The right and left coronary arteries arise from the ascending aorta in its anterior and left posterior sinuses. The levels of the coronary ostia are variable. The two arteries, as indicated by their name, form an oblique inverted crown, in which an anastomotic circle in the atrioventricular groove is connected by marginal and interventricular (descending) loops intersecting at the cardiac apex. This is, of course, only an approximation. The degree of anastomosis varies and is usually insignificant. The main arteries and major branches are usually subepicardial, but those in the atrioventricular and interventricular grooves are often deeply sited, and occasionally hidden by overlapping myocardium or embedded in it.
The term ‘dominant' is used to refer to the coronary artery giving off the posterior interventricular (descending) branch, which supplies the posterior part of the ventricular septum and often part of the posterolateral wall of the left ventricle. The dominant artery is usually the right (60%). Anastomoses between right and left coronary arteries are abundant during fetal life, but are much reduced by the end of the first year of life. Anastomoses providing collateral circulation may become prominent in conditions of hypoxia and in coronary artery disease. An additional collateral circulation is provided by small branches from mediastinal, pericardial and bronchial vessels.
The calibre of coronary arteries, both main stems and larger branches, based on measurements of arterial casts or angiograms, ranges between 1.5 and 5.5 mm for the coronary arteries at their origins. The left exceed the right in 60% of hearts, the right being larger in 17%, and both vessels being approximately equal in 23%. The diameters of the coronary arteries may increase up to the 30th year.
Right coronary artery
The right coronary artery arises from the anterior (‘right coronary') aortic sinus: the ostium is below the margin of the cusps in 10%. The artery is usually single, but as many as four right coronary arteries have been observed. It passes at first anteriorly and slightly to the right between the right auricle and pulmonary trunk, where the sinus usually bulges. It reaches the atrioventricular groove and descends in this almost vertically to the right (acute) cardiac border, curving around it into the posterior part of the groove, where the latter approaches its junction with both interatrial and interventricular grooves, a region appropriately termed the crux of the heart. The artery reaches the crux and ends a little to the left of it, often by anastomosing with the circumflex branch of the left coronary artery. In a minority of individuals, the right coronary artery ends near the right cardiac border (10%), or between this and the crux (10%); more often (20%) it reaches the left border, replacing part of the circumflex artery.
Branches of the right coronary supply the right atrium and ventricle and, variably, parts of the left chambers and atrioventricular septum. The first branch (arising separately from the anterior aortic sinus in 36% of individuals) is the arteria coni arteriosi or conus artery. This is sometimes termed a ‘third coronary' artery, but as a similar vessel comes from the left coronary, it is more correctly named the right conus artery. It ramifies anteriorly on the lowest part of the pulmonary conus and upper part of the right ventricle. It may anastomose with a similar left coronary branch from the left anterior descending artery to form the ‘anulus of Vieussens', which is a tenuous anastomotic ‘circle' around the right ventricular outflow tract.
The first segment of the right coronary artery (between its origin and the right margin of the heart) gives off anterior atrial and ventricular branches. These vessels diverge widely, approaching a right angle in the case of ventricular arteries, which is in marked contrast to the more acute origins of the left coronary ventricular branches. The anterior ventricular branches, usually two or three, ramify towards the cardiac apex, which they rarely reach unless the right marginal artery is included in this group of branches, as it is by some authors. The right marginal artery is greater in calibre than the other anterior ventricular arteries and long enough to reach the apex in most hearts (93%). When it is very large, the remaining anterior ventricular branches may be reduced to one, or may be absent. Up to three small posterior ventricular branches, commonly two, arise from the second segment of the right coronary artery between the right border and crux and supply the diaphragmatic aspect of the right ventricle. Their size is inversely proportional to that of the right marginal artery, which usually extends to the diaphragmatic surface of the heart. As the right coronary approaches the crux, it normally produces one to three posterior interventricular branches (occasionally there are none). One, the posterior interventricular artery, lies in the interventricular groove. It is usually single (70%), and flanked either to the right or left or bilaterally by parallel branches from the right coronary artery. When these flanking vessels exist, branches of the posterior interventricular artery are small and sparse. The posterior interventricular artery is replaced by a left coronary branch in 10% of individuals.
Although the atrial branches of the right coronary artery are sometimes described as anterior, lateral (right or marginal) and posterior groups, they are usually small single vessels 1mm in diameter. The right anterior and lateral branches are occasionally double, very rarely triple, and mainly supply the right atrium. The posterior branch is usually single and supplies the right and left atria. The artery of the sinoatrial node is an atrial branch, distributed largely to the myocardium of both atria, mainly the right. Its origin is variable: it comes from the circumflex branch of the left coronary in 35%. However, more commonly it arises from the anterior atrial branch of the right coronary artery, less often from its right lateral part, least often from its posterior atrioventricular part. This ‘nodal' artery thus usually passes back in the groove between the right auricular appendage and aorta. Whatever its origin, it usually branches around the base of the superior vena cava, typically as an arterial loop from which small branches supply the right atrium. A large ‘ramus cristae terminalis' traverses the sinu-atrial node; it would seem more appropriate to name this branch the ‘nodal artery', as most of the currently named vessel actually supplies the atria and serves as the ‘main atrial branch'.
Septal branches of the right coronary artery are relatively short, and leave the posterior interventricular branch to supply the posterior interventricular septum. They are numerous, but do not usually reach the apical parts of the septum. The largest posterior septal artery, usually the first, commonly arises from the inverted loop which is said to characterize the right coronary artery at the crux. It supplies the atrioventricular node in 80% of hearts.
Small recurrent atrioventricular branches are given off from the ventricular branches of the right coronary artery as they cross the atrioventricular groove; they supply the adjoining atrial myocardium.
Left coronary artery
The left coronary artery is larger in calibre than the right, and supplies a greater volume of myocardium, including almost all the left ventricle and atrium, except in so-called ‘right dominance', when the right coronary artery partly supplies a posterior region of the left ventricle. The left coronary artery usually supplies most of the interventricular septum. It arises from the left posterior (left ‘coronary') aortic sinus; the ostium is below the margin of the cusps in 15%, and may be double, leading into major initial branches, usually the circumflex and anterior interventricular (descending) arteries. Its initial portion, between its ostium and its first branches, varies in length from a few millimetres to a few centimetres. The artery lies between the pulmonary trunk and the left atrial auricle, emerging into the atrioventricular groove, in which it turns left. This part is loosely embedded in subepicardial fat and usually has no branches, but may give off a small atrial ramus and, rarely, the sinu-atrial nodal artery.
Reaching the atrioventricular groove, the left coronary divides into two or three main branches: the anterior interventricular artery is commonly described as its continuation. This artery descends obliquely forward and to the left in the interventricular groove, sometimes deeply embedded in or crossed by bridges of myocardial tissue, and by the great cardiac vein and its tributaries. Almost invariably, it reaches the apex. It terminates there in one-third of specimens, but more often it turns round the apex into the posterior interventricular groove, and passes one-third to one-half of the way along its length, where it meets the terminal twigs of the posterior interventricular branches of the right coronary artery.
The anterior interventricular artery produces right and left anterior ventricular and anterior septal branches, and a variable number of corresponding posterior branches. Anterior right ventricular branches are small and rarely number more than one or two; the right ventricle is supplied almost wholly by the right coronary artery.
From two to nine large left anterior ventricular arteries branch at acute angles from the anterior interventricular artery and cross the anterior aspect of the left ventricle diagonally; larger terminals reach the rounded (obtuse) left border. One is often large and may arise separately from the left coronary trunk, which then ends by trifurcation. This left diagonal artery, reported in 33–50% or more individuals, is sometimes duplicated (20%). A small left conus artery frequently leaves the anterior interventricular artery near its origin, and anastomoses on the conus with its counterpart from the right coronary artery and with the vasa vasorum of the pulmonary artery and aorta. The anterior septal branches leave the anterior interventricular artery almost perpendicularly, and pass back and down in the septum, usually supplying its ventral two-thirds. Small posterior septal branches from the same source supply the posterior one-third of the septum for a variable distance from the cardiac apex.
The circumflex artery, comparable to the anterior interventricular artery in calibre, curves left in the atrioventricular groove, continuing round the left cardiac border into the posterior part of the groove and ending left of the crux in most hearts, but sometimes continuing as a posterior interventricular artery. Proximally, the left atrial auricle usually overlaps it. In 90%, a large ventricular branch, the left marginal artery, arises perpendicularly from the circumflex artery and ramifies over the rounded ‘obtuse' margin, supplying much of the adjacent left ventricle, usually to the apex. Smaller anterior and posterior branches of the circumflex artery also supply the left ventricle. Anterior ventricular branches (from one to five, commonly two or three) course parallel to the diagonal artery, when it is present, and replace it when it is absent. Posterior ventricular branches are smaller and fewer; the left ventricle is partly supplied by the posterior interventricular artery. When this is small or absent, it is accompanied or replaced by an interventricular continuation of the circumflex artery, which is frequently double or triple. The circumflex artery may supply the left atrium via anterior, lateral and posterior atrial branches.
The circumflex artery has inconstant branches. The artery to the sino-atrial node (35%) is usually derived from the anterior circumflex segment, less often from the circum-marginal. It passes over and supplies the left atrium, encircling the superior vena cava like a right coronary nodal branch. It sends a large branch to (and through) the node, but is predominantly atrial in distribution. The artery to the atrioventricular node, the terminal branch in 20%, arises near the crux, in which case the circumflex usually supplies the posterior interventricular artery, an example of so-called ‘left dominance'. Kugel's anastomotic artery (‘arteria anastomotica auricularis magna') has been described as a constant circumflex branch, usually from its anterior part, which traverses the interatrial septum (near its ventricular border) to establish direct or indirect anastomosis with the right coronary; its existence has been questioned.
Details of coronary distribution require integration into a concept of total cardiac supply. Most commonly, the right coronary artery supplies all of the right ventricle (except a small region to the right of the anterior interventricular groove); a variable part of the diaphragmatic aspect of the left ventricle; the posteroinferior one-third of the intraventricular septum; the right atrium and part of the left atrium; the conduction system as far as the proximal parts of the right and left crura. Left coronary distribution is reciprocal, and includes most of the left ventricle; a narrow strip of right ventricle; the anterior two-thirds of the interventricular septum; most of the left atrium. As noted previously, variations in the coronary arterial system mainly affect the diaphragmatic aspect of the ventricles and reflect the relative ‘dominance' of supply by the left or the right coronary artery. The term is misleading, because the left artery almost always supplies a greater volume of tissue than the right. In ‘right dominance', the posterior interventricular artery is derived from the right coronary artery; in ‘left dominance' it is derived from the left coronary artery. In the so-called ‘balanced' pattern, branches of both arteries run in or near the posterior interventricular groove.
Less is known about variations in atrial supply because the small vessels involved are not easily preserved in the corrosion casts that are used for analysis. In more than 50% of individuals, the right atrium is supplied only by the right coronary artery, and in the remainder the supply is dual. More than 62% of left atria are supplied mainly by the left coronary artery, 27% by the right coronary artery (in each group a small accessory supply from the other coronary artery exists), and 11% are supplied almost equally by both arteries. Sinu-atrial and atrioventricular supplies also vary. The sinu-atrial node is supplied by the right (51–65%) or left (35–45%) coronary arteries, and fewer than 10% of nodes receive a bilateral supply. The atrioventricular node is supplied by the right (80–90%) or left (10–20%) coronary arteries.
Anastomoses between branches of the coronary arteries, both subepicardial and myocardial, and between these arteries and extracardiac vessels, are of prime medical importance. Clinical experience suggests that anastomoses cannot rapidly provide collateral routes sufficient to circumvent sudden coronary obstruction, and the coronary circulation is assumed to be end-arterial. Nevertheless, it has long been established that anastomoses do occur, particularly between fine subepicardial branches, and they may increase during individual life. Analyses of coronary radiographs and resin corrosion casts, and the results of radio-opaque perfusion studies, have revealed intra- and inter-coronary anastomoses in vessels up to 100–200 μm in calibre. The most frequent sites of extramural anastomoses are the apex, the anterior aspect of the right ventricle, the posterior aspect of the left ventricle, crux, interatrial and interventricular grooves, and between the sinoatrial nodal and other atrial vessels. The functional value of such anastomoses must vary, but they appear to become more effective in slowly progressive pathological conditions. Their structure is uncertain: most observations that have been made on corrosion casts suggest that anastomotic vessels are relatively straight in normal hearts, but greatly coiled in hearts that have been subject to coronary occlusion. Little has been recorded of their microscopic structure; they appear little more than endothelial tubes, without muscles or elastic tissue.
Extracardiac anastomoses may connect various coronary branches with other thoracic vessels via the pericardial arteries and arterial vasa vasora of vessels which link the heart with the systemic and pulmonary circulations. The effectiveness of these connections as collateral routes in coronary occlusion is unpredictable.
Coronary arteriovenous anastomoses and numerous connections between the coronary circulation and cardiac cavities, producing so-called ‘myocardial sinusoids' and ‘arterioluminal' vessels, have been reported; their importance in coronary disease is uncertain.
Coronary artery disease
Atherosclerosis is characterized by deposition of lipid and accumulation of macrophages in the intima. Endothelial dysfunction leads to the recruitment of inflammatory cells into the vascular wall and the release of various cytokines and adhesion molecules which propagate the process of atherosclerosis. Lipid accumulation and smooth muscle proliferation lead to the formation of an atheromatous plaque. The formation of the plaque itself may cause stenosis of the coronary arteries, and reduces coronary blood flow (classically on exertion). The plaques are also susceptible to rupture with concomitant thrombus formation, which leads to acute occlusion of one of the coronary arteries and may cause myocardial infarction. Plaques may rupture as a result of fatigue within the fibrous cap, but are also more vulnerable when the lipid content is greater than 40% of the composition of the plaque. Superficial erosion of the plaque may also promote critical thrombus formation.
Assessment of coronary artery disease is possible via a number of radiological techniques, including MRI, positron emission tomography, scintigraphy and ultrasound, and invasively by coronary arteriography (which displays the anatomy and delineates regions of stenosis).
Coronary angiography may be performed by introducing a catheter through the femoral, radial or brachial arteries. The femoral artery is punctured with a needle 3 cm below the inguinal ligament while the leg is held adducted and slightly externally rotated. The exact position is guided by palpation of the femoral arterial pulse, and the needle is inserted at an angle of 45°. After arterial puncture, a fine guidewire is inserted through the needle and fed into the artery. The catheter is then inserted over the guidewire and manipulated via the iliac artery into the aorta, up the aortic arch and located in the ascending aorta. The brachial or radial artery may be used for percutaneous access to the circulation. Once the catheter is located in the ascending aorta, a variety of guidewires (straight tip, left and right curved catheters, and pigtail catheters) are used to enter the coronary vessels for selective arteriography and interventions.
Angiography is performed with standard high osmolality contrast medium with cineangiography. In selected patients, new-generation low osmolality contrast medium may also be used. All the coronary arteries are catheterized and evaluated in a variety of views to obtain a full evaluation of their anatomy and to determine the location and degree of any stenoses. The ostium of the left coronary artery arises from the left sinus and is best viewed in the direct frontal and left anterior oblique directions. The right anterior oblique view is useful in demonstrating the diagonal branches and anterior interventricular (descending) coronary artery. The right coronary artery originates from the right sinus of Valsalva and is usually visualized in the right anterior oblique and left anterior oblique views. Pressure and oxygen saturations can be measured via the catheter. Changes in pressure across valves allow the degree of stenosis to be measured. Coronary blood flow and relative flow reserve can also be calculated. Significant stenosis may be treated initially by balloon angioplasty followed by insertion of stents. The balloon exerts pressure against the plaque in the arterial wall and fractures and splits the plaque. The splinting effect of the plaque and elastic recoil are reduced, resulting in an increase in the arterial lumen. Insertion of a stent reduces the rate of re-stenosis.
Atherosclerosis causing greater than 60% stenosis of the terminal diameter of the coronary arteries is likely to cause a significant reduction in myocardial perfusion. Patients with high-grade lesions or left main-stem coronary artery or triple-vessel disease with impaired left ventricular function are usually considered for coronary artery bypass grafting. The common grafts that are used are the saphenous veins and internal thoracic (mammary) arteries. Other grafts that are occasionally used are the radial, ulnar, gastroepiploic and inferior epigastric arteries.
The left internal thoracic artery grafts have a greater patency rate than saphenous vein grafts. Approximately 15% of saphenous vein grafts occlude in 1 year and from then on at an annual rate of 1–2% in the first 6 years and 4% thereafter; 40–50% of saphenous vein grafts have occluded by 10 years. In contrast, only about 10% of left internal thoracic artery grafts will have occluded at 10 years.
The common surgical approach is via a midline sternotomy. If the internal thoracic artery is used as a donor graft, it is divided distally (maintaining its proximal origin from the subclavian artery) and anastomosed to the coronary artery distal to the stenosis. If saphenous vein grafts are used, they must be anastomosed both proximal and distal to the coronary artery, to bridge the site of the stenosis.
In selected cases, minimally invasive direct coronary artery bypass grafting is performed, but the approach is dependent on the vessel being grafted. The anterior approach is via mini-thoracotomy over the fourth intercostal space underneath the nipple for grafting the mid-left anterior interventricular (descending) and diagonal branches. The anterolateral approach is via an incision in the third intercostal space from the midclavicular line to the anterior axillary line and is used for grafting early marginal branches of the circumflex system. The lateral approach allows grafting of the circumflex vessels via a lateral thoracotomy measuring only 10 cm in size through the fifth or sixth intercostal space. Extrathoracic approaches that are occasionally used include the subxiphoid approach for the distal right coronary artery and posterior interventricular (descending) artery. Port access surgery allows for full re-vascularization with cardiopulmonary bypass, but obviates the need for midline sternotomy.
The heart is drained by the coronary sinus and its tributaries, the anterior cardiac veins and the small cardiac veins. The coronary sinus and its tributaries return blood to the right atrium from the entire heart (including its septa) except for the anterior region of the right ventricle and small, variable parts of both atria and the left ventricle. The anterior cardiac veins drain an anterior region of the right ventricle and, when the right marginal vein joins this group, a region around the right cardiac border, and end principally in the right atrium. The small cardiac veins (Thebesius' veins) open into the right atrium and ventricle and, to a lesser extent, the left atrium and sometimes the left ventricle.
Variation in cardiac veins
Attempts to categorize variations in cardiac venous circulation into ‘types' have not produced any accepted pattern. There are major variations concerning the general directions of drainage. The coronary sinus may receive all the cardiac veins (except the small veins), including the anterior cardiac veins (33%), which may be reduced by diversion of some into the small cardiac vein and then to the coronary sinus (28%). The remainder (39%) represents the ‘normal' pattern, as described above.
The large majority of cardiac veins drain into the wide coronary sinus, 2 or 3 cm long, lying in the posterior atrioventricular groove between the left atrium and ventricle. The sinus opens into the right atrium between the opening of the inferior vena cava and the right atrioventricular orifice; the opening is guarded by an endocardial fold (semilunar valve of the coronary sinus; Fig. 56.4A). Its tributaries are the great, small and middle cardiac veins, the posterior vein of the left ventricle and the oblique vein of the left atrium; all except the last have valves at their orifices. Fig. 56.19 The principal veins of the heart.
Great cardiac vein
The great cardiac vein begins at the cardiac apex, ascends in the anterior interventricular groove to the atrioventricular groove and follows this, passing to the left and posteriorly to enter the coronary sinus at its origin. It receives tributaries from the left atrium and both ventricles, including the large left marginal vein that ascends the left aspect (obtuse border) of the heart.
Small cardiac vein
The small cardiac vein lies in the posterior atrioventricular groove between the right atrium and ventricle and opens into the coronary sinus near its atrial end. It receives blood from the posterior part of the right atrium and ventricle. The right marginal vein passes right, along the inferior cardiac margin (acute border). It may join the small cardiac vein in the atrioventricular groove, but more often opens directly into the right atrium.
Middle cardiac vein
The middle cardiac vein begins at the cardiac apex, and runs back in the posterior interventricular groove to end in the coronary sinus near its atrial end.
Posterior vein of the left ventricle
The posterior vein of the left ventricle is found on the diaphragmatic surface of the left ventricle a little to the left of the middle cardiac vein. It usually opens into the centre of the coronary sinus, but sometimes opens into the great cardiac vein.
Oblique vein of the left atrium
The small vessel that is the oblique vein of the left atrium descends obliquely on the back of the left atrium to join the coronary sinus near its end. It is continuous above with the ligament of the left vena cava. The two structures are remnants of the left common cardinal vein.
Anterior cardiac veins
The anterior cardiac veins drain the anterior part of the right ventricle. Usually two or three, sometimes even five, they ascend in subepicardial tissue to cross the right part of the atrioventricular groove, passing deep or superficial to the right coronary artery. They end in the right atrium, near the groove, separately or in variable combinations. A subendocardial collecting channel, into which all may open, has been described. The right marginal vein courses along the inferior (acute) cardiac margin, draining adjacent parts of the right ventricle, and usually opens separately into the right atrium. It may join the anterior cardiac veins or, less often, the coronary sinus. Because it is commonly independent, it is often grouped with the small cardiac veins, but it is larger in calibre, being comparable to the anterior cardiac veins or even wider.
Small cardiac veins
The existence of small cardiac veins, opening into all cardiac cavities, has been confirmed, but they are more difficult to demonstrate than larger cardiac vessels. Their numbers and size are highly variable: up to 2 mm in diameter opening into the right atrium and 0.5 mm into the right ventricle. Numerous small cardiac veins have been identified in the right atrium and ventricle, but they are rare in the left atrium and left ventricle.
Cardiac venous anastomosis
There are widespread anastomoses at all levels of the cardiac venous circulation, on a scale exceeding that of the arteries and amounting to a veritable venous plexus. Not only are adjacent veins often connected, but connections also exist between tributaries of the coronary sinus and those of the anterior cardiac veins. Abundant anastomoses occur at the apex and its anterior and posterior aspects. Like coronary arteries, cardiac veins connect with extracardiac vessels, chiefly the vasa vasorum of the large vessels continuous with the heart.
Lymphatic drainage of the heart
Cardiac lymphatic vessels form subendocardial, myocardial and subepicardial plexuses, the first two draining into the third. Efferents from the subepicardial plexuses form the left and right cardiac collecting trunks. Two or three left trunks ascend the anterior interventricular groove, receiving vessels from both ventricles. On reaching the atrioventricular groove, they are joined by a large vessel from the diaphragmatic surface of the left ventricle, which first ascends in the posterior interventricular groove and then turns left along the atrioventricular groove. The vessel formed by this union ascends between the pulmonary artery and the left atrium, and usually ends in an inferior tracheobronchial node. The right trunk receives afferents from the right atrium and the right border and diaphragmatic surface of the right ventricle. It ascends in the atrioventricular groove, near the right coronary artery, and then anterior to the ascending aorta to end in a brachiocephalic node, usually on the left.