OVERVIEW

The overall shape of the heart is that of a three-sided pyramid located in the middle mediastinum. 

When viewed from its apex, the three sides of the ventricular mass are appreciated readily.

 Two of the edges are named: The acute margin lies inferiorly and describes a sharp angle between the sternocostal and diaphragmatic surfaces. The obtuse margin lies superiorly, and is much more diffuse. The posterior margin is unnamed, but is also diffuse in its transition.

One-third of the cardiac mass lies to the right of the midline, and two-thirds to the left. The long axis of the heart is oriented from the left upper gastrium to the right shoulder. The short axis, which corresponds to the plane of the atrioventricular groove, is oblique and is oriented closer to the vertical than the horizontal plane.

Anteriorly, the heart is covered by the sternum and the costal cartilages of the third, fourth, and fifth ribs. The lungs contact the lateral surfaces of the heart, whereas, posteriorly, the heart abuts onto the pulmonary hila. The right lung overlies the right surface of the heart and reaches to the midline. In contrast, the left lung retracts from the midline in the area of the cardiac notch. The heart has an extensive diaphragmatic surface inferiorly. Posteriorly, the heart lies on the esophagus and the tracheal bifurcation, and bronchi that extend into the lung. The sternum lies anteriorly and provides rigid protection to the heart during blunt trauma and is aided by the cushioning effects of the lungs.

The vagus and phrenic nerves descend through the mediastinum in close relationship to the heart. 

They enter through the thoracic inlet, with the phrenic nerve located anteriorly on the surface of the anterior scalene muscle and lying just posterior to the internal thoracic artery (internal mammary artery) at the thoracic inlet. In this position, it is vulnerable to injury during dissection and preparation of the internal thoracic artery for use in coronary arterial bypass grafting. On the right side, the phrenic nerve courses on the lateral surface of the superior caval vein, again in harm's way during dissection for venous cannulation for cardiopulmonary bypass. The nerve then descends anterior to the pulmonary hilum before reflecting onto the right diaphragm, where it branches to provide its innervation. In the presence of a left-sided superior caval vein, the left phrenic nerve is directly applied to its lateral surface. The nerve passes anterior to the pulmonary hilum and eventually branches on the surface of the diaphragm. The vagus nerves enter the thorax posterior to the phrenic nerves and course along the carotid arteries. On the right side, the vagus gives off the recurrent laryngeal nerve that passes around the right subclavian artery before ascending out of the thoracic cavity. The right vagus nerve continues posterior to the pulmonary hilum, gives off branches of the right pulmonary plexus, and exits the thorax along the esophagus. On the left, the vagus nerve crosses the aortic arch, where it gives off the recurrent laryngeal branch. The recurrent nerve passes around the arterial ligament before ascending in the tracheoesophageal groove. The vagus nerve continues posterior to the pulmonary hilum, gives rise to the left pulmonary plexus, and then continues inferiorly out of the thorax along the esophagus. A delicate nerve trunk known as the subclavian loop carries fibers from the stellate ganglion to the eye and head. This branch is located adjacent to the subclavian arteries bilaterally. Excessive dissection of the subclavian artery during shunt procedures may injure these nerve roots and cause Horner's syndrome.

SURGICAL INCISIONS

RELATIONSHIP OF THE CARDIAC CHAMBERS AND GREAT ARTERIES

The surgical anatomy of the heart is best understood when the position of the cardiac chambers and great vessels is known in relation to the cardiac silhouette. The atrioventricular junction is oriented obliquely, lying much closer to the vertical than the horizontal plane. This plane can be viewed from its atrial aspect 

 if the atrial mass and great arteries are removed by a parallel cut just above the junction. The tricuspid and pulmonary valves are widely separated by the inner curvature of the heart lined by the transverse sinus. Conversely, the mitral and aortic valves lie adjacent to one another, with fibrous continuity of their leaflets. The aortic valve occupies a central position, wedged between the tricuspid and pulmonary valves. Indeed, there is fibrous continuity between the leaflets of the aortic and tricuspid valves through the central fibrous body. With careful study of this short axis, several basic rules of cardiac anatomy become apparent. First, the atrial chambers lie to the right of their corresponding ventricles. Second, the right atrium and ventricle lie anterior to their left-sided counterparts. The septal structures between them are obliquely oriented. Third, by virtue of its wedged position, the aortic valve is directly related to all of the cardiac chambers. Several other significant features of cardiac anatomy can be learned from the short axis section. The position of the aortic valve minimizes the area of septum where the mitral and tricuspid valves attach opposite each other. Because the tricuspid valve is attached to the septum further toward the ventricular apex than the mitral valve, a part of the septum is interposed between the right atrium and the left ventricle to produce the muscular atrioventricular septum. The central fibrous body, where the leaflets of the aortic, mitral, and tricuspid valves all converge, lies cephalad and anterior to the muscular atrioventricular septum. The central fibrous body is the main component of the fibrous skeleton of the heart and is made up, in part, by the right fibrous trigone, a thickening of the right side of the area of fibrous continuity between the aortic and mitral valves, and in part by the membranous septum, the fibrous partition between the left ventricular outflow tract and the right heart chambers. 

The membranous septum itself is divided into two parts by the septal leaflet of the tricuspid valve, which is directly attached across it. 

Thus, the membranous septum has an atrioventricular component between the right atrium and left ventricle, as well as an interventricular component. Removal of the noncoronary leaflet of the aortic valve demonstrates the significance of the wedged position of the left ventricular outflow tract in relation to the other cardiac chambers. The subaortic region separates the mitral orifice from the ventricular septum; this separation influences the position of the atrioventricular conduction tissues and the position of the leaflets and tension apparatus of the mitral valve.

CARDIAC CHAMBERS AND VALVES

The right atrium has three basic parts: the appendage, the vestibule, and the venous component. 

Externally, the right atrium is divided into the appendage and the venous component, which receives the systemic venous return. The junction of the appendage and the venous component is identified by a prominent groove, the terminal groove. This corresponds internally to the location of the terminal crest. The right atrial appendage has the shape of a blunt triangle, with a wide junction to the venous component across the terminal groove. The appendage also has an extensive junction with the vestibule of the right atrium; the latter structure is the smooth-walled atrial myocardium that inserts into the leaflets of the tricuspid valve. The most characteristic and constant feature of the morphology of the right atrium is that the pectinate muscles within the appendage extend around the entire parietal margin of the atrioventricular junction. 

These muscles originate as parallel fibers that course at right angles from the terminal crest. The venous component of the right atrium extends between the terminal groove and the interatrial groove. It receives the superior and inferior caval veins and the coronary sinus.

The sinus node lies at the anterior and superior extent of the terminal groove, where the atrial appendage and the superior caval vein are juxtaposed. The node is a spindle-shaped structure that usually lies to the right or lateral to the superior cavoatrial junction. 

In approximately 10 percent of cases, the node is draped across the cavoatrial junction in horseshoe fashion.The blood supply to the sinus node is from a prominent nodal artery that is a branch of the right coronary artery in approximately 55 percent of individuals, and a branch of the circumflex artery in the remainder. Regardless of its artery of origin, the nodal artery usually courses along the anterior interatrial groove toward the superior cavoatrial junction, frequently within the atrial myocardium. At the cavoatrial junction, its course becomes variable and may circle either anteriorly or posteriorly, or rarely both anteriorly and posteriorly, around the cavoatrial junction to enter the node. Uncommonly, the artery arises more distally from the right coronary artery and courses laterally across the atrial appendage. This places it at risk of injury during a standard right atriotomy. The artery also may arise distally from the circumflex artery to cross the dome of the left atrium, where it is at risk of injury when utilizing a superior approach to the mitral valve. Incisions in either the right or left atrial chambers always should be made with this anatomic variability in mind. In our experience, these vessels can be identified by careful gross inspection and should prompt appropriate modification of surgical incisions.

The most common incision into the right atrium is made into the atrial appendage parallel and anterior to the terminal groove. Opening the atrium through this incision confirms that the terminal groove is the external marking of the prominent terminal crest. Anteriorly and superiorly, the crest curves in front of the orifice of the superior caval vein to become continuous with the so-called “septum secundum,” which, in reality, is the superior rim of the oval fossa. When the right atrium is inspected through this incision, there appears to be an extensive septal surface between the tricuspid valve and the orifices of the caval veins. This “septal” surface includes the opening of the oval fossa and the orifice of the coronary sinus. The apparent extent of the septum is spurious, as the true septum between the atrial chambers is virtually confined to the oval fossa .  

The superior rim of the fossa, although often referred to as the “septum secundum,” is an extensive infolding between the venous component of the right atrium and the right pulmonary veins. The inferior rim is directly continuous with the so-called sinus septum that separates the orifices of the inferior caval vein and the coronary sinus.

The region around the coronary sinus is where the right atrial wall overlies the atrioventricular muscular septum. Removing the floor of the coronary sinus reveals the anterior extension of the atrioventricular groove in this region. Only a small part of the anterior rim of the oval fossa is a septal structure. The majority is made up of anterior atrial wall overlying the aortic root. Thus, dissection outside the limited margins of the oval fossa will penetrate the heart to the outside, rather than provide access to the left atrium via the septum.

In addition to the sinus node, another major area of surgical significance is occupied by the atrioventricular node. This structure lies within the triangle of Koch, which is demarcated by the tendon of Todaro, the septal leaflet of the tricuspid valve, and the orifice of the coronary sinus.

 The tendon of Todaro is a fibrous structure formed by the junction of the eustachian valve and the thebesian valve (the valves of the inferior caval vein and coronary sinus, respectively). The entire atrial component of the atrioventricular conduction tissues is contained within the triangle of Koch, which must be avoided to prevent surgical damage to atrioventricular conduction. The atrioventricular bundle (of His) penetrates directly at the apex of the triangle of Koch before it continues to branch on the crest of the ventricular septum. 

The key to avoiding atrial arrhythmias is careful preservation of the sinus and atrioventricular nodes and their blood supply. No advantage is gained in attempting to preserve nonexistent tracts of “specialized atrial conduction tissue,” although it makes sense to avoid prominent muscle bundles where parallel orientation of atrial myocardial fibers favors preferential conduction.

Like the right atrium, the left atrium has three basic components: the appendage, vestibule, and venous component. 

Unlike the right atrium, the venous component is considerably larger than the appendage and has a narrow junction with it that is not marked by a terminal groove or crest. There also is an important difference between the relationship of the appendage and vestibule between the left and right atria. As shown, the pectinate muscles within the right atrial appendage extend all around the parietal margin of the vestibule. In contrast, the left atrial appendage has a limited junction with the vestibule, and the pectinate muscles are located almost exclusively within the appendage. 

The larger part of the vestibule that supports and inserts directly into the mural leaflet of the mitral valve is directly continuous with the smooth atrial wall of the pulmonary venous component.

Because the left atrium is posterior and tethered by the four pulmonary veins, the chamber is relatively inaccessible. Several approaches are used by surgeons to gain access. The most common is an incision just to the right of and parallel to the interatrial groove, anterior to the right pulmonary veins. This incision can be carried beneath both the superior and inferior caval veins parallel to the interatrial groove, to provide wide access to the left atrium. A second approach is through the dome of the left atrium. If the aorta is pulled anteriorly and to the left, an extensive trough may be seen between the right and left atrial appendages. An incision through this trough, between the pulmonary veins of the upper lobes, provides direct access to the left atrium. When this incision is made, it is important to remember the location of the sinus node artery, which may course along the roof of the left atrium if it arises from the circumflex artery. The left atrium also can be reached via a right atrial incision and an opening in the atrial septum.

When the interior of the left atrium is visualized, the small size of the mouth of the left atrial appendage is apparent. It lies to the left of the mitral orifice as viewed by the surgeon. The majority of the pulmonary venous atrium usually is located inferiorly away from the operative field. The vestibule of the mitral orifice dominates the operative view. The septal surface is located anteriorly, with the true septum relatively inferior.

The mitral valve is supported by two prominent papillary muscles located in anterolateral and posteromedial positions. The two leaflets of the mitral valve have markedly different appearances. 

The aortic (or anterior) leaflet is short, relatively square, and guards approximately one-third of the circumference of the valvar orifice. This leaflet is in fibrous continuity with the aortic valve and, because of this, is best referred to as the “aortic leaflet,” since it is neither strictly anterior nor superior in position. The other leaflet is much shallower but guards approximately two-thirds of the circumference of the mitral orifice. As it is connected to the parietal part of the atrioventricular junction, it is most accurately termed the mural leaflet, but is often termed the posterior leaflet. It is divided into a number of subunits that fold against the aortic leaflet when the valve is closed. Although generally there are three, there may be as many as five or six scallops in the mural leaflet.

Unlike the tricuspid valve, the mitral valve leaflets are supported by a rather dense collagenous annulus, although it may take the form of a sheet rather than a cord. This annulus usually extends parietally from the fibrous trigones, the greatly thickened areas at either end of the area of fibrous continuity between the leaflets of the aortic and mitral valves. 

The area of the valvar orifice related to the right fibrous trigone and central fibrous body is most vulnerable with respect to the atrioventricular node and penetrating bundle. 

The mid-portion of the aortic leaflet of the mitral valve is related to the commissure between the noncoronary and left coronary cusps of the aortic valve. An incision through the atrial wall in this area may be extended into the subaortic outflow tract, and may be useful for enlarging the aortic annulus during replacement of the aortic valve. 

The circumflex coronary artery is adjacent to the left half of the mural leaflet, whereas the coronary sinus is adjacent to the right half of the mural leaflet. 

These structures can be damaged during excessive dissection, or by excessively deep placement of sutures during replacement or repair of the mitral valve. When the circumflex artery is dominant, the entire attachment of the mural leaflet may be intimately related to this artery.

The morphology of both the right and left ventricles can be understood best by subdividing the ventricles into three anatomically distinct components: the inlet, apical trabecular, and outlet portions. This classification is more helpful than the traditional division of the right ventricle into the “sinus” and “conus” parts. The inlet portion of the right ventricle surrounds the tricuspid valve and its tension apparatus. A distinguishing feature of the tricuspid valve is the direct attachment of its septal leaflet. The apical trabecular portion of the right ventricle extends out to the apex. Here, the wall of the ventricle is quite thin and vulnerable to perforation by cardiac catheters and pacemaker electrodes.

The outlet portion of the right ventricle consists of the infundibulum, a circumferential muscular structure that supports the leaflets of the pulmonary valve. Because of the semilunar shape of the pulmonary valvar leaflets, this valve does not have an annulus in the traditional sense of a ring-like attachment. The leaflets have semilunar attachments that cross the musculo-arterial junction in a corresponding semi-lunar fashion.

 Therefore, instead of a single annulus, three rings can be distinguished anatomically in relation to the pulmonary valve. Superiorly, the sinotubular ridge of the pulmonary trunk marks the level of peripheral apposition of the leaflets (the commissures). A second ring exists at the ventriculo-arterial junction. A third ring can be constructed by joining together the basal attachments of the three leaflets to the infundibular muscle. None of these rings, however, corresponds to the attachments of the leaflets, which must be semilunar to permit the valve to competently open and close. In fact, these semilunar attachments, which mark the hemodynamic ventriculo-arterial junction, extend from the first ring, across the second, down to the third, and back in each cusp.

A distinguishing feature of the right ventricle is a prominent muscular shelf, the supraventricular crest, which separates the tricuspid and pulmonary valves. 

In reality, this muscular ridge is the posterior part of the subpulmonary muscular infundibulum that supports the leaflets of the pulmonary valve. In other words, it is part of the inner curve of the heart. Incisions through the supraventricular crest run into the transverse septum and may jeopardize the right coronary artery. Although this area is often considered the “outlet” component of the interventricular septum, in fact the entire subpulmonary infundibulum, including the ventriculoinfundibular fold, can be removed without entering the left ventricular cavity. This is possible because the leaflets of the pulmonary and aortic valves are supported on separate sleeves of right and left ventricular outlet muscle. There is an extensive external tissue plane between the walls of the aorta and the pulmonary trunk, 

and the leaflets of the pulmonary and aortic valves have markedly different levels of attachments within their respective ventricles. This feature enables enucleation of the pulmonary valve, including its basal attachments within the infundibulum, during the Ross procedure without creating a ventricular septal defect. When the infundibulum is removed from the right ventricle, the insertion of the supraventricular crest between the limbs of the septomarginal trabeculation is visible. 

This trabeculation is a prominent muscle column that divides superiorly into anterior and posterior limbs. The anterior limb runs superiorly into the infundibulum and supports the leaflets of the pulmonary valve. The posterior limb extends backwards beneath the ventricular septum and runs into the inlet portion of the ventricle. The medial papillary muscle arises from this posterior limb. The body of the septomarginal trabeculation runs to the apex of the ventricle, where it divides into smaller trabeculations. Two of these trabeculations may be particularly prominent. One becomes the anterior papillary muscle and the other crosses the ventricular cavity as the moderator band.

The left ventricle can be subdivided into three components, as was the right ventricle. The inlet component surrounds, and is limited by, the mitral valve and its tension apparatus. The two papillary muscles occupy anterolateral and posteromedial positions and are positioned rather close to each other. The leaflets of the mitral valve have no direct septal attachments because the deep posterior diverticulum of the left ventricular outflow tract displaces the aortic leaflet away from the inlet septum. The apical trabecular component of the left ventricle extends to the apex, where the myocardium is surprisingly thin. The trabeculations of the left ventricle are quite fine compared with those of the right ventricle. 

This characteristic is useful for defining ventricular morphology on diagnostic ventriculograms.

The outlet component supports the aortic valve and consists of both muscular and fibrous portions. This is in contrast to the infundibulum of the right ventricle, which is composed entirely of muscle. The septal portion of the left ventricular outflow tract, although primarily muscular, also includes the membranous portion of the ventricular septum. The posterior quadrant of the outflow tract consists of an extensive fibrous curtain that extends from the fibrous skeleton of the heart across the aortic leaflet of the mitral valve, and supports the leaflets of the aortic valve in the area of aortomitral continuity. 

The lateral quadrant of the outflow tract is again muscular and consists of the lateral margin of the inner curvature of the heart, delineated externally by the transverse sinus. The left bundle of the cardiac conduction system enters the left ventricular outflow tract posterior to the membranous septum and immediately beneath the commissure between the right and non-coronary leaflets of the aortic valve. After traveling a short distance down the septum, the left bundle divides into anterior, septal, and posterior divisions.

The aortic valve is a semilunar valve, morphologically quite similar to the pulmonary valve. Likewise, it does not have a discrete annulus. Because of its central location, the aortic valve is related to each of the cardiac chambers and valves. 

A thorough knowledge of these relationships is essential to understanding aortic valve pathology and many congenital cardiac malformations. The aortic valve consists primarily of three semilunar leaflets. As with the pulmonary valve, attachments of the leaflets extend across the ventriculo-arterial junction in a curvilinear fashion. Each leaflet, therefore, has attachments to the aorta and within the left ventricle. 

Behind each leaflet, the aortic wall bulges outward to form the sinuses of Valsalva. The leaflets themselves meet centrally along a line of coaptation, at the center of which is a thickened nodule, the nodule of Arantius. Peripherally, adjacent to the commissures, the line of coaptation is thinner and normally may contain small perforations. During systole, the leaflets are thrust upward and away from the center of the aortic lumen, whereas, during diastole, they fall passively into the center of the aorta. With normal valvar morphology, all three leaflets meet along lines of coaptation and support the column of blood within the aorta to prevent regurgitation into the ventricle. Two of the three aortic sinuses give rise to coronary arteries, from which arise their designations as right, left, and non-coronary sinuses.

By sequentially following the line of attachment of each leaflet, the relationship of the aortic valve to its surrounding structures can be clearly understood. Beginning posteriorly, the commissure between the non-coronary and left coronary leaflets is positioned along the area of aorto-mitral valvar continuity. The fibrous sub-aortic curtain is beneath this commissure. To the right of this commissure, the non-coronary leaflet is attached above the posterior diverticulum of the left ventricular outflow tract. Here the valve is related to the right atrial wall. As the attachment of the non-coronary leaflet ascends from its nadir toward the commissure between the non-coronary and right coronary leaflets, the line of attachment is directly above the portion of the atrial septum containing the atrioventricular node. The commissure between the non-coronary and right coronary leaflets is located directly above the penetrating atrioventricular bundle and the membranous ventricular septum. 

The attachment of the right coronary leaflet then descends across the central fibrous body before ascending to the commissure between the right and left coronary leaflets. Immediately beneath this commissure, the uppermost part of the subaortic outflow tract is formed by the wall of the aorta. An incision through this area passes into the space between the facing surfaces of the aorta and pulmonary trunk. As the facing left and right leaflets descend from this commissure, they are attached to the outlet muscular component of the left ventricle. Only a small part of this area in the normal heart is a true outlet septum, since both pulmonary and aortic valves are supported on their own sleeves of myocardium. Thus, although the outlet components of the right and left ventricle face each other, an incision below the aortic valve enters low into the infundibulum of the right ventricle. As the lateral part of the left coronary leaflet descends from the facing commissure to the base of the sinus, it becomes the only part of the aortic valve that is not intimately related to another cardiac chamber.

The anatomy of the aortic valve and its relationship to surrounding structures is important to successful replacement of the aortic valve, particularly when elargement of the aortic root is required. The Konno-Rastan aortoventriculoplasty involves opening and enlarging the anterior portion of the sub-aortic region. The incisions for this procedure begin with an anterior longitudinal aortotomy that extends through the commissure between the right and left coronary leaflets. Anteriorly, the incision is extended across the base of the infundibulum. The differential level of attachment of the aortic and pulmonary valve leaflets permits this incision without damage to the pulmonary valve. 

Posteriorly, the incision extends through the most medial portion of the supraventricular crest into the left ventricular outflow tract. By closing the resulting ventricular septal defect with a patch, the aortic outflow tract is widened to allow implantation of a larger valvar prosthesis. A second patch is used to close the defect in the right ventricular outflow tract.

Alternative methods to enlarge the aortic outflow tract involve incisions in the region of aortomitral continuity. In the Manouguian procedure, 

a curvilinear aortotomy is extended posteriorly through the commissure between the left and non-coronary leaflets down to, and occasionally into, the aortic leaflet of the mitral valve. A patch is used to augment the incision posteriorly. When the posterior diverticulum of the outflow tract is fully developed, this incision can be made without entering other cardiac chambers, although not uncommonly the roof of the left atrium is opened. The Nicks procedure for enlargement of the aortic root involves an aortotomy that passes through the middle of the non-coronary leaflet into the fibrous sub-aortic curtain and may be extended into the aortic leaflet of the mitral valve. This incision also may open the roof of the left atrium. When these techniques are used, any resultant defect in the left atrium must be closed carefully.

As discussed previously, the differential level of attachment of aortic and pulmonary valves, as well as the muscular nature of their support, allows the pulmonary valve to be harvested and used as a replacement for the aortic valve in the Ross procedure.  This procedure can be combined with the incisions of the Konno-Rastan aortoventriculoplasty to repair left ventricular outflow tract obstructions in young children with a viable autograft that has potential for growth and avoids the need for anticoagulation.

Accurate understanding of left ventricular outflow tract anatomy is also important in the treatment of aortic valvar endocarditis.  Because of the central position of the aortic valve relative to the other valves and cardiac chambers , 

abscess formation can produce fistulas between the aorta and any of the four chambers of the heart. Patients may, therefore, present with findings of left heart failure, left-to-right shunting, and/or complete heart block in addition to the usual signs of sepsis and systemic embolization.

THE CORONARY ARTERIES 

The right and left coronary arteries originate behind their respective aortic valvar leaflets. 

The orifices usually are located in the upper third of the sinuses of Valsalva, although individual hearts may vary markedly. Because of the oblique plane of the aortic valve, the orifice of the left coronary artery is superior and posterior to that of the right coronary artery. The coronary arterial tree is divided into three segments; two (the left anterior descending artery and the circumflex artery) arise from a common stem. The third segment is the right coronary artery. The “dominance” of the coronary circulation (right vs. left) usually refers to the artery from which the posterior descending artery originates, not the absolute mass of myocardium perfused by the left or right coronary artery. Right dominance occurs in 85–90 percent of normal individuals. Left dominance occurs slightly more frequently in males than females.

The main stem of the left coronary artery courses from the left sinus of Valsalva anteriorly, inferiorly, and to the left between the pulmonary trunk and the left atrial appendage. 

Typically it is 10–20 millimeters in length but can extend to a length of 40 millimeters. The left main stem can be absent, with separate orifices in the sinus of Valsalva for its two primary branches (1 percent of patients). The main stem divides into two major arteries of nearly equal diameter: the left anterior descending artery and the circumflex artery.

The left anterior descending (or interventricular) coronary artery continues directly from the bifurcation of the left main stem, coursing anteriorly and inferiorly in the anterior interventricular groove to the apex of the heart. 

Its branches include the diagonals, the septal perforators, and the right ventricular branches. The diagonals, which may be two to six in number, course along the anterolateral wall of the left ventricle and supply this portion of the myocardium. The first diagonal generally is the largest and may arise from the bifurcation of the left main stem (formerly known as the intermediate artery). The septal perforators branch perpendicularly into the ventricular septum. Typically there are three to five septal perforators; the initial one is the largest and commonly originates just beyond the take-off of the first diagonal. This perpendicular orientation is a useful marker for identification of the left anterior descending artery on coronary angiograms. The septal perforators supply blood to the anterior two-thirds of the ventricular septum. Right ventricular branches, which may not always be present, supply blood to the anterior surface of the right ventricle. In approximately 4 percent of hearts, the left anterior descending artery bifurcates proximally and continues as two parallel vessels of approximately equal size down the anterior interventricular groove. Occasionally, the artery wraps around the apex of the left ventricle to feed the distal portion of the posterior interventricular groove. Rarely, it extends along the entire length of the posterior groove to replace the posterior descending artery.

The left circumflex coronary artery arises from the left main coronary artery roughly at a right angle to the anterior interventricular branch. It courses along the left atrioventricular groove and, in 85–95 percent of patients, terminates near the obtuse margin of the left ventricle. 

In 10–15 percent of patients, it continues around the atrioventricular groove to the crux of the heart to give rise to the posterior descending artery (left dominance). 

The primary branches of the left circumflex coronary artery are the obtuse marginals. They supply blood to the lateral aspect of the left ventricular myocardium, including the posteromedial papillary muscle. Additional branches supply blood to the left atrium and, in 40–50 percent of hearts, the sinus node. When the circumflex coronary artery supplies the posterior descending artery, it also supplies the atrioventricular node.

The right coronary artery courses from the aorta anteriorly and laterally before descending in the right atrioventricular groove and curving posteriorly at the acute margin of the right ventricle. 

In 85–90 percent of hearts, the right coronary artery crosses the crux, where it makes a characteristic U-turn before bifurcating into the posterior descending artery and the right posterolateral artery. In 50–60 percent of hearts, the artery to the sinus node arises from the proximal portion of the right coronary artery. The blood supply to the atrioventricular node (in patients with right dominant circulation) arises from the mid-portion of the U-shaped segment. The posterior descending artery runs along the posterior interventricular groove, extending for a variable distance toward the apex of the heart. It gives off perpendicular branches, the posterior septal perforators, that course anteriorly in the ventricular septum. Typically, these perforators supply the posterior one-third of the ventricular septal myocardium.

The right posterolateral artery gives rise to a variable number of branches that supply the posterior surface of the left ventricle. The circulation of the posteoinferior portion of the left ventricular myocardium is quite variable. It may consist of branches of the right coronary artery, the circumflex artery, or both. The acute marginal arteries branch from the right coronary artery along the acute margin of the heart, before its bifurcation at the crux. These marginals supply the anterior free wall of the right ventricle. In 10–20 percent of hearts, one of these acute marginal arteries courses across the diaphragmatic surface of the right ventricle to reach the distal ventricular septum. The right coronary artery supplies important collaterals to the left anterior descending artery through its septal perforators. In addition, its infundibular (or conus) branch, which arises from the proximal portion of the right coronary artery, courses anteriorly over the base of the ventricular infundibulum and may serve as a collateral to the anterior descending artery. Kugel's artery is an anastomotic vessel between the proximal right coronary and the circumflex coronary artery that can also provide a branch that runs through the base of the atrial septum to the crux of the heart, where it supplies collateral circulation to the atrioventricular node. 

THE CORONARY VEINS

A complex network of veins drains the coronary circulation. An extensive degree of collateralization amongst these veins and the coronary arteries, and the paucity of valves within coronary veins, enables the use of retrograde coronary sinus cardioplegia for intraoperative myocardial protection. The venous circulation can be divided into three systems: the coronary sinus and its tributaries, the anterior right ventricular veins, and the thebesian veins.

The coronary sinus predominantly drains the left ventricle and receives approximately 85 percent of coronary venous blood. It lies within the posterior atrioventricular groove and empties into the right atrium at the lateral border of the triangle of Koch. 

The orifice of the coronary sinus is guarded by the crescent-shaped thebesian valve. The named tributaries of the coronary sinus include the anterior interventricular vein, which courses parallel to the left anterior descending coronary artery. Adjacent to the bifurcation of the left main stem, the anterior interventricular vein courses leftward in the atrioventricular groove, where it is referred to as the great cardiac vein. It receives blood from the marginal and posterior left ventricular branches before becoming the coronary sinus at the origin of the oblique vein (of Marshall) at the posterior margin of the left atrium. The posterior interventricular vein, or middle cardiac vein, arises at the apex, courses parallel to the posterior descending coronary artery, and extends proximally to the crux. Here, this vein drains either directly into the right atrium or into the coronary sinus just prior to its orifice. The small cardiac vein runs posteriorly through the right atrioventricular groove.

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