John Kennedy III and Louis Louis
This chapter is a revision and update of that included in previous editions of the TSRA Review written by Ghulam Murtaza (2nd edition), Ashok Babu (1st edition), and Ramesh Singh (1st edition).
Surface Anatomy
The heart (Latin: Cor; Greek: Cardia) lies in the central chest with the majority of its mass to the left of midline. The pulmonary valve is situated under the third left sternocostal joint. The aortic valve is immediately inferomedial to it. The mitral valve is obliquely oriented behind the left half of the sternum at the level of 4th intercostal space. The tricuspid valve is almost vertically oriented behind the right half of sternum at the level of 4th and 5th sternocostal cartilages. The cardiac apex extends 9 cm to the left from midline at the level of 5th intercostal space.
The internal thoracic (mammary) artery lays a centimeter lateral to the sternal edge on either side and bifurcates into superior epigastric and costal branches at the sixth intercostal space.
The Heart In-Situ
The heart is most commonly accessed via median sternotomy, though a thoracotomy may also be used in specific circumstances such as minimally invasive cardiac surgery. Directly behind the sternum lies the right ventricle. It is important to note that in repeat sternotomies, the right ventricle may be adhered to the posterior surface of the sternum.
Both the vagus and phrenic nerves are in close relation to the heart and at risk for injury during cardiac surgery. At the level of the thoracic inlet, the phrenic nerve lies just posterior to the internal mammary arteries and is at risk during internal mammary artery dissection for coronary artery bypass grafting. On the right side of the heart, the phrenic nerve courses on the lateral surface of the superior vena cava and pericardium, anterior to the hilum of the lung, and continues on to the diaphragm. Care must be taken with electrocautery around the SVC to avoid injuring the nerve during bi-caval venous cannulation for cardiopulmonary bypass. The vagus nerve runs posteriorly to the hilum and is generally not at risk. On the left side, the vagus nerve passes anteriorly to the aortic arch before giving off the left recurrent laryngeal nerve and then dives posteriorly to the hilum. Both nerves are at risk during distal arch dissection.
Before opening the pericardium, the thymus—laying anterior to the heart—is divided with particular attention being paid to the innominate vein. With thymus dissected, the surgeon can turn their focus to the pericardium.
Pericardium
The pericardium is composed of a fibrous external sac lined by a smooth, serous pericardium. The outer fibrous sac is attached to the central tendon of the diaphragm via the pericardiophrenic ligament and the sternum via the superior and inferior sternopericardial ligaments. The heart, during its development, forms a loop and invaginates into the serous pericardium causing it to reflect onto itself. This reflection leads to formation of two distinct serous pericardial layers: an outer parietal pericardium and an inner visceral pericardium. The space between the two, the pericardial cavity, is filled with a thin film of serous fluid which acts as a lubricant. The parietal pericardium lines the outer fibrous sac, while the visceral pericardium lies in direct contact with the heart and extends onto the tunica adventitia of the great vessels. As this inner layer extends onto the aorta, several folds, known as Rindfleisch’s folds, form a 1-3 cm long and 2-4 mm wide band of fat approximately 2-3 cm above the aortic origin. This fatty band of tissue (incorrectly referred to in cardiac surgical vernacular as the “fat stripe” of the aorta) is often removed and used a site for proximal anastomoses in coronary artery bypass grafting or as a cannulation site for antegrade cardioplegia. Another fold, known as the ligament of Marshall, sits posteriorly between the left atrial appendage and the left pulmonary veins. This remnant of the left SVC contains neurovascular structures, including the oblique vein of Marshall, and serves as a source of paroxysmal atrial fibrillation. Two sinuses, the transverse sinus and the oblique sinus, are formed by the reflections between the two serous layers. The transverse sinus lies between the aorta and pulmonary artery anteriorly and between the superior vena cava and superior pulmonary veins posteriorly. As the heart grows further, the pulmonary veins spread out and form the oblique sinus – a blind pouch bound by the pericardial reflection from the pulmonary veins and inferior vena cava.
External Surface
The heart sits atop the diaphragm and resembles a pyramid. The anterior or sternocostal surface combines with the inferior or diaphragmatic surface to form the acute margin on the right. On the left, the sternocostal surface and left lateral surface form the obtuse margin. The most anterior chamber upon opening the pericardium is the right ventricle. The left ventricle is mostly hidden from view by the right ventricle but extends inferiorly and laterally to the left to form the apex. Superior to the right ventricle is the right atrial appendage. Tracing this posteriorly leads to the remainder of the right atrium along with the superior and inferior vena cava. With the heart retracted to the left, the left atrium and right pulmonary veins can be seen below (from the surgeon’s perspective) the right atrium. The fatty interatrial groove, known as Waterston’s (David Waterson, a mid-20th century pediatric surgeon) or Sondergaard’s groove, is a key landmark as development of this plane permits access to the left atrium.
Fibrous Skeleton
The ventricular myocardial fibers are organized in a single band, forming two loops (basal and apical) extending from the PA to the aorta. The muscle fibers themselves travel in a way that forms reciprocal spirals resulting in a helical heart structure. The helical nature of the muscle fibers results in a “concentric constriction” of the left ventricle, while the right ventricle “wrings itself out” using the muscular intraventricular septum to beat against. They are attached to the central fibrous skeleton, which provides electrical insulation between the ventricles and the atria. The valves of the heart are supported by the fibrous skeleton and are juxtaposed so that the aortic valve is wedged between the mitral and tricuspid valves. The pulmonary valve, anterior and slightly leftward, is suspended in the muscle without a fibrous annulus. It is attached to the central skeleton only through an infundibular tendon. The junction of the remaining three valvular annuli forms the right (posterior) fibrous trigone. The left (anterior) trigone is between the aortic and mitral annuli. The adjacent halves of the non-coronary and left sinuses of the aortic valve are supported by the aorto-mitral continuity (curtain) between the two trigones.
Right Atrium (RA) and Tricuspid Valve
The right atrium consists of an appendage, a vestibule, and a venous component. The venous component (sinus venarum) is an untrabeculated portion derived from the sinus venosus, and receives the SVC and IVC. This is separated from the wide-based, trabeculated appendage (primitive atrium) by the crista terminalis internally and the corresponding sulcus terminalis externally. The crista terminalis represents the right sinoatrial valve, and its lower portion forms the Thebesian and Eustachian valves (the latter being a remnant of a structure that directed incoming oxygenated blood away from the right atrium and through the foramen ovale) guarding the coronary sinus and the IVC orifices, respectively. Its extension toward the antero-septal commissure of the tricuspid valve is called the tendon of Todaro. The tendon of Todaro, the coronary sinus opening, and the septal portion of the tricuspid valve annulus form the triangle of Koch. The AV node lies in its center; the proximal portion of the bundle of His extends toward the apex of this triangle. The interatrial septum is composed of the fossa ovalis and the surrounding septum secundum which forms a muscular rim (limbus) around the fossa. Folds of atrial musculature and the right pulmonary veins make up the superior portion of the septum secundum, while the inferior portion is in close proximity to the ostium of the coronary sinus. The vestibule is a smooth-walled portion adjoined with the appendage that leads to the tricuspid valve.
The tricuspid valve has three leaflets; the most distinctive is the septal leaflet. The anterior most portion of the septal leaflet is attached to the ventricular septum. Because of this, tricuspid regurgitation due to right atrial enlargement is unlikely to involve this portion of the annulus. The anterior leaflet is the largest leaflet and is supported by a large anterior papillary muscle arising from the anterior ventricular wall. The posterior leaflet is supported by several small muscles attached to the diaphragmatic wall of the right ventricle. Septal papillary muscles originating from the septal band are sometimes referred to as the muscle of Lancisi or Luschka.
Right Ventricle (RV) & Pulmonary Valve
The RV is a coarsely trabeculated, crescent-shaped, thin-walled tube-like structure. It is comprised of a larger inlet portion (sinus), an apical trabecular portion, and a smaller outlet portion. As the name suggests, the inlet portion surrounds the tricuspid valve. From here, blood flows to the thin-walled apical portion. This section is section is lined with rough muscular ridges, known as trabeculae carneae. These trabeculae are derived from the septomarginal and septoparietal trabeculations. The body of the septomarginal trabeculation (or septal band) courses towards the RV apex, giving rise to the smaller trabeculae carne as well as two larger trabeculae — the anterior papillary muscle for the tricuspid valve and moderator band. The moderator band, named so because it was (incorrectly) thought to limit or moderate the size of the RV, contains the right branch of the bundle of His stretches across the RV from the septum to the anterior papillary muscle.
The y-septomarginal trabeculation additionally serves as the anteroinferior border between the rough, trabeculated inflow portion of the RV and its smooth outflow tract. As its body runs posterosuperior along the interventricular septum, it bifurcates superiorly into an anterior limb and towards the RV inflow portion into a posterior limb. This posterior limb goes on to form the medial or septal papillary muscle for the tricuspid valve. The anterior limb joins with the infundibulum, a smooth, funnel-shaped structure comprising the majority of the outflow tract, to provide support for the tricuspid valve’s leaflets. The posterior portion of the infundibulum sits nestled between the septomarginal trabeculation’s limbs and is known as the supraventricular crest (or crista supraventriculari). This structure separates the tricuspid and pulmonary valves and provides additional support for the pulmonary valve’s leaflets.
The pulmonary valve is composed of three semi-lunar leaflets. Unlike the tricuspid and mitral valves, the pulmonary valve lacks a true annulus. Instead, the leaflets are supported by the infundibulum, supraventricular crest, and the anterior limb of the septomarginal trabeculation.
Left Atrium (LA) & Mitral Valve
The LA is the only midline chamber of the heart. Similar to the right atrium, it also consists of an appendage, vestibule, and venous component. The left atrial appendage is long and has a narrow base (finger-like) at its junction with the vestibule. The atrial wall between the left pulmonary veins and the appendage is sometimes confused with a thrombus on an echocardiogram and is referred to as the “Coumadin ridge.” Surgical access to the left atrium is most commonly obtained through the interatrial groove (Sondergaard’s or Waterson’s groove). Other approaches include an incision through the dome of the left atrium, an incision in the right atrium and through the atrial septum, or a combination of both. The sinus node artery is commonly taken in theses surgical approaches, and the resultant sinus node dysfunction contributes to the need for a permanent pacemaker in approximately 12 percent of patients whose mitral valve is approached via the superior septal technique.
The mitral valve has an anterior (aortic) and posterior (mural) leaflet connected by two commissures formed by A3 and P3 are caudad or rightward. The anterior leaflet is suspended below the two trigones. It is attached to one-third of the annular circumference, but it covers two thirds of the total commissures with each leaflet divided into three segments (Carpentier’s classification). From a surgeon’s view, the anterolateral commissure formed by A1 and P1 are cephalad or leftward the posteromedial valve area. The mitral valve annulus has critical surgical relationships to a number of important structures, all of which are at risk during mitral valve surgery. The annulus surrounding the anterolateral commissure and posterior leaflet courses along the free wall of the LA through the atrioventricular groove and is in close proximity to the circumflex artery and coronary sinus. The posteromedial commissure attaches to the right fibrous trigone near the Bundle of His (“PosteroMedial injuries results in a PaceMaker”). The anterior leaflet is connected to the fibrous skeleton in close proximity to the left and non- coronary sinuses of the aortic valve and the non-left commissure that attaches the two. Both papillary muscles through the chordae tendineae support each leaflet. Chordae are classified into primary (attached to the free edge of the leaflet), secondary (attached to the ventricular surface of the leaflet) and tertiary (arising from the ventricular wall or base of the papillary muscles and inserting into the base of the posterior leaflet).
Left Ventricle (LV) & Aortic Valve
Like the RV, the LV consists of an inlet, trabeculated apical, and outlet portion. Each of these components, however, differs from their RV counterparts. The large size of the mitral valve and its components limits the size of the inlet region. The trabeculated apical portion features much finer trabeculations in comparison to the RV. The outlet portion lies anteromedially to the anterior mitral valve leaflet. Unlike the RV’s purely muscular outlet portion, the LV’s outlet portion features muscular, membranous, and fibrous components. The septal region of the outlet portion is mostly muscular, but also contains membranous contributions from the interventricular septum. Moving posteriorly, the outlet transitions to a fibrous component as it runs in continuity with the fibrous skeleton of the heart. Continuing in a counterclockwise fashion, the lateral aspect of the outflow tract is composed primarily of muscular tissue. The anterior leaflet of the mitral valve separates the inlet from the outflow tract. Because of this location, a redundant or tall anterior leaflet be pulled into the left ventricular outflow tract leading to obstruction and systolic anterior motion (SAM).
The aortic valve is morphologically similar in that it too is composed of three semi-lunar leaflets and lacks a true annulus. At the point of coaptation, the leaflets are slightly thickened and marked by the nodules of Arantius at their midpoint. The commissures are higher in the aortic wall and mark the sinotubular junction, and the nadirs of the cusps sit slightly below this level. Each space within these cusps is known as a sinus of Valsalva. Two of the three sinuses contain ostium for the coronary arteries and their accompanying cusps are named accordingly: the right coronary cusp, the left coronary cusp, and the non-coronary cusp. The right and left coronary cusps give rise to the right and left coronary arteries, respectively. The non-coronary cusp sits in the most posterior position. The commissures are often referred to as non-right, non-left, left-right (or a similar variant) to describe the leaflets they sit between. The non-left commissure is in continuity with the anterior leaflet of the mitral valve via the aortomitral curtain, a component of the heart’s fibrous skeleton situated between the right and left fibrous trigones. Aggressive removal of the aortic valve in this region during an aortic valve replacement can cause the anterior mitral leaflet to become detached. Components of the conduction system are located below the non-right commissure in the muscular septum (below the membranous septum) and may be injured in aortic valve replacement surgery by annular stitches that are thrown too deeply.
Blood Supply
The right coronary artery (RCA) and left main coronary artery (LMCA) arise from the corresponding aortic sinuses immediately below the sinotubular junction.
The LMCA is a short vessel and travels laterally between the pulmonary artery and the left atrial appendage to bifurcate into the left anterior descending (LAD) artery and circumflex artery. The LAD travels in the anterior interventricular groove toward the apex. Along its course, it gives off perpendicular septal perforators (supplying the anterior two-thirds of the septum) and diagonal branches to the left ventricular anterior wall. The RV branches may connect with the vessels from the RCA to form the loop of Vieussens.
The circumflex artery travels in the left AV groove and gives off obtuse marginal branches to the lateral and inferior wall. In a left dominant circulation (10-15%), the circumflex artery gives rise to the posterior descending artery (PDA) and the AV nodal branch.
The RCA travels in the right AV groove. It gives off a conal branch, a sinoatrial (SA) nodal artery, anterior ventricular branches, and an acute marginal artery (supplies distal PDA territory in 10-20%). At the cardiac crux, in patients with a right dominant circulation (70-85%), it gives off an AV nodal branch and bifurcates into the PDA and the posterolateral artery.
Kugel’s artery is a proximal branch of the RCA or circumflex artery. It travels within the atrial septum and anastomoses with both the SA nodal and AV nodal arteries.
The anterolateral papillary muscle is supplied by branches of both the LAD and circumflex artery, unlike the posteromedial papillary muscle, which is supplied by the terminal branches of the RCA or circumflex artery.
The coronary veins usually have no valves. The anterior interventricular vein drains into the great cardiac vein in the AV groove and becomes the coronary sinus posteriorly. It opens in the RA near the IVC and is cannulated for retrograde cardioplegia. The Thebesian veins drain 15% of the coronary blood flow into the adjacent chambers. A persistent left SVC (when present) tends to drain into the coronary sinus, interfering with adequate retrograde cardioplegia unless it is controlled.
Conduction System
The SA node is composed of specialized myocardium located right under the epicardium lateral to the cavoatrial junction (at the base of the right atrial appendage). There are three preferential internodal pathways to the AV node, which is situated in the center of the triangle of Koch. The bundle of His travels toward the anteroseptal commissure of the tricuspid valve and perforates the right trigone of the central fibrous body. On the ventricular side, it travels between the membranous and muscular septum, where it divides into two bundle branches. The left bundle branch fibers are fanned out on the left side of the septum and are organized into two branches that travel to the bases of the papillary muscles. The right bundle branch crosses the moderator band toward the right ventricular wall.
Suggested Readings
- Kouchoukos, Nicholas T., et al., editors. “Anatomy, Dimensions, and Terminology.” Kirklin/Barratt-Boyes Cardiac Surgery, 4th ed., vol. 1, Elsevier/Saunders, 2013.
- Mill, Michael R., et al. Cardiac Surgery in the Adult, edited by Lawrence H. Cohn and David H. Adams, 5th ed., McGraw Hill Education, 2018.
- Youssef, Samuel J. “Looking at the Heart.” TSRA Primer of Cardiothoracic Surgery, edited by Samuel J. Youssef and Jason A. Williams, Thoracic Surgery Residents Association, 2013.
