absent or equivocal in another.
Continuous monitoring is necessary for identification when arrhythmias are intermittent. Ambulatory (Holter) electrocardiography is normally performed for periods of 24 to 48 h using a portable recorder. High-speed or automatic replay facilities enable the identification of intermittent arrhythmias, as well as the quantification of extrasystoles and assessment of parameters of heart rate variability. Interpretation of recordings requires knowledge of possible artefacts, such as those caused by movement, or variations in tape speed in recorders that use magnetic tape. It is important to allow for physiological variability in the sinus rate, also appreciating that minor abnormalities such as extrasystoles or brief (3–4 beat) runs of supraventricular arrhythmias are usually of no significance. Ambulatory electrocardiographic recordings are of most value when they provide correlation between the patient’s symptoms and the cardiac rhythm at that moment. Patients should be issued with a diary card and asked to note any symptoms suggestive of arrhythmia during the recording.
Alternative strategies for the detection of infrequent arrhythmias include the use of a patient-activated recorder, which is applied and activated during symptoms, or an external or implanted loop recorder. Loop recorders continually record the electrocardiographic signal, but only have sufficient memory to retain a few minutes of data. In the event of symptoms, the patient activates the device, thus ‘fixing’ the previous few minutes of recording for subsequent analysis. External loop recorders are usually used for up to 7 days, while an implanted event recorder can last for up to 18 months.
|Table 1 Principles of ECG diagnosis of arrhythmias|
|Obtain 12-lead or multichannel recordings if possible|
|Atrial activity||P-waves visible?|
|Normal P-wave morphology and axis?|
|Ventricular activity||Ventricular rate?|
|Regular or irregular?|
|Normal QRS morphology and duration?|
|Bundle-branch block or bizarre QRS morphology?|
|Variation in QRS morphology/axis?|
|Atrioventricular relationship||PR interval—fixed or varied?|
|Atrial versus ventricular rate?|
Cardiac electrophysiological study
More detailed investigation of cardiac arrhythmias is undertaken by invasive cardiac electrophysiological testing. Multipolar electrodes are inserted transvenously to record electrograms from the atrium, ventricle, His bundle, and commonly from the coronary sinus. The site of conduction delays within the heart may be identified, or accessory pathways localized. Sustained arrhythmias may be initiated and terminated by extrastimuli , and their pattern of activation in the heart studied in detail. Electrophysiological mapping is an essential part of radiofrequency ablation (see below), and modern three-dimensional mapping systems have facilitated ablation of complex arrhythmias.
Aetiology and mechanisms
Bradycardia is defined as a ventricular rate of less than 60/min, and results from a reduction in the rate of normal sinus pacemaker activity, or from disturbances of atrioventricular (AV) conduction. Sinus bradycardia may be physiological, e.g. during sleep in young people, and in athletes. Pathological bradyarrhythmias can result from intrinsic degenerative disease of the sinus or AV node, or the conducting system. Bradycardia may also be due to extraneous factors such as sympathetic withdrawal, vagal stimulation, drug effects, myocardial ischaemia/infarction, infiltration, or surgical trauma and also miscellaneous conditions such as hypothyroidism, hypothermia, jaundice, or raised intracranial pressure.
General principles of management
The principal indications for active intervention in bradycardia are symptomatic (disturbances of consciousness, fatigue, lethargy, dyspnoea, or bradycardia-induced tachyarrhythmias) or prognostic (prevention of sudden cardiac death). Particular attention should be given to the history and ECG documentation of the rhythm disturbance. Drugs interfering with sinoatrial or AV nodal function should be withdrawn if possible, although under certain circumstances (e.g. tachycardia–bradycardia syndrome) it may be necessary to combine pacemaker implantation with continued drug therapy.
Acute management of bradycardia
General principles can be applied to patients presenting with overt bradycardia, regardless of aetiology (Table 2). In the presence of haemodynamic compromise, immediate attempts to increase heart rate should be employed. Transient increases in sinus rate or the ventricular escape rate in complete AV block may be achieved with atropine or isoproterenol (isoprenaline). However, drug treatment is only of temporary value, and temporary cardiac pacing is indicated for persistent bradycardia (see ‘Pacemaker therapy’, below). Temporary pacing is also indicated where frequent Stokes–Adams attacks are occurring. Pacing can be performed transcutaneously using an external pacing system in the emergency situation if facilities for transvenous pacing are not immediately available.
|Table 2 General principles of acute management of the patient with bradycardia|
|Assess the patient||Respiratory status|
|Examine the ECG||Sinus rate|
|QRS morphology and duration|
|If haemodynamic compromise||Atropine|
|Look for precipitants||Ischaemia/infarction|
Following stabilization, factors causing or contributing to the presentation should be sought and corrected, especially acute ischaemia and infarction, concomitant drug therapy, or electrolyte disorders. Analysis of the ECG will allow identification of the conduction disorder and plans for long-term management can be instituted.
Specific causes of brachycardia
Sinoatrial disease, often referred to as ‘sick sinus syndrome’, results in inappropriate sinus bradycardia, sinus pauses, or junctional rhythm in the absence of extrinsic factors. The condition is most commonly caused by idiopathic degeneration of the sinus nodal cells, particularly in older people, and is associated in about 20% of cases with idiopathic bundle branch fibrosis (see below). Occasionally, sinoatrial disease is caused by ischaemia due to obstruction of the right coronary artery. Conduction block may occur between the sinus node and the atrium (sinoatrial block), resulting in ‘dropped’ P-waves. More prolonged suppression of sinus node activity results in periods of sinus arrest, which are terminated by an escape beat from the sinus node, AV junction, or ventricle. Where the sinus rate is permanently slower than the junctional rate, continuous AV junctional rhythm will be present. Patients with sinoatrial disease have an increased predisposition to atrial tachyarrhythmias (tachycardia–bradycardia syndrome), and prolonged pauses may follow termination of tachycardia.
Sinoatrial disease can cause symptomatic bradycardia, dizziness, or syncope, but may be asymptomatic. The diagnosis is normally made from 12-lead or ambulatory ECG recording. Investigation should focus on excluding extrinsic causes of bradycardia, and on demonstrating the correlation between bradycardia or pauses and symptoms. Pacemaker implantation is indicated for the relief of symptoms (see below). Prognosis is not improved by pacemaker implantation in sinus nodal disease and thus pacemaker implantation in asymptomatic patients is not indicated.
Patients with carotid sinus hypersensitivity and symptoms of presyncope or syncope should undergo permanent pacemaker implantation (see below). In patients with recurrent vasovagal syncope, the optimal treatment is uncertain. Medical therapy with agents as diverse as α-agonists, β-blockers, vagolytic agents (disopyramide, hyoscine), ephedrine, or antidepressants is often tried, but the evidence base for the efficacy of drug therapy is weak. Spontaneous resolution of symptoms occurs in many patients. Nonpharmacological treatments, such as isometric manoeuvres (e.g. leg crossing or arm tensing) at the onset of symptoms may reduce the severity of episodes and prevent syncope. There is little evidence to support pacemaker implantation even in those with predominant bradycardia as the response to tilt testing, but it may be considered in selected individuals with intractable symptoms.
Atrioventricular conduction disorders
Impairment of AV conduction may occur either within the AV node (intranodal) or within the His–Purkinje system (infranodal). Intranodal block is not associated with QRS abnormalities, while distal (infranodal) block is commonly associated with bundle branch block.
The causes of AV block are shown in Bullet list 1. The commonest is idiopathic fibrosis of the His–Purkinje system, which occurs with increasing frequency from the seventh decade of life onwards, is associated with sinoatrial disease in up to 25% of cases, and results in progressive impairment of AV conduction.
Atrioventricular block may occur acutely in myocardial infarction. Inferior myocardial infarction predominantly affects AV nodal conduction by vagal overactivity, and possibly adenosine release from ischaemic myocardium. First-degree, second-degree type I (Wenckebach), or third-degree AV block may occur, but are commonly transient. Spontaneous recovery of normal conduction generally occurs within 7 to 10 days. By contrast, AV block secondary to anterior myocardial infarction is normally due to extensive infarction of the interventricular septum involving both the left and right bundle branches. This may result in type II second-degree block or complete AV block, with a lower probability of recovery of normal conduction.
Any drug slowing AV conduction may potentially produce AV block. The risk is greater when such drugs are used in combination. Intravenous verapamil in patients already receiving β-adrenoceptor blockers is particularly hazardous. Vagally mediated conduction disturbances occur as a physiological finding in highly trained athletes, and in young people during sleep, or in neurocardiogenic syncope. Atrioventricular conduction disturbances arise in structural congenital heart disease such as endocardial cushion defects, but also as an isolated congenital abnormality, commonly in association with maternal systemic lupus erythematosus.
First-degree atrioventricular block
First-degree AV block is defined as a PR interval greater than 0.20 s. This produces no symptoms and does not require treatment, although the risk of progression to higher-degree AV block should be considered.
In second-degree AV block, there is intermittent failure of conduction from atrium to ventricle. In type I (Wenckebach) second-degree block, a characteristic pattern of increasing PR interval duration followed by a non-conducted P-wave is seen. The QRS morphology is commonly normal. Type I (Wenckebach) second-degree AV block usually indicates block in the AV node, and is normally associated with a reliable subsidiary pacemaker and a low risk of progression to complete heart block. In most instances pacemaker implantation is not necessary unless recurrent presyncope or syncope suggest the occurrence of an intermittent higher-degree block. By contrast, in type II second-degree AV block (commonly called Mobitz type II AV block) there is a sudden failure of conduction, without a preceding increase in the PR interval. Regular non-conducted P-waves may result in high-degree block, with 2:1 or 3:1 conduction. Type II second-degree AV block is generally indicative of extensive infranodal conduction abnormality, with a high risk of progression to complete AV block. Guidelines therefore recommend permanent pacemaker implantation even in the absence of symptoms.
Bullet list 1 Causes of atrioventricular block
- ◆ Idiopathic conducting system fibrosis
- ◆ Acute myocardial ischaemia/infarction
- ◆ Infiltration—calcific aortic stenosis, sarcoidosis, scleroderma, syphilis, tumour
- ◆ Infection—diphtheria, rheumatic fever, endocarditis, Lyme disease
- ◆ Drugs—digoxin, verapamil or diltiazem, β-blockers, antiarrhythmic drugs
- ◆ Surgical trauma, radiofrequency ablation
- ◆ Congenital heart block, congenital heart disease
- ◆ Vagal—athletic heart, carotid sinus, and vasovagal syndrome
- ◆ Myotonic dystrophy
Third-degree atrioventricular block
The characteristic feature of third-degree (complete) AV block is dissociation between atrial and ventricular activity. The ventricular rate is regular and slower than the atrial rate. An escape rhythm arising above the bifurcation of the bundle of His will produce a narrow QRS morphology, commonly with a relatively stable escape rhythm (50–60/min). A more distal escape rhythm results in widened, bundle branch block morphology complexes with a slower escape rate (20–30/min). When complete AV block coexists with atrial fibrillation, it is recognized by the presence of a slow, regular ventricular response. High-degree AV block can be intermittent, and the resting ECG may be normal or only show evidence of mild conducting system disturbance such as first-degree AV block or bundle branch block. If there is clinical suspicion, ambulatory ECG recording is required, for prolonged periods if necessary.
The presence of complete AV block, except in the context of an acutely reversible condition, should be regarded as an indication for permanent pacemaker implantation. This is urgent in patients who are having Stokes–Adams attacks; their prognosis is poor without pacemaker implantation, and markedly improved by permanent pacing, after which outcome will depend on the presence and extent of any underlying cardiac disease. Permanent pacing also improves prognosis in asymptomatic patients with complete AV block. One exception to this general rule is congenital complete heart block, where the escape rhythm is often relatively fast (50–60/min) with a narrow QRS morphology. Many patients remain asymptomatic well into adult life, although there is a small risk of syncope or sudden death. Pacemaker implantation should be considered if there are symptoms, if there are abrupt pauses, if the average heart rate is below 50/min, or in patients over 40 years of age.
The term asystole is used when the electrocardiogram shows a complete cessation of both atrial and ventricular activity. This appearance may be mimicked by disconnected ECG cables or other artefacts, but since asystole causes cardiac arrest the distinction is virtually always obvious.
The basis of pacemaker therapy is the local depolarization of the myocardium by an electric current passed through an electrode in contact with the heart (atrium or ventricle). Activation of the remainder of the atria or ventricles occurs by direct cell-to-cell conduction. The minimum current necessary to stimulate the heart during diastole is known as the pacing threshold. Pacemaker systems comprise one or more intracardiac catheter electrodes, introduced into the heart via the venous system, and a pulse generator, which contains the circuitry for generating and timing the pacing stimulus, as well as for sensing spontaneous cardiac depolarizations. The pacing stimulus is delivered between the active pole at the tip of the electrode catheter and an indifferent electrode sited either on the same catheter 1–2 cm proximal to the tip (bipolar pacing), or utilizing the can of an implanted pulse generator (unipolar pacing). Satisfactory pacing requires stable electrode contact with the myocardium. The standard sites for endocardial atrial and ventricular pacing are the right atrial appendage and the right ventricular apex respectively, although screw-in active fixation leads allow placement at other atrial and ventricular sites.
An external pulse generator is used for temporary pacing. For permanent pacing, it is usually implanted deep to the subcutaneous fat layer in the prepectoral region. The generator contains a timer set to deliver pacing stimuli at a preset pulse interval (e.g. 1000 ms). Pacemakers normally operate in the demand mode, whereby if spontaneous activation of the cardiac chamber is sensed via the electrode, the delivery of a pacing stimulus is inhibited and the timer circuit of the generator is reset. Pacing in the fixed rate mode results in the delivery of stimuli regardless of the spontaneous activity of the chamber being paced.
Temporary ventricular pacing
Temporary pacing is indicated in patients with bradycardia causing haemodynamic compromise, or as a prelude to permanent pacemaker implantation in those with significant recurring symptoms, or high-risk AV block. Facilities for radiographic screening, continuous electrocardiographic monitoring, and defibrillation are required. The pacing electrode is introduced under aseptic conditions via an intravascular sheath into the subclavian, internal jugular, or femoral vein and the tip advanced under radiographic guidance to the right ventricular apex. Nonsustained ventricular tachycardia, or occasionally ventricular fibrillation, may occur during catheter manipulation. Once the electrode is at an acceptable site, pacing is initiated, and the minimum output necessary to achieve stable ventricular capture is determined. The pacing threshold should normally be less than 1 V, at a pulse width of between 0.5 and 2 ms. If the pacing threshold is unsatisfactory, the electrode is repositioned until an acceptable site is found. Care should be taken to determine that the electrode is stable by asking the patient to take deep breaths or to cough while pacing at threshold. The electrode is then secured at the site of insertion and the pulse generator set to an output of at least 3 V above the pacing threshold.
Permanent pacemaker implantation
Permanent pacing electrodes are normally inserted via the left or right subclavian or cephalic vein. Once the electrode is in a satisfactory position, it is secured and connected to the implanted pulse generator. Most pulse generators are powered by lithium batteries and have a life of approximately 6 to 8 years, after which the generator is replaced. The rate, output voltage, pulse width, and other pacemaker functions can be modified noninvasively by means of telemetry via a transmitter/receiver placed on the skin over the pulse generator. The amplitude and pulse width of the pacing stimulus are usually set at nominal values (e.g. 5 V, 1 ms), but are adjustable and can be reduced to prolong the life of the battery, providing there is a sufficient safety margin between the pulse generator output and the pacing threshold.
The nomenclature used to describe pacing mode is given in Table 3. Atrial demand (AAI) pacing is used for sinoatrial disease in the absence of AV block. Ventricular pacing (VVI) is the simplest and technically easiest mode of pacing, and is required for AV conduction disturbances. However, VVI pacing does not permit AV synchrony or an increase in pacing rate in response to an increase in sinus (atrial) rate. Dual-chamber (DDD) pacemakers have electrodes in both the right atrium and ventricle. If the sinus cycle length is greater than the pulse interval, atrial demand pacing occurs. Following the atrial stimulus, a programmable AV delay commences. If no spontaneous ventricular depolarization is sensed before the end of this interval, a pacing stimulus is delivered via the ventricular electrode. If the sinus cycle length is shorter than the pulse interval, no atrial stimulus is given, but the AV delay is triggered by the sensed atrial activity, followed by a paced ventricular beat, if a conducted ventricular activation does not occur. By this means, the ventricular rate tracks the atrial rate up to a programmable maximum, allowing the heart to increase its rate in a physiological manner in response to metabolic demand. An alternative, and simpler, approach to achieve a rate response is the use of an activity sensor such as an accelerometer in the pulse generator. Such devices detect bodily movement and increase the pacing rate according to a programmable algorithm. Rate response can be utilized in either single- or dual-chamber pacemakers, and is designated by the suffix ‘R’ (e.g. AAIR, VVIR, DDDR).
|Table 3 Pacemaker mode nomenclature|
|D||Dual (A and V)||D||Dual (A and V)||D||Dual (I and T)|
See text for examples.
Complications of pacemaker insertionThe advantage of DDD pacing over VVI pacing lies in the maintenance of AV synchrony and rate responsiveness, but this is achieved at the expense of increased complexity, complications, and cost. DDD pacing reduces the risk of atrial fibrillation by virtue of pacing the atrium and avoiding retrograde atrial activation via the AV node and has a lower incidence of the pacemaker syndrome (see below). However, large-scale randomized trials comparing DDD with VVI(R) pacing have failed to substantiate survival benefits from DDD pacing, at least during follow-up periods of up to 3 years.
Complications of temporary or permanent pacemaker implantation include those of central venous cannulation (e.g. pneumothorax), perforation of the heart by the electrode tip leading to pericardial effusion and cardiac tamponade, and macroscopic or microscopic displacement of the electrode resulting in an increase in the pacing threshold or failure to capture. A chest radiograph should always be taken after pacemaker insertion to exclude pneumothorax and to confirm that the electrode position is satisfactory.
Permanent pacing may be complicated by the development of infection around the pulse generator, or by mechanical erosion of the generator through the skin. Once infection is established, or the skin is breached, it is almost never possible to eradicate infection with antibiotics: removal and replacement of the pacing system is required. The development of oedema and inflammation around the implanted electrode tip may result in a steady rise in the pacing threshold over the first few weeks, which can lead to an increase of the pacing threshold such that capture is lost, although the process is normally mild and self-limiting.
Demand pacemakers require an adequate intracardiac signal to recognize activation of the chamber in question, to inhibit output. The pacing stimulus will not be suppressed (‘undersensing’) if the intracardiac signal is of insufficient amplitude, resulting in inappropriate pacemaker firing. This phenomenon is commoner in atrial pacing, owing to the lower amplitude of atrial compared with ventricular electrograms. Conversely, detection of extraneous electrical activity (e.g. skeletal muscle activity) via the pacing electrode can result in inappropriate inhibition of the pacemaker output (oversensing). Oversensing is commoner with unipolar than bipolar pacing modes because of the inclusion of the pulse generator can in the electrical circuit, and its proximity to the pectoral muscles. For the same reason, unipolar pacemaker systems are more prone to the problem of local skeletal muscle stimulation. Damage to the conductor or insulation of the pacing electrode may occur due to trauma at the site of ligation or to compression between the clavicle and first rib. This may result in oversensing, skeletal muscle stimulation, or short-circuiting leading to premature battery depletion.
Patients receiving AAI pacemakers may subsequently develop AV block, resulting in a recurrence of syncope and requiring upgrade of the pacing system to a DDD unit. Some patients with VVI pacemakers, particularly those with sinoatrial rather than AV disease, will manifest retrograde ventriculoatrial conduction during ventricular pacing. This sometimes causes symptoms of fatigue, dizziness, or hypotension (‘pacemaker syndrome’), which are associated with the presence of atrial cannon waves occurring as a result of simultaneous atrial and ventricular contraction. Upgrade of the system to a dual-chamber unit is necessary if symptoms are troublesome. Newer pacing systems allow DDD pacemakers to act as single-chamber atrial pacemakers, automatically switching to dual-chamber pacing should AV conduction fail, providing the benefits of atrial pacing with less risk of pacemaker syndrome.Follow-up
Many patients with long-standing heart block treated by permanent pacing have no underlying cardiac rhythm, hence failure of the pacing system for whatever reason may be fatal and patients require follow-up in a pacemaker clinic. As well as detection of the complications described above, the function of such a clinic is to assess the status of the pulse generator battery, and to maximize its life by programming the pulse generator output to the minimum consistent with a satisfactory safety margin. The design of pulse generators and the battery characteristics normally allow prediction of the expected replacement date several months if not years ahead. However, premature battery depletion or pacemaker failure does occur, and patients should therefore be assessed at least annually by the clinic.
Mechanisms of arrhythmogenesis
The principal mechanisms responsible for tachyarrhythmias are those of abnormal automaticity, triggered activity, or re-entry. There is a complex interaction between the underlying substrate, such as previous myocardial infarction, a triggering event such as an extrasystole, and modulating influences, of which sympathetic stimulation and myocardial ischaemia are the most important.
Abnormal automaticity is defined as an inappropriate increase in the rate of discharge of a tissue that has physiological pacemaker properties (sinus node, AV node, or Purkinje fibres) or the pathological development of automaticity in atrial or ventricular myocytes. Such abnormalities are most commonly seen in the presence of ischaemia, sympathetic stimulation, or drug toxicity, especially digoxin. Automatic tachycardias are characterized by an absence of initiation by extrasystoles, either spontaneously or during electrophysiological testing.
The term ‘triggered activity’ is used to define an impulse initiation associated with a preceding action potential, and can be induced in vitro in tissues that do not demonstrate physiological automaticity. Two characteristic forms of depolarization may cause triggered activity.
These occur during the plateau phase of the action potential, prior to repolarization, and are more evident at slow heart rates, particularly in the presence of hypokalaemia and hypomagnesaemia. Mutations in cardiac Na+ or K+ channels, or drugs that prolong myocardial repolarization by inhibiting one or more components of the outward potassium current, I K, (including class IA and class III antiarrhythmics, tricyclic antidepressants, antihistamines, organophosphorous insecticides, and many others) predispose to the appearance of early after-depolarizations in vitro. These changes are associated with the congenital and acquired long QT syndromes and the arrhythmia torsades de pointes (see below).
These are subthreshold depolarizations occurring after full repolarization of the action potential. Their amplitude is increased by tachycardia or intracellular calcium overload, and may reach a threshold at which an action potential is generated, potentially initiating a sustained tachycardia. Delayed after-depolarizations can be induced experimentally by digitalis overload, and are the likely mechanism of digitoxic arrhythmias.
Most clinically important sustained tachycardias, whether of atrial, junctional, or ventricular origin, appear to arise on the basis of re-entry. The establishment of a re-entry tachycardia requires the presence of a potential circuit comprising two limbs with different refractoriness and conduction properties. A premature beat can be conducted in one limb of the circuit, but the other limb may still be refractory, resulting in unidirectional conduction block. If conduction is sufficiently slow, the tissue distal to the site of block in the refractory limb will have regained excitability before the arrival of the depolarizing wavefront, and conducts the activity retrogradely. This results in reactivation of the initial conducting pathway and thus a circus movement tachycardia is established. Macro re-entry is defined as the occurrence of a re-entry circuit over a large area of the heart, such as in the presence of an accessory pathway. Micro re-entry occurs in a relatively small area of the heart, for example at the border zone of an old myocardial infarction, where conduction velocity is markedly slowed. The characteristic feature of a re-entrant tachycardia is that an appropriately timed extrastimulus can induce unidirectional block and initiate the arrhythmia. The tachycardia may be terminated by extrastimuli that depolarize the tissue ahead of the circulating wave front and thus interrupt the circus movement.
Differential diagnosis of tachycardias
The first and most important step in the diagnosis and management of tachycardias is to determine whether the arrhythmia arises within the atria and/or AV junction, or from the ventricles. An essential element in the differential diagnosis is to distinguish between tachycardias with normal QRS-complex morphology and duration (‘narrow-complex tachycardias’), and those where the QRS complexes are abnormal in morphology and increased in duration (‘wide-complex tachycardias’).
Narrow-complex tachycardias arise through mechanisms that result in ventricular activation via the AV node and His–Purkinje system and therefore show normal QRS morphology and duration (≤0.12 s) during tachycardia. Careful study of all leads of the electrocardiogram is necessary to assess regularity of QRS complexes and to identify the presence of atrial activity (P-waves). The relationship of the PR to the RP interval is helpful in determining mechanism of narrow-complex tachycardias. In supraventricular tachycardias (see below), P waves may not be visible, or may occur immediately following the QRS complex. A long RP interval is found in atrial tachycardia, atypical AV nodal re-entry tachycardia and AV re-entry involving a slowly conducting accessory pathway as the retrograde limb. Atrial flutter waves are most commonly evident in the inferior limb leads or in lead V1.
Few areas in cardiology cause more difficulty, or result in more mismanagement, than the diagnosis of wide-complex tachycardias. Whereas it is safe to assume that virtually all narrow-complex tachycardias have a supraventricular origin, wide-complex tachycardias (QRS duration ≥0.12 s) may arise either from the ventricle or from supraventricular mechanisms, the latter occurring if there is bundle branch block, either pre-existing or functional (aberration) as a result of the high rate. An additional cause of aberrant conduction is activation of the ventricles via an accessory pathway.
If the wide QRS morphology during tachycardia is identical to that in sinus rhythm, then a supraventricular origin is likely, with fixed bundle branch block. However, no ECG in sinus rhythm may be available, and difficulties in diagnosis and management arise when ventricular tachycardia is not recognized and is misdiagnosed as ‘SVT with aberration’. This usually happens as a result of a number of failings and misconceptions, the commonest being that the clinical context is not considered:
- ◆ The age of the patient—middle-aged or elderly individuals presenting with a recent history of wide-complex tachycardia, and who give a history of myocardial infarction or congestive heart failure, are more likely to have ventricular than supraventricular tachycardia. However, ventricular tachycardia can also arise in young patients.
- ◆ The haemodynamic status of the patient—it is often assumed that ventricular tachycardia should cause haemodynamic collapse, whereas patients may in fact be haemodynamically stable if the rate is not excessively fast or if underlying cardiac function is good. Conversely, supraventricular tachycardias may cause syncope, hypotension, or shock if sufficiently rapid, or if there is underlying heart disease.
- ◆ The nature of the episodes of palpitation—it is often not appreciated that ventricular tachycardia can present with a typical history of paroxysmal self-terminating episodes, just as in the case of supraventricular tachycardia.
The importance of making a correct diagnosis in wide-complex tachycardia is twofold. First, inappropriate acute therapy of the tachyarrhythmia can be avoided. In particular, the use of verapamil in ventricular tachycardia misdiagnosed as supraventricular tachycardia is associated with a high risk of haemodynamic collapse as a result of its negative inotropic effect, coupled with its lack of efficacy in terminating ventricular tachycardia. Secondly, if the original arrhythmia has been misdiagnosed, then the adverse prognostic significance of ventricular tachycardia will be overlooked. Appropriate investigation and long-term management may not be instituted. It is therefore important that a diagnosis of SVT with aberration is made only if the ECG displays typical left or right bundle branch block. In addition to attention to the history and 12-lead ECG, the response to transient AV nodal blockade with adenosine will assist diagnosis in many patients (Table 4).
General principles of management
Many cardiac arrhythmias are benign and require no intervention. The main indications for treatment are to relieve symptoms, or to prevent complications such as myocardial ischaemia, cardiac failure, embolism, or arrhythmic sudden death. Precipitating factors such as myocardial ischaemia/infarction, infection, thyrotoxicosis, alcohol, electrolyte disorders, or drug toxicity must be sought and treated if possible. The therapy indicated will commonly be influenced by the presence of underlying structural heart disease such as myocardial ischaemia/infarction or left ventricular dysfunction and can include drug therapy, device implantation, or radiofrequency ablation.
Acute management of tachycardia
Assessment of the patient’s cardiorespiratory status takes precedence. R-wave synchronized, direct current (DC) cardioversion under general anaesthesia or deep sedation is the most effective and immediate means of terminating sustained tachycardias, and should be employed when the tachycardia is associated with haemodynamic compromise. Although atrial flutter may respond to low-energy cardioversion (50–100 J), other arrhythmias normally require energies of 100 to 360 J for termination (100–150 J for biphasic shocks).
|Table 4 Diagnostic use of intravenous adenosine|
||Terminates tachycardia by anterograde (AV) block|
In patients with haemodynamically stable tachycardias, manoeuvres that produce transient vagal stimulation such as the Valsalva manoeuvre or carotid sinus massage may be employed. Similarly, adenosine (see below) is used pharmacologically to produce transient slowing or block of the sinus node or AV node. Vagal manouevres or adenosine will often terminate arrhythmias dependent on the AV node, and are also useful diagnostic tools, since transient interruption of AV nodal conduction may reveal the tachycardia mechanism (Table 4. Atrial tachyarrhythmias will not normally be terminated by vagal stimulation or adenosine, but an increase in AV block reveals the underlying atrial rhythm.
Re-entry tachycardias may be terminated by the delivery of appropriately timed extrastimuli that depolarize part of the re-entry circuit prior to the arrival of the wave front and interrupt the arrhythmia. Simple overdrive pacing can be effective in the termination of atrial flutter, AV nodal re-entry, AV (orthodromic) re-entry tachycardia, or sustained ventricular tachycardia. The cardiac chamber in question is paced for brief periods, e.g. 6 to 12 beats, at a rate just above that of the tachycardia, with repeated attempts sometimes necessary at gradually increasing rates. Overdrive atrial or ventricular pacing may result in degeneration into atrial and ventricular fibrillation respectively, hence facilities for immediate defibrillation must be available. Implantable antitachycardia pacing facilities are incorporated into implantable cardioverter-defibrillators (see below).
Treatments for tachycardias
Antiarrhythmic drug therapy
The Vaughan Williams classification is based on the effects of antiarrhythmic drugs in isolated normal tissue, and although many drugs act by more than one mechanism, the classification is still in widespread use. The effects of the major classes of antiarrhythmic drug activity at the tissue level, and the associated electrocardiographic changes, are listed in Table 5. Individual drugs are described in Table 6.
Class I antiarrhythmic drugs act by inhibiting the rapid inward sodium current. Class Ia agents (e.g. quinidine, procainamide, and disopyramide) increase the cardiac action potential duration and have intermediate effects on the onset and recovery kinetics of the sodium channel and hence on intracardiac conduction. Class Ib agents (e.g. lidocaine and mexiletine) shorten the cardiac action potential duration and have very rapid offset kinetics that result in minimal slowing of normal intracardiac conduction. Class Ic drugs (e.g. flecainide and propafenone) have no major effect on action potential duration, but produce the most long-lasting effect on cardiac sodium channel kinetics and the most marked slowing of intracardiac conduction.
Class II activity is defined as antagonism of the arrhythmogenic effects of catecholamines. The commonest agents in this class are the competitive β-adrenoceptor blockers. Other agents such as propafenone have a weak β-receptor blocking activity, and amiodarone (see below) exhibits a noncompetitive sympatholytic effect.
The class III mode of antiarrhythmic activity comprises lengthening of the cardiac action potential duration and hence of the effective refractory period. Drugs in this class possess a broad spectrum of activity against atrial, supraventricular, and ventricular arrhythmias. Currently available class III agents act by inhibiting the rapid component of the outward potassium current I Kr. Dofetilide and ibutilide are examples of drugs with ‘pure’ class III antiarrhythmic actions. Sotalol is a non-selective β-adrenoceptor antagonist that also possesses class III activity. Amiodarone possesses antiarrhythmic activity in all four Vaughan Williams classes.
Class IV drugs (e.g. verapamil and diltiazem) reduce the inward calcium current ICa in sinoatrial and AV nodal tissues. They are used to prevent or interrupt re-entry arrhythmias involving the AV node (e.g. AV nodal re-entry tachycardia), or to slow the ventricular response in atrial fibrillation or flutter. The dihydropyridine calcium antagonists, such as amlodipine and nifedipine, have no antiarrhythmic action.
The antiarrhythmic activity of digoxin is not explained within the Vaughan Williams classification and appears to be mediated predominantly through vagal stimulation. It is used to slow ventricular rate in atrial fibrillation.
Adenosine, a naturally occurring purine nucleoside, is used pharmacologically to produce transient slowing or block of the sinus node or atrioventricular node. It is of particular value in view of its extremely short plasma half-life (c.2 s), which confers safety. It must be administered by rapid intravenous bolus injection, using incremental doses from 3 to 12 mg, to achieve the desired therapeutic effect. Adenosine is contraindicated in pre-excited atrial fibrillation or in severe asthma and cautioned in patients with known pre-excitation syndrome (see below).
External electrical cardioversion, as described above, can be used electively to restore normal rhythm in patients with persistent arrhythmia. Failure of external cardioversion of atrial fibrillation occurs in some patients as a result of various factors, including increased transthoracic impedance due to obesity, prolonged atrial fibrillation, left ventricular dysfunction, and left atrial dilatation. Internal cardioversion can be successful in many of these patients. The procedure involves the introduction of specialized electrode catheters that permit DC-shock delivery between electrodes in the right atrium and the pulmonary artery or coronary sinus, providing a current field that achieves depolarization of both atria.
|Table 5 Classification of antiarrhythmic drug activity|
|ECG effect||Tissue effect|
|HR||PR||QRS||QT||SA node||Atrium||AV node||Ventricle|
ECG effect: +, increases; –, decreases; 0, no effect; HR, heart rate.
Tissue effect: +, antiarrhythmic activity; –, potential adverse or proarrhythmic effect; 0 no effect
|Table 6 Commonly used antiarrhythmic drugs|
|Principal indication||Dose||Adverse effects|
|Quinidine||AF cardioversion||–||1–2 g/day||Hypersensitivity, GI symptoms, QT prolongation, hypotension|
||2 mg/kg||300–600 mg/day||Negative inotropy, QT prolongation, parasympathetic blockade (accelerated AV conduction, urinary retention, dry mouth, blurred vision)|
||100 mg/5 min up to 1000 mg1–6 mg/min||2–6 g/day||Hypotension, QT prolongation, GI upset, lupus syndrome|
||100 mg bolus 1–4 mg/min||Ineffective||CNS—confusion, dysarthria, fits|
||2 mg/kg||100–300 mg/day||Proarrhythmia, negative inotropy, CNS disturbance|
||–||450–900 mg/day||Proarrhythmia, negative inotropy, CNS disturbance, bronchoconstriction|
|Various, e.g. atenolol||
||–||50–100 mg/day||Bradycardia, -ve inotropy, cold extremities, bronchoconstriction, lethargy|
||2 mg/kg||160–480 mg/day||Bradycardia, negative inotropy, cold extremities, bronchoconstriction, lethargy, QT prolongation|
||300 mg in30 min, then 1200 mg/24 h||0.6–1.2 g/day loading first 2 weeks, then100–400 mg/day||Bradycardia, photosensitivity, skin pigmentation, hypo- or hyperthyroidism, alveolitis, hepatitis, peripheral neuropathy, epidydimitis|
||5–10 mg||240–480 mg/day||Negative inotropy, AV block, flushing, constipation|
|Digoxin||AF rate control||0.125–0.5 mg/day ineffective||Anorexia, nausea, vomiting, AV block, atrial and ventricular arrhythmias|
|Adenosine||SVT termination||3–12 mg by incremental bolus||Flushing, chest pain, bronchospasm, transient AV block|
AF, atrial fibrillation; SVT, supraventricular tachycardia (atrioventricular nodal and atrioventricular re-entrant tachycardia); VT, ventricular tachycardia; WPW, Wolff–Parkinson–White syndrome.
Patients identified as being at high risk of sudden cardiac death, e.g. a history of spontaneous or inducible sustained ventricular arrhythmias or out-of-hospital cardiac arrest, may be treated with an implantable cardioverter–defibrillator (ICD). A transvenous rate-sensing/shocking electrode is introduced via the subclavian vein to the right ventricular apex, with the generator implanted in the pectoral region. If a heart rate above the limit programmed by the device is recognized, a shock is delivered between the intracardiac shocking electrode and the generator casing. Some devices also include a right atrial electrode to sense atrial activation. This improves the distinction between sinus or atrial tachyarrhythmias and ventricular tachycardia, and reduces the risk of an inappropriate shock being delivered. A third lead lying in a tributary of the coronary sinus can be implanted to pace the left ventricle and help restore electromechanical synchrony in those with heart failure, reduced ejection fraction and evidence of dyssynchrony (cardiac resynchronization therapy). An ICD can be programmed to deliver initial antitachycardia ventricular pacing for tolerated tachycardias, with shock delivery available for faster rates or if pace-termination fails. ICDs are expensive, complex, and require regular specialist follow-up.
Selective ablation of part of a re-entry circuit, an arrhythmic focus, or of the AV node is used increasingly in the management of arrhythmias, and offers the opportunity of curative treatment. Radiofrequency energy is delivered between the tip of an intracardiac electrode positioned at the appropriate site and an indifferent surface electrode placed over the scapula. The energy produces a localized necrotic lesion 2 to 3 mm in diameter, which results in local conduction block. Current indications for radiofrequency ablation are listed in Table 7, and specific issues are discussed below in relation to individual arrhythmias.
|Table 7 Indications for radiofrequency ablation|
|Pre-excited AF||Accessory pathway||+++|
|AVNRT||Slow pathway||+++||0.5–1 % risk of CHB|
|Atrial flutter||TVA–IVC isthmus||+++|
|Focal atrial tachycardia||Tachycardia focus||++|
|Paroxysmal AF||Pulmonary vein isolation||++||High recurrence rate|
|Persistent AF||Extensive LA ablation||+||Often requires >1 procedure|
|Permanent AF||AV node||+++||Requires permanent pacing|
|Scar-related ventricular tachycardia||Re-entry circuit||+||High recurrence rate|
|Focal ventricular tachycardia||Site of origin||++||Especially RVOT focus|
AVRT, atrioventricular (orthodromic) re-entry tachycardia; AF, atrial fibrillation; AVNRT, atrioventricular nodal re-entry tachycardia; LA, left atrial; CHB, complete heart block; TVA, tricuspid valve annulus; IVC, inferior vena cava; RVOT, right ventricular outflow tract.
The ‘maze’ procedure for atrial fibrillation involves creating a series of lines of conduction block in the left and right atria, either by incisions or by ablation. This prevents the development of atrial re-entry circuits while permitting AV conduction. Surgical management of recurrent ventricular tachycardia by mapping and resection of the re-entry circuit is occasionally performed, but has been largely superseded by ablation or ICD therapy.
Specific causes of arrythmias
The term extrasystole is used to describe a premature beat arising from a focus other than the sinus node. Extrasystoles are also described as premature beats, premature contractions, premature depolarizations, or ectopic beats.
Atrial extrasystoles are recognized by a premature P-wave of different morphology from the sinus P-wave, which can be hidden within the ST segment or T wave of the preceding sinus beat. Premature atrial extrasystoles that occur before full recovery of the AV node will be followed by prolongation of the PR interval, or, if sufficiently premature, complete failure of conduction. Nonconducted atrial extrasystoles must be distinguished from sinus arrest or second-degree AV block.
An atrial extrasytole will commonly reset the sinoatrial node, such that the next sinus beat occurs earlier than expected with respect to the preceding sinus beat, and the pause is less than compensatory.
Atrial extrasystoles are a common finding in healthy people, particularly with increasing age, but are more frequent in the presence of increased atrial pressure or stretch such as in cardiac failure or chronic mitral valve disease. Patients should be reassured that the arrhythmia is benign and that drug treatment is rarely necessary. If treatment is required on symptomatic grounds, β-adrenergic blockers may be used, but class I antiarrhythmic drugs should be avoided in view of their proarrhythmic risk.
Junctional extrasystoles are identified by the appearance of a premature, normal QRS complex in the absence of a preceding atrial extrasystole. The atria as well as the ventricles may be activated, resulting in an inverted P-wave simultaneous with the QRS complex, or inscribed within the ST segment. The significance and management of junctional extrasystoles are similar to those of atrial extrasystoles.
Ventricular extrasystoles are identified by the appearance of a bizarre, wide QRS complex not preceded by a P-wave. There is commonly ST segment depression and T wave inversion. Ventricular extrasystoles may be intermittent, or occur with a fixed relationship to the preceding normal beats, i.e. 1:2, 1:3 (bigeminy or trigeminy). Ventricular extrasystoles occur in otherwise normal hearts, but are found particularly in the presence of structural heart disease. They occur commonly in the acute phase of myocardial infarction, but are also seen in the postinfarction phase, and in the presence of severe left ventricular hypertrophy or dysfunction of whatever cause. Extrasystoles may produce symptoms that require treatment in a minority of cases. The safest option is β-blockade.
Studies of patients with paroxysmal atrial fibrillation suggest that the arrhythmia may be triggered by one or more rapidly discharging foci, which are commonly situated in the pulmonary veins.
The underlying mechanism for maintenance of fibrillation is thought to be re-entry, with multiple wavelets (probably a minimum of six) circulating through the atria. Rapid atrial activation induces a process of electrical remodelling, which renders cardioversion and maintenance of sinus rhythm more difficult (‘atrial fibrillation begets atrial fibrillation’). The initial mechanism of remodelling is thought to be intracellular calcium overload resulting in shortening of the atrial refractory period, although more prolonged atrial tachyarrhythmias result in downregulation of calcium entry and dedifferentiation of atrial myocytes towards a fetal phenotype. Structural changes, including interstitial fibrosis, also occur and further perpetuate the arrhythmia.
The prevalence of atrial fibrillation increases with advancing age and may be as high as 5 to 10% in very elderly individuals. There are numerous causes of the arrhythmia (Bullet list 2ox 16.4.2), but in many instances no obvious aetiological factor can be identified, and the patient is described as having ‘lone’ atrial fibrillation. Atrial fibrillation carries adverse prognostic significance, in part through its association with organic heart disease but also as an important risk factor for the development of stroke and systemic embolism as a result of stasis and thrombus formation in the left atrium. The risk of stroke is particularly high in patients with mitral stenosis or mitral valve replacement and chronic atrial fibrillation.
Atrial fibrillation results in loss of the atrial contribution to left ventricular filling, which can result in a worsening of heart failure. More commonly, symptoms and impairment of left ventricular function (‘tachycardiomyopathy’) arise as a result of a rapid uncontrolled ventricular rate. In addition, uncontrolled atrial fibrillation can cause further impairment of ventricular filling in mitral stenosis and conditions associated with left ventricular diastolic dysfunction, or the development of angina in patients with coexisting coronary artery disease.
Bullet list 2 Aetiology of atrial fibrillation
- ◆ Increased atrial pressure—mitral valve disease, congestive heart failure,
- ◆ left ventricular hypertrophy, restrictive cardiomyopathy, pulmonary embolism
- ◆ Atrial volume overload—atrial septal defect
- ◆ Myocardial ischaemia/infarction
- ◆ Thyrotoxicosis
- ◆ Alcohol
- ◆ Sinoatrial disease
- ◆ Infiltration—constrictive pericarditis, tumour
- ◆ Infection— systemic, e.g. pneumonia
- ◆ Infection— cardiac: myo/pericarditis
- ◆ Retrograde activation—WPW syndrome, ventricular pacing
- ◆ Cardiac or thoracic surgery
- ◆ Idiopathic—‘lone’ atrial fibrillation
The characteristic ECG findings in atrial fibrillation of recent onset are of rapid, irregular ‘f’ waves at a rate of 350 to 600/min. These are associated with an irregular ventricular response because of variable conduction through the AV node. With increasing duration of chronic atrial fibrillation, the amplitude of the ‘f’ waves diminishes until they are no longer visible. Under these circumstances, atrial fibrillation is diagnosed by the absence of P-waves and the irregular ventricular response.
Atrial fibrillation is classified into three patterns: paroxysmal, persistent, or permanent. In paroxysmal atrial fibrillation, spontaneously terminating attacks of palpitation last anything from a few seconds to a few days. The ventricular rate is often rapid and the patient may be severely symptomatic. The term ‘persistent atrial fibrillation’ is used to describe instances where the arrhythmia is not self-terminating, but where sinus rhythm can be restored by electrical or pharmacological cardioversion. Permanent atrial fibrillation describes the situation where restoration of sinus rhythm is no longer possible. At this stage, the ventricular rate is often slower and the patient may be unaware of the irregular pulse or of palpitations.
Appropriate management of atrial fibrillation depends on the presence or absence of symptoms, haemodynamic status, duration of arrhythmia and the presence of factors affecting the successful maintenance of sinus rhythm. Atrial fibrillation of recent onset may terminate spontaneously, particularly if associated with an acute febrile illness. However, outside of the context of an acute febrile illness, an attempt to restore sinus rhythm should be made unless the arrhythmia is obviously long-standing (>48 h) or is associated with advanced organic heart disease. Underlying precipitating factors such as thyrotoxicosis should be corrected before attempting cardioversion.
Chemical cardioversion may be achieved with class Ia, Ic, or III agents. Class Ia agents accelerate the ventricular rate by virtue of their anticholinergic action on the AV node and must be used in combination with AV nodal blocking agent (e.g. digoxin, beta-blocker, or calcium channel blocker). For patients without significant underlying heart disease, the current drugs of choice are the class Ic agents (e.g. flecainide 2 mg/kg intravenously over 30 min). Class III drugs are somewhat less effective but are safer in the presence of left ventricular dysfunction or ischaemic heart disease. Options include sotalol (1.5 mg/kg intravenously over 30 min) or amiodarone (300 mg intravenously over 30 min followed by 1200 mg/24 h until cardioversion). The pure class III agent ibutilide is approved for this indication in the United States of America. Normally, only one drug should be tried in any individual patient. If drug therapy fails, direct current (DC) cardioversion is commonly effective.
Given that atrial fibrillation is a risk factor for the development of intracardiac thrombus formation, cardioversion, by chemical or electrical means, should not be attempted if arrhythmia has been present for longer than 48 h. Anticoagulation plus rate control with a β-blocker, calcium channel blocker, or digoxin should be considered in these circumstances. However, in the presence of haemodynamic compromise, the benefit of achieving sinus rhythm may outweigh the potential risk of embolism and attempt to restore sinus should be made. Transoesphageal echocardiography is useful in this situation to exclude left atrial thrombus.
Prophylaxis against thromboembolism should be considered in all patients with atrial fibrillation, as stroke risk exists with paroxysmal, persistent, or permanent atrial fibrillation. Chronic anticoagulation with warfarin is indicated in patients in atrial fibrillation with rheumatic mitral valve disease. Meta-analysis of trials of patients with nonrheumatic atrial fibrillation shows that warfarin anticoagulation with a target range for the international normalized ratio (INR) of between 2.0 and 3.0 reduces the risk of thromboembolic events by about 60%. Aspirin is a significantly less effective alternative, achieving a risk reduction of around 20%. As such, the choice of antithrombotic prophylaxis depends on balancing the risk of thromboembolism against the risk of haemorrhagic complications, as well as the local facilities for anticoagulant control. The presence of high risk features such as previous stroke, history of heart failure, age over 75 years, hypertension, and diabetes favours warfarin, although the balance of risk vs benefit has to be assessed on an individual basis (Bullet list 3.
Paroxysmal atrial fibrillation
Paroxysmal atrial fibrillation is a self-terminating, recurrent arrhythmia, often associated with marked symptoms of palpitations. The goal of treatment is the maintenance of sinus rhythm and the amelioration of symptoms. In patients with infrequent paroxysms, drug therapy may not be necessary, or a ‘pill in the pocket’ approach can be used with selected patients without structural heart disease. For this, the patient takes a dose of an antiarrhythmic drug after the onset of arrhythmia, e.g. flecainide 200 to 300 mg, if this has previously been shown to be safe and effective under hospital supervision.
In those with recurrent paroxysmal atrial fibrillation, prophylactic therapy should be considered. No drug is entirely satisfactory and a β-blocker is often prescribed as first-line therapy. If this is ineffective, other antiarrhythmic therapy should be started. Class Ic agents (flecainide or propafenone) are effective and reasonably safe in the absence of underlying ischaemia or left ventricular dysfunction. Sotalol (80–160 mg twice daily) is also effective and well tolerated. Amiodarone is effective but can be associated with significant adverse effects and should be reserved for when the above measures fail. In the tachycardia–bradycardia syndrome, implantation of a permanent pacemaker may be required to control bradycardia and to allow antiarrhythmic therapy for the treatment of tachycardia. Finally, radiofrequency ablation should be considered for those in whom pharmacological therapy has failed. Foci arising from the pulmonary vein are targeted by placing lesions at the pulmonary vein antrum to achieve electrical isolation, with clinical success rates of 70 to 80%.
Bullet list 3 Thromboembolic risk and anticoagulation in atrial fibrillation
- ◆ Previous transient ischaemic attack or stroke, or thromboembolic event
- ◆ Age ≥ 75 years with hypertension, diabetes or vascular disease
- ◆ Clinical evidence of valve disease or heart failure, or impaired left ventricular function on echocardiography
- ◆ Age ≥ 65 years of age not in high-risk group
- ◆ Age <75 years with hypertension, diabetes, or vascular disease
- ◆ Age <65 years with no history of stroke, embolism, hypertension, diabetes or vascular disease
- ◆ Warfarin (target INR 2.0–3.0) if no contraindications and if practicable; if contraindications, consider aspirin
- ◆ Either warfarin or aspirin. In view of insufficient evidence, treatment may be determined on an individual basis.
- ◆ Echocardiography may be helpful.
- ◆ Aspirin 75–300 mg daily
Data from NICE Clinical Guideline 36, Atrial fibrillation: the management of atrial fibrillation.
Persistent atrial fibrillation
Persistent atrial fibrillation is not self-terminating, usually requires electrical cardioversion to achieve sinus rhythm, and has a high recurrence rate even after successful cardioversion. The key decision is whether to employ a rhythm or rate control strategy. The AFFIRM trial showed no overall benefit of a rhythm control strategy. In general, a rate control strategy should be employed in asymptomatic or mildly symptomatic individuals, in older people, and in those with contraindications to antiarrhythmic therapy or cardioversion. This group should be treated as having permanent atrial fibrillation. In more severely symptomatic or younger patients, or in those with atrial fibrillation due to a treated precipitant, a rhythm control strategy may be more appropriate. However, treatment choice has to be tailored to the individual and both options should be discussed with the patient. Prophylaxis of thromboembolism should be considered in both groups.
If a rhythm control strategy is adopted, elective cardioversion should be scheduled. Given that cardioversion may be associated with embolism, patients undergoing this procedure should ideally be treated with warfarin for at least 4 weeks beforehand and warfarin should be continued for a minimum of 4 weeks afterwards since atrial mechanical function recovers slowly. There is a high risk of recurrent atrial fibrillation (up to 50% at 1 year) and antiarrhythmic prophylaxis should be considered. First-line therapy is often a simple β-blocker followed by a class Ic agent if there is no structural heart disease. Amiodarone may also be considered, and treatment prior to cardioversion increases the likelihood of its success. Finally, radiofrequency ablation may be employed but this requires more extensive left atrial ablation compared to paroxysmal atrial fibrillation, with a lower success rate, and often requires more than one procedure.
Permanent atrial fibrillation
In permanent atrial fibrillation, restoration of sinus rhythm is not feasible or is unsuccessful and chronic management involves control of ventricular rate. Traditionally, the mainstay of treatment has been digoxin, at a dose titrated to achieve adequate slowing in the ventricular rate at rest, with therapeutic plasma concentrations. However, despite adequate rate control at rest, patients commonly have an uncontrolled heart rate on exercise. Control of rate response with other AV nodal blocking drugs such as β-blockers or verapamil is associated with improved rate control which is especially important if the duration of diastole is critical, as in mitral stenosis or ischaemic heart disease. Often a combination of AV nodal blocking drugs is required. In cases where adequate rate control cannot be achieved despite combination therapy, radiofrequency ablation of the AV node and implantation of a permanent pacemaker is an option, although this commits the patient to lifelong pacemaker therapy.
Atrial flutter is caused by a macro re-entrant circuit in the right atrium, which produces a typical electrocardiographic ‘sawtooth’ pattern of atrial activity with a rate close to 300/min. In the common form of the arrhythmia, flutter waves are negative in leads II, III, and aVF and positive in lead V1. Atrial flutter may be associated with either a regular or irregular ventricular response. Flutter with 2:1 AV conduction produces a regular tachycardia of 150/min and should always be considered in the differential diagnosis of a regular, narrow-QRS tachycardia of this rate. Occasionally, flutter occurs with 1:1 AV conduction producing a ventricular rate approaching 300/min. Class I antiarrhythmic drugs may predispose to this by causing a relative slowing of the atrial rate and allowing 1:1 conduction through the AV node. The flutter waves may not be seen easily with faster ventricular rates, and transient slowing of AV conduction may be necessary to make the diagnosis.
The underlying causes of atrial flutter are the same as those of atrial fibrillation (Bullet list 2). Although atrial flutter may last for many months or occasionally years, it usually degenerates into chronic atrial fibrillation unless cardioversion is undertaken. Atrial flutter also carries a risk of thromboembolism, and anticoagulation is indicated before and after cardioversion as for atrial fibrillation. It is important to attempt to terminate atrial flutter since the ventricular rate is often poorly controlled by AV nodal blocking drugs. Termination may be achieved by chemical or electrical cardioversion as described above for atrial fibrillation. Bursts of atrial overdrive pacing at a rate approximately 10% above the atrial flutter rate are also used: this may restore sinus rhythm or precipitate atrial fibrillation. Prophylaxis against atrial flutter is undertaken using the same agents as in paroxysmal atrial fibrillation; indeed the conditions often coexist and patients may manifest either flutter or fibrillation at different times. Treatment of atrial flutter by radiofrequency ablation creates a line of conduction block between the tricuspid valve annulus and the inferior vena cava, interrupting the isthmus through which the re-entry circuit must pass. This achieves cure in 90% of cases and is increasingly used as a first-line therapy.
Atrial tachycardia usually results in an atrial rate between 120 and 250/min. There may be a degree of AV block, although 1:1 AV conduction can occur. The ECG shows regular P-waves which do not show the same ‘sawtooth’ appearance as in atrial flutter. Atrial tachycardia may occur as a result of sinus node re-entry, with sudden paroxysms of tachycardia with a normal P-wave morphology. Automatic atrial tachycardia manifests an abnormal P-wave morphology, commonly with a prolonged PR interval. The rate characteristically accelerates or ‘warms up’ before reaching a rate of 125 to 200/min. Atrial tachycardia with AV conduction block is a manifestation of digitalis toxicity. Multifocal atrial tachycardia, in which rapid, irregular P-waves of three or four different morphologies are seen, may occur in severely ill elderly patients or in association with acute exacerbation of pulmonary disease.
Management includes drug treatment or cardioversion, as for atrial fibrillation. Focal atrial tachycardia may be amenable to treatment with radiofrequency ablation with success rates approaching 75%, although recurrence rate is high.
Although all atrial arrhythmias are by definition supraventricular in origin, the term supraventricular tachycardia is commonly reserved for those in which the AV node is an obligate part of a re-entry circuit—AV nodal re-entrant tachycardia (AVNRT) or AV re-entry tachycardia (AVRT)—also known as junctional re-entry tachycardias. Correct recognition of these arrhythmias has achieved additional importance with the development of effective curative measures.
Atrioventricular nodal re-entry tachycardia
This is the commonest cause of paroxysmal re-entry tachycardia manifesting regular, normal QRS complexes. The basis of the arrhythmia is the presence of two functionally distinct pathways in the region of the AV node. The ‘fast’ pathway conducts more rapidly, but has a longer refractory period. The ‘slow’ pathway has slower conduction properties but a shorter refractory period. During sinus rhythm, AV nodal conduction occurs via the fast pathway with a normal PR interval. If a sufficiently premature atrial extrasystole arises, conduction in the fast pathway is blocked, but slow pathway conduction may continue, resulting in an abrupt increase in the AH interval as recorded in the His bundle electrogram. This corresponds to an increased PR interval on the surface ECG. If conduction down the slow pathway is sufficiently delayed to allow the fast pathway to recover excitability before activation reaches the distal end of the pathways, retrograde activation occurs via the fast pathway. The stage is then set for a re-entry circuit with anterograde conduction via the slow pathway and retrograde conduction via the fast pathway (‘slow/fast AV nodal re-entry’). Characteristically, anterograde activation of the ventricles and retrograde activation of the atria occur virtually simultaneously, resulting in the P-wave being ‘buried’ within the QRS complex, or producing a very small distortion of the terminal QRS, recognition of which requires careful comparison with the ECG during sinus rhythm.
A less common variant of AV nodal re-entry tachycardia may arise where anterograde conduction during tachycardia is via the fast pathway with retrograde conduction via the slow pathway (‘fast/slow AV nodal re-entry’). Under these circumstances, the atrium is activated well after the QRS complex, characteristically producing an inverted P′ wave, with the RP′ interval greater than the P′ interval during tachycardia.
Atrioventricular nodal re-entry tachycardia commonly presents for the first time in childhood or adolescence, although it may appear at any age. The natural history is of episodic paroxysmal tachycardia. Attacks occur at random intervals, although clustering of attacks may occur interposed with periods of relative freedom from symptoms. Atrioventricular nodal re-entry tachycardia has no specific association with other organic heart disease. Palpitations are normally well tolerated unless the tachycardia is particularly rapid, prolonged, or if the patient has other heart disease.
Termination of an attack of AV nodal re-entry tachycardia is achieved by producing transient AV nodal block. This may be achieved by vagotonic manoeuvres, by intravenous adenosine (3–12 mg), or by intravenous verapamil (5–10 mg). Drug prophylaxis of AV nodal re-entry tachycardia is undertaken with β-blockers, a combined β-blocker/class III agent such as sotalol, or AV nodal blocking drugs such as verapamil or digoxin. Curative treatment of AV nodal re-entry tachycardia by radiofrequency ablation is increasingly used as a first-line therapy, and is indicated if patients are refractory to drugs, intolerant of side effects, or unwilling to take long-term medication. Radiofrequency energy is delivered to the ‘slow’ pathway, which lies between the compact AV node and the tricuspid annulus. Ablation at this site is normally curative in over 90% of cases, but carries a small risk (0.5–1%) of inducing complete heart block.
Atrioventricular re-entry tachycardia
In contrast to AV nodal re-entry tachycardia, the substrate for AV re-entry is the presence of a second atrioventricular connection, separate from the AV node. This accessory pathway can lie anywhere along the mitral or tricuspid annuli. Anterograde pathway conduction produces ventricular pre-excitation and is discussed in the ‘Pre-excitation syndromes’ section below. However, most pathways conduct only in the retrograde (ventriculoatrial) direction and are termed ‘concealed’, since there is no clue to their presence on the resting ECG. The anterograde limb of the re-entrant circuit is the AV node, with retrograde atrial activation occurring over the accessory pathway. This is termed orthodromic tachycardia and normally produces a narrow complex QRS morphology. Retrograde atrial activation can be identified by the presence of a characteristic inverted P′ wave early in the ST segment, an important diagnostic feature of AV re-entry tachycardia. Rarely, an accessory pathway with slow retrograde conduction may allow a stable, incessant re-entrant circuit with a long RP interval, referred to as permanent junctional reciprocating tachycardia.
Features are similar to AV nodal re-entry tachycardia, although accessory pathways are the more common tachycardia substrate in children. Patients have a similar relapsing course of symptoms interspersed with periods of relative quiescence. Multiple pathways can be present within the same patient and are more common if there is coexisting Ebstein’s anomaly.
As with AV nodal re-entry tachycardia, the AV node is an obligate part of the circuit and attacks may be aborted by vagotonic manoeuvres or with intravenous adenosine or verapamil. Antiarrhythmic therapy may be effective, but radiofrequency ablation offers high success rates with low incidence of complications and should be considered early in a patient’s treatment.
Pre-excitation syndromes (Wolff–Parkinson–White syndrome)
The term ‘pre-excitation’ refers to the premature activation of the ventricle via one or more accessory pathways that bypass the normal AV node and His–Purkinje system. The commonest of the pre-excitation syndromes is the Wolff–Parkinson–White syndrome, in which accessory pathways with electrophysiological properties of normal myocardium may lie at any point in the AV ring, the commonest sites being in the left free wall or the posteroseptal region. The characteristic electrocardiographic appearance is of early activation of the ventricular myocardium adjacent to the insertion of the accessory pathway. There is no AV delay via the pathway, hence the PR interval is shortened, but slow intraventricular conduction results in slurred initiation of the QRS complex (the delta wave), before the remainder of the ventricle is excited via the normal His–Purkinje system. The ECG appearances of a delta wave occur in approximately 1.5 per 1000 of the population, but many individuals never experience paroxysmal tachycardias. The degree of pre-excitation during sinus rhythm is variable: it may be intermittent if the refractory period of the accessory pathway is close to the sinus cycle length, or inapparent if the delta wave is obscured due to rapid AV nodal conduction. In such instances, transient slowing of AV nodal conduction (e.g. by adenosine) will enhance the proportion of the ventricle excited by the accessory pathway and reveal pre-excitation.
Mechanisms of orthodromic and antidromic tachycardia
A premature atrial extrasystole may find the pathway refractory but be conducted through the AV node to the ventricles. If sufficient delay has occurred by the time the ventricular insertion of the accessory pathway is depolarized, the pathway may have recovered excitability and allow retrograde activation from the ventricle to atrium, with the establishment of a re-entry circuit. Since the circuit involves activation of the ventricles via the His–Purkinje system, the QRS morphology during re-entry tachycardia is normal, unless a rate-related bundle branch block develops.
A rare form of AV re-entry tachycardia has anterograde conduction via the accessory pathway and retrograde conduction via the AV node (antidromic tachycardia). The QRS morphology of this tachycardia is grossly abnormal with appearances dependent upon the site of insertion of the accessory pathway.
Pre-excited atrial fibrillation
The major prognostic concern in Wolff–Parkinson–White syndrome is pre-excited atrial fibrillation. Conduction via an accessory pathway with a short refractory period, bypassing the normal AV nodal slowing, may result in very rapid ventricular conduction that can degenerate into ventricular fibrillation. The degree of pre-excitation during atrial fibrillation varies, giving a characteristic pattern of an irregular ventricular response with QRS morphology ranging from normal to fully pre-excited. The risk of sudden death is increased if the shortest R–R interval is <250 ms during pre-excited atrial fibrillation, and is an indication for urgent cardioversion and early radiofrequency ablation.
Management of the symptomatic patient with ventricular pre-excitation
The AV node is a part of the re-entry circuit in both ortho- and antidromic tachycardia, and adenosine and other AV nodal blocking drugs may be effective. However, adenosine may precipitate pre-excited atrial fibrillation and should be used with caution. In patients with known Wolff–Parkinson–White syndrome presenting with AV re-entrant tachycardia, drugs which also act on the accessory pathway such as flecainide or sotalol may be preferred. In pre-excited atrial fibrillation, AV nodal blocking drugs such as digoxin or verapamil should be avoided, because of the risk of ventricular fibrillation; treatment should be with antiarrhythmic therapy such as flecainide or by DC cardioversion. Patients with symptomatic Wolff–Parkinson–White syndrome should be offered radiofrequency ablation as first-line therapy. This abolishes the risk of pre-excited atrial fibrillation as well as preventing further attacks of AV re-entry tachycardia. Careful mapping of the AV annulus using an electrode catheter is necessary to identify the site of the accessory pathway, at which the interval between the atrial and ventricular electrograms is at a minimum. Passage of the radiofrequency current causes heating of the catheter tip and results in the disappearance of accessory pathway conduction within a few seconds. The success rate of ablation varies according to the location of the pathway, but is usually over 90% in experienced hands.
Approach to the asymptomatic patient with ventricular pre-excitation
Patients with Wolff–Parkinson–White syndrome should be evaluated carefully for the risk of pre-excited atrial fibrillation, even in the absence of symptoms. The risk of sudden death due to rapid pre-excited atrial fibrillation is very low among those who have not had any symptomatic tachycardias, but is higher in symptomatic patients. If pre-excitation is intermittent, this indicates a long refractory period of the pathway and a low risk of life-threatening tachycardias. Abrupt disappearance of the delta wave in response to exercise testing, or during Holter monitoring, or with the administration of a class Ia or Ic antiarrhythmic drug, also suggests a low risk. Some centres advocate diagnostic electrophysiological studies to identify a high-risk group with short pathway refractory periods and inducible tachycardia or pre-excited atrial fibrillation. The general tendency is for accessory pathway conduction to become slower with increasing age, and spontaneous disappearance of conduction is well documented.
Other pre-excitation syndromes
Other forms of pre-excitation include the Mahaim pathway, a direct AV or atriofasicular connection with slow conduction properties similar to AV nodal tissue. Evidence for direct atrionodal pathways associated with a short PR interval but no delta wave (Lown–Ganong–Levine syndrome) remains controversial and has not been established histologically.
Ventricular tachycardia is defined as the presence of three or more consecutive ventricular beats at a rate of 120/min or greater. It is considered to be sustained if an individual salvo lasts for 30 s or more, and nonsustained if the duration is between 3 beats and 30 s. Monomorphic ventricular tachycardia demonstrates a consistent QRS morphology during each paroxysm,. Polymorphic ventricular tachycardia demonstrates a constantly changing QRS morphology, often without discrete QRS complexes. Polymorphic ventricular tachycardia may degenerate into ventricular fibrillation and the electrocardiographic distinction between the two is difficult. Torsades de pointes is a polymorphic VT in association with QT interval prolongation and is discussed in more detail later in the chapter.
The presence of AV dissociation is a particularly important feature to seek in a wide-complex tachycardia as it makes the diagnosis of ventricular tachycardia virtually certain. A careful search for P-waves perturbing the QRS complex or T-waves is necessary, ideally using multilead recordings. Occasionally, a fortuitously timed P-wave allows the development of a capture beat of normal QRS morphology without interrupting the tachycardia. A fusion beat occurs when activation of the ventricle is partly via the normal His–Purkinje system and partly from the tachycardia focus. Fusion and capture beats are diagnostic of ventricular tachycardia, but are commonly present only if the ventricular rate is relatively slow. Where dissociated P-wave activity cannot be recognized with certainty on the surface ECG, direct recording of atrial activity by an oesophageal or right atrial electrogram may aid the diagnosis. Although AV dissociation is diagnostic of ventricular tachycardia, it is not invariable. Retrograde ventriculoatrial conduction may occur, giving either 1:1 conduction or higher degrees of block.
The QRS duration in ventricular tachycardia is commonly greater than 0.12 s, and values greater than 0.14 s are particularly suggestive of ventricular tachycardia. Although the QRS morphology may superficially resemble left or right bundle branch block, the morphology is commonly atypical. Ventricular tachycardia arising from the right ventricular free wall has a left bundle branch block-like pattern, whereas left ventricular free wall tachycardias show right bundle branch block morphology. The presence of concordant positive or negative QRS complexes across the chest leads is suggestive of ventricular tachycardia, as is the existence of extreme axis deviation.
Sustained monomorphic ventricular tachycardia commonly occurs in the presence of structural heart disease, but also arises in structurally normal hearts. It rarely occurs in the acute phase of myocardial infarction, but may be seen in the subacute phase (>48 h), or may arise many years later, particularly in association with left ventricular dilatation and aneurysm formation. The arrhythmia also occurs in other conditions associated with ventricular dilatation or fibrosis such as dilated cardiomyopathy, hypertrophic cardiomyopathy, or previous ventriculotomy (e.g. following repair of Fallot’s tetralogy). Ventricular tachycardia may degenerate into ventricular fibrillation (see below). Sustained monomorphic tachycardia can occur as a proarrhythmic response to antiarrhythmic drugs, particularly class I agents.
Although ventricular tachycardia normally occurs in individuals with overt heart disease, it is also seen in young and apparently healthy subjects. In these, occult cardiac disease or cardiac genetic syndromes should be considered (see below). There remain a few patients with documented ventricular tachycardia in whom no structural heart disease is evident on clinical, ECG, or echocardiographic examination. The tachycardia may arise from the outflow tract of the right or (rarely) left ventricle, or from one of the fascicles of the left bundle branch, and is amenable to radiofrequency ablation.
Acute management of ventricular tachycardia
Rapid ventricular tachycardia may present with cardiac arrest, syncope, shock, anginal chest pain, or left ventricular failure, but slower tachycardias in patients with preserved cardiac function may be well tolerated. Sustained ventricular tachycardia is a medical emergency. If the patient is pulseless or unconscious, immediate DC cardioversion is necessary. If the patient is conscious but hypotensive, urgent DC cardioversion under general anaesthesia or deep sedation is used. Haemodynamically tolerated tachycardias may be terminated by drug therapy. Adenosine may be administered if there is diagnostic uncertainty, but is likely to be ineffective (see Table 5). Intravenous lidocaine (lignocaine) 100 mg, repeated if necessary after 5 min, may restore sinus rhythm in about 30% of patients, and is usually well tolerated. Sotalol 1.5 mg/kg intravenously is more effective, but its use is restricted by its negative inotropic action. Second-line drugs for the termination of ventricular tachycardia include procainamide, disopyramide, and amiodarone. Amiodarone has a relatively slow onset of action but may be effective if the tachycardia is well tolerated. Flecainide is contraindicated in view of the risk of developing incessant tachycardia. Verapamil should be avoided as it may cause clinical deterioration. The only exception to this is in the rare instance of patients with structurally normal hearts who have ventricular tachycardia that is known to respond to verapamil, e.g. left ventricular fascicular tachycardia. All antiarrhythmic drugs have significant negative inotropic actions that may further impair the haemodynamic status of the patient if sinus rhythm is not restored. For this reason, no more than one antiarrhythmic drug should normally be given before recourse to alternative therapy, usually DC cardioversion. Overdrive termination of ventricular tachycardia following insertion of a temporary pacing lead may be effective, particularly if the tachycardia is relatively slow. Facilities for cardioversion must be available in view of the risk of acceleration or degeneration into ventricular fibrillation.
Ventricular tachycardia is a potentially life-threatening condition. Unless the acute episode was clearly precipitated by some transient or reversible factor, there is a high probability of recurrent attacks, which may result in sudden death. Prognosis is worse if the arrhythmia was poorly tolerated, or if there is severe left ventricular dysfunction. The 3-year cardiac survival rate varies from 80% in patients in whom arrhythmia induction is suppressed by antiarrhythmic drug therapy to 40% in those in whom no effect of suppression is achieved and/or empirical therapy is used.
Clinical evaluation of the patient after restoration of sinus rhythm should be supported by electrocardiography, echocardiography, and/or radionuclide ventriculography. In those with ischaemic heart disease, exercise testing should be undertaken to identify the presence of reversible ischaemia, which may act as a trigger to ventricular tachycardia, and coronary arteriography to determine the extent of arterial disease. Unless there is a clear precipitating factor such as drug toxicity, electrolyte abnormality, or acute ischaemia, the risk of sudden death is high and patients should be considered for an ICD. A meta-analysis of three secondary prevention trials of patients resuscitated from ventricular fibrillation or ventricular tachycardia causing haemodynamic compromise showed defibrillators to be better than antiarrhythmic drug therapy in preventing death from any cause.
Patients with left ventricular dysfunction of any cause are at risk of sudden death from ventricular tachycardia or fibrillation and implantable defibrillators are appropriate for a subgroup of these patients as part of a primary prevention strategy. Those with nonsustained ventricular tachycardia, in whom sustained tachycardia can be induced at electrophysiological testing, have a better survival with defibrillator implantation compared with drug therapy. The Sudden Cardiac Death in Heart Failure Trial (SCD-HeFT) expanded the indications to include patients with class II/III heart failure and an ejection fraction <30%, even in the absence of known arrhythmia. Patients with a QRS duration greater than 120 ms appear to derive the largest benefit.
Implantable defibrillator therapy is not affordable in all countries, and not appropriate for patients with other conditions causing a severely limited prognosis. Medical therapy is necessary for many patients, but is limited by a relative lack of evidence from randomized controlled trials. β-Adrenoceptor blockers are comparable to conventional antiarrhythmic agents in the prevention of recurrent ventricular tachyarrhythmias. Since they have been shown to reduce the risk of sudden death in unselected survivors of myocardial infarction and in patients with chronic heart failure, they should be used routinely in the prophylaxis of ventricular tachycardia if tolerated. Of the conventional antiarrhythmic agents, there is evidence that the class III drugs sotalol and amiodarone are superior to class I antiarrhythmic agents, which should not be used for this indication. However, sotalol and amiodarone have not been tested against placebo or conventional β-adrenoceptor blockers in randomized trials, although observational studies suggest they are of benefit in the prevention of arrhythmic death.
Radiofrequency ablation is used in the management of ventricular tachycardia, particularly in those with no structural heart disease. Right ventricular outflow tract and fascicular tachycardia are particularly amenable to ablation. Location and ablation of critical areas of slow conduction in ventricular tachycardias of an ischaemic origin is feasible but technically difficult. Success rates are lower than for other types of ablation and this approach is often reserved for the treatment of frequently recurrent tachycardia in patients with implantable defibrillators.
Direct surgical management of recurrent ventricular tachycardia involves aneurysmectomy, endocardial mapping, and resection of the subendocardial area containing the micro re-entry circuit. The indications for surgery have been reduced considerably since the advent of the ICD, since the surgical mortality is up to 10 to 15%, compared with 0.5% for defibrillator implantation. Where medically intractable ventricular tachyarrhythmias are associated with very poor left ventricular function, the only possible therapeutic option is cardiac transplantation.
Nonsustained ventricular tachycardia
The mechanism and causes of nonsustained ventricular tachycardia are similar to those of sustained ventricular tachycardia. There is often slight variation in the RR interval, particularly if the salvo involves only a few beats. Short salvos of nonsustained ventricular tachycardia are often asymptomatic. Apart from the instances where nonsustained ventricular tachycardia produces troublesome symptoms, the major clinical significance of the arrhythmia is as a risk marker for sustained ventricular tachycardia or sudden cardiac death in patients with left ventricular dysfunction or hypertrophy. Patients with structural heart disease, in particular those with severe left ventricular dysfunction, with QRS duration >120 ms or inducible ventricular arrhythmias, should be considered for an implantable defibrillator as primary prevention of sudden cardiac death. If no significant organic heart disease is present, and the patient is asymptomatic, no treatment is indicated as long-term follow-up of such patients indicates a good prognosis with no excess risk of sudden death.
Polymorphic ventricular tachycardia
Polymorphic ventricular tachycardia is an unstable rhythm with varying QRS morphology. It is most commonly seen in the acute phase of myocardial infarction and is due to unstable re-entry circuits. As such, it either undergoes spontaneous termination or degenerates into ventricular fibrillation. If episodes of polymorphic ventricular tachycardia are frequent in the early hours of myocardial infarction, they can be suppressed by intravenous lidocaine (lignocaine), although there is no evidence that this improves survival. Short, infrequent episodes are commonly left untreated.
Torsades de pointes and the long-QT syndromes
Torsades de pointes is a characteristic type of polymorphic ventricular tachycardia with a typical undulating variation in QRS morphology as a result of variation in axis. It occurs in association with a prolonged QT interval during sinus rhythm. Long-QT syndromes may be acquired or congenital. The latter are discussed later in the chapter.
Although class Ia and III antiarrhythmic drugs are the best-known causes of acquired long QT syndrome, a very large number of noncardiac drugs inhibit the outward potassium current I Kr, and may cause significant lengthening of the QT interval singly or in combination (Table 8). Episodes of torsades de pointes are often multifactorial in origin, with prolongation of the QT interval by an Ikr inhibitor in association with predisposing factors such as bradycardia or pauses, hypokalaemia, or hypomagnesaemia. All of these predispose to early after-depolarizations in vitro and this mechanism appears to be the likely cause of torsades de pointes in the acquired syndromes. The prognosis of the acquired long QT syndromes is excellent, provided the underlying predisposing factors are identified and corrected. However, it is increasingly recognized that there is a genetic predisposition to the development of acquired long QT syndrome in the face of predisposing factors, leading to the concept that patients developing acquired long QT syndrome have reduced ‘repolarization reserve’ as a result of a forme fruste of the congenital syndrome.
|Table 8 Causes or contributory factors in acquired long-QT syndromes|
|Drug induced||Antiarrhythmic drugs —classes ia, iii|
|Macrolide antibiotics —erythromycin|
|Vasodilators —prenylamine, ketanserin, lidoflazine|
|Psychotropics —tricyclic/tetracyclic antidepressants, antipsychotics|
|Antihistamines —terfenadrine, astemizole|
|Cholinergic antagonists —cisapride|
|Synthetic opioid —methadone|
|Electrolyte disturbances||Hypokalaemia, hypomagnesaemia, hypocalcaemia|
|Metabolic||Hypothyroidism, starvation, anorexia nervosa, liquid protein diet|
|Bradycardia||Sinoatrial disease, AV block|
|Toxins||Organophosphorous insecticides, heavy metal poisoning|
Torsades de pointes is an atypical ventricular tachycardia characterized by a continuously varying QRS axis (‘twisting of points’). Episodes of torsades are commonly repetitive and normally self-terminating, although they may degenerate into ventricular fibrillation. Paroxysms of torsades de pointes are associated in the preceding beats with evidence of marked QT prolongation, and frequently with morphological abnormalities of the T-wave such as T–U fusion, gross increases in T-wave amplitude, or T-wave alternans. In the acquired long-QT syndromes a slowing of the heart rate, and in particular a postextrasystolic pause, is often associated with initiation of the arrhythmia. This produces a characteristic ‘short–long–short’ sequence of initiation.
The common clinical presentation is of recurrent dizziness or syncope, and the condition may easily be misdiagnosed as self-terminating polymorphic ventricular tachycardia or ventricular fibrillation unless the characteristic morphology of torsades de pointes and the associated QT interval prolongation is recognized. It is essential to discontinue predisposing drugs or other agents and to avoid empirical antiarrhythmic drug therapy, which may worsen the arrhythmia. Individual paroxysms of torsades de pointes are normally self-limiting, but if they are persistent, cardiac arrest will occur and emergency defibrillation is necessary. Intravenous magnesium sulphate (8 mmol over 10–15 min, repeated if necessary) is a safe and effective emergency measure for the prevention of recurrent paroxysms of tachycardia. If torsades de pointes is associated with bradycardia and pauses, the heart rate should be increased to between 90 and 100/min by atrial or ventricular pacing or isoproterenol (isoprenaline) infusion. Hypokalaemia should be sought and corrected if necessary.
Accelerated idioventricular rhythm
The term ‘accelerated idioventricular rhythm’ is used to describe a continuous ventricular rhythm with a rate less than 120/min. Idioventricular rhythm commonly occurs in the setting of acute myocardial infarction and appears to be a marker of successful reperfusion therapy. No active treatment is necessary.
Ventricular fibrillation is defined as a chaotic, disorganized arrhythmia with no identifiable QRS complexes. The mechanism is of multiple, unstable re-entry circuits. The electrocardiographic pattern depends on the duration of fibrillation: recent-onset fibrillation is described as ‘coarse’, with a peak-to-peak amplitude of around 1 mV (1 cm). With increasing duration of cardiac arrest, the amplitude of ventricular fibrillation diminishes and such ‘fine’ ventricular fibrillation is less likely to be amenable to successful electrical defibrillation.
Ventricular fibrillation may occur during acute myocardial ischaemia often initiated by an R on T extrasystole, and is the principal cause of death in the first 2 h following acute myocardial infarction. Ventricular fibrillation during myocardial infarction is subdivided into primary, occurring without warning in an otherwise stable patient, and secondary, where fibrillation occurs in the context of left ventricular failure or cardiogenic shock. Ventricular fibrillation occurring in chronic heart disease is most commonly a result of degeneration of rapid ventricular tachycardia, whose causes have been described above. Rarer causes of fibrillation are listed in Bullet list 4.
Ventricular fibrillation is rarely self-terminating, and normally causes cardiac arrest with the rapid onset of pulselessness, unconsciousness, and apnoea.
Patients who survive an episode of ventricular fibrillation should be assessed carefully to determine the risk of recurrence. If ventricular fibrillation has occurred in the first few hours of a typical ST-elevation myocardial infarction, the risk of recurrent cardiac arrest is low, and no specific prophylactic therapy other than assessment and treatment of residual ischaemia and conventional postinfarction β-blockade is indicated. However, in many instances ventricular fibrillation arises as a result of acute ischaemia in patients with known, extensive heart disease who have not sustained an acute infarction. These patients remain at high risk of recurrent ventricular fibrillation, and should be evaluated fully by exercise testing and coronary arteriography with a view to revascularization, and managed with an ICD or antiarrhythmic therapy as discussed in the section on ventricular tachycardia.
Bullet list 4 Causes of ventricular fibrillation
- ◆ Acute myocardial ischaemia
- ◆ Acute myocardial infarction—primary or secondary
- ◆ Advanced organic heart disease with poor LV or RV function
- ◆ Severe LV hypertrophy
- ◆ Ventricular tachycardia/torsades de pointes
- ◆ Electrical—electrocution, lightning, unsynchronized DC shock, competitive ventricular pacing
- ◆ Pre-excited atrial fibrillation
- ◆ Profound bradycardia
- ◆ Hypoxia, acidosis
- ◆ Genetic syndromes (e.g. long QT-syndrome, Brugada syndrome)
Congenital long-QT syndromes
The congenital long-QT syndromes are inherited conditions due to mutations in genes encoding ion channel proteins. They are mainly autosomal dominant and are subclassified according to the underlying gene defect (Table 9). Most cases are either LQT1 or LQT2, due to mutations affecting either the slow (I Ks) or rapid (I Kr) components of the outward potassium current. In the less common LQT3, the inward sodium current (I Na) is affected. Lengthening of ventricular repolarisation, and hence of the QT interval, occur as a result either of reduced outward current flow via I Kr or I Ks or increased duration of current flow via I Na. The arrhythmia, torsades de pointes, has characteristics consistent with triggered activity.
Attacks of torsades de pointes in the congenital syndromes are commonly associated with sympathetic stimulation such as exercise, waking, or fright, and are associated with increases in sinus rate. Cardiac events are particularly associated with exercise in LQT1, with auditory stimulation in LQT2, and can occur during sleep in LQT3. Paroxysms may produce syncope, which if prolonged may be complicated by convulsion, leading to misdiagnosis as epilepsy. A family history of recurrent syncope or sudden death may be obtained. Sinus bradycardia is commonly seen in these syndromes.
|Table 9 Congenital long-QT syndromes|
|Subtype||Chromosome||Gene||Protein||Ion current affected||Frequency|
The prognosis of untreated congenital long-QT syndrome is poor, with a high incidence of sudden death in childhood. Increased risk of syncope or sudden death is associated with a corrected QT interval (QTc) of more than 500 ms. Males with LQT3 are at increased risk regardless of the degree of QT interval prolongation. LQT1 has a better prognosis than other subtypes. Episodes of torsades de pointes and T wave alternans on Holter monitoring also confer a higher risk.
β-Blockers are highly effective in LQT1 but are less protective in LQT2 and LQT3. Selective high left stellate ganglionectomy has been employed successfully in cases with recurrent events despite β-blockers. Permanent pacing at rates of 70 to 80/min, in combination with β-blockers, may also be effective in reducing symptoms but defibrillator implantation is necessary for resistant cases, and is commonly used as first-line therapy if episodes of torsades de pointes have resulted in cardiac arrest or in those thought to be at high risk of sudden death.
This is a recently described entity with autosomal dominant inheritance characterized by a gain of function mutation in the outward potassium currents (I Kr and I Ks). It produces a markedly shortened QTc, often less than 280 ms, and predisposes to atrial and ventricular fibrillation.
The Brugada syndrome is an autosomal dominant condition which has a risk of sudden cardiac death associated with characteristic ECG abnormalities and a structurally normal heart. There is an unusual pattern of variable ST-segment elevation and partial right bundle branch block in the right precordial leads, associated with a risk of polymorphic ventricular tachycardia and sudden death. Mutations of genes encoding the rapid sodium channel (SCN5a), causing partial inactivation, have been identified in about 20% of patients. Patients with a history of syncope or a family history of sudden death should be considered for defibrillator therapy.
Catecholaminergic polymorphic ventricular tachycardia
This is a rare arrhythmia characterized by polymorphic or bidirectional ventricular tachycardia occurring in situations of strenuous exercise, psychological stress or emotion, often presenting in childhood. It is associated with mutations of genes involved in controlling intracellular calcium handling. Mutations of the cardiac ryanodine receptor have autosomal dominant transmission, whereas mutations of the gene encoding for calsequestrin have autosomal recessive transmission. The resting ECG has no diagnostic features and the heart is structurally normal. β-Blockers may prevent syncope but an ICD may be indicated for recurrent symptoms or high risk of cardiac arrest.
Hypertrophic cardiomyopathy has a prevalence of 0.2% in the population, and is associated with a wide range of mutations encoding structural or regulatory proteins of the cardiac myofibrillar apparatus. The mode of inheritance is autosomal dominant in 70% of cases, with variable penetrance. Although symptoms are often related to impaired haemodynamics, left ventricular hypertrophy and myofibrillar disarray increase the risk of re-entrant arrhythmias and sudden death. Patients with sustained ventricular tachycardia or fibrillation should be considered for defibrillator therapy. Risk assessment should be performed in all patients with hypertrophic cardiomyopathy. Unexplained syncope, nonsustained ventricular tachycardia, ventricular septal thickness greater than 3 cm, a family history of sudden cardiac death, and a hypotensive response to exercise are all associated with increased risk. An ICD may be considered if two or more high-risk features are present.
Arrhythmogenic right ventricular cardiomyopathy
Arrhythmogenic right ventricular cardiomyopathy (dysplasia) is an autosomal dominant condition associated with replacement of the right ventricular free wall with fat and fibrous tissue. These patients may have no symptoms or signs of cardiac disease, but typical ECG changes (epsilon wave in V1, or T wave inversion in the right precordial leads) are associated with variable degrees of dilatation of the right ventricle demonstratable by echocardiography or MRI. This creates a substrate for ventricular tachycardia and fibrillation and many patients will ultimately require defibrillator therapy.