TSRA Content:
Author: Hannah L. McMullen, MD
This is a revision and update from the previous edition of the TSRA Primer in Cardiothoracic Surgery written by Samuel J. Youssef, MD.
The Basics
Developing a full understanding of the anatomy of the heart is an essential goal for beginning trainees. Direct hands-on dissection and study allows thorough inspection in 3D of the spatial relationship of the cardiac valves, chambers, and vasculature. This tactile learning is necessary to allow additional study in atlases and textbooks. This chapter will serve as a brief introductory road map of the heart structures and their relational anatomy to accompany hands-on experience.
After the Sternotomy
Once the sternotomy is performed and the sternum retracted, the first organ encountered is the thymus gland. The amount of thymic tissue varies among different individuals; children usually have a prominent thymus while many adults have nearly complete involution of the gland. The thymus will need to be divided to gain access to the pericardium, beginning by clipping and dividing the venous branches between the lobes. Extreme caution must be utilized when dividing the most cephalad aspect of the thymus along the path of the innominate vein (often referred to as ‘big blue’ and sitting horizontally at the superior aspect of the thymus gland). This structure should be identified and protected during the dissection. Usually, the innominate vein is simply retracted to the upper limit of the dissection, but some operations (complex aortic arch repair) may require division of this vessel.
Opening the Pericardium
After the identification of the innominate vein and division of the thymus, the underlying pericardium is exposed. The pericardium is opened with cautery or sharp dissection and followed inferiorly to the diaphragm (where it is T’d off to each side) and superiorly to the aorta. This is the outer layer: the parietal pericardium. The inner layer of pericardium which envelopes the heart and great vessels directly is the visceral pericardium. The two recesses formed by the folds of these layers of pericardium around the heart are important surgical landmarks. The transverse sinus is a recess behind the aorta and pulmonary trunk and is accessible behind these structures. The oblique sinus is a recess behind the left atrium; access to this space is between the pulmonary veins and the inferior vena cava.
Cardiac Great Vessels
Once the pericardium is suspended, the heart sits like a pyramid in the chest and rests with a broad base on the surface of the diaphragm. The great vessels can be appreciated emerging superiorly from the heart, with the aorta anterior to the pulmonary trunk, and the left and right pulmonary arteries extending to each side. The superior vena cava can be followed to the right atrium, but the inferior vena cava is not easily visualized prior to mobilization of the heart from its in situ position.
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Cardiac Surface Anatomy
Underneath the pericardium lies the beating heart. The right atrium is the smaller auricle closest to the surgeon’s side; this is the most superior cardiac chamber. The left atrium is the most posterior chamber, lying deep to the right atrium. When the right atrium is pulled towards the left shoulder, the interatrial groove will come into view. This fat plane (Sondergaard’s groove) is developed to separate the right and left atrium, starting from the junction between the superior pulmonary vein and left atrium and dissecting towards the oblique sinus. Remember that the inferior vena cava delivers blood to the right atrium, so staying behind (deep to) the inferior vena cava at the oblique sinus will keep you in the groove between the right and left atrium and prevent inadvertent entry to the right atrium.
The most anterior surface of the heart is largely comprised of the right ventricle. Schematics normally depict the right ventricle and left ventricle side by side. In situ as viewed intraoperatively, the right ventricle actually sits on top of the left ventricle. The most inferior portion of the left ventricle forms the apex of the heart- in close proximity to the diaphragm. The atrioventricular groove separates each atrium from its ventricle, and the interventricular groove separates the right and left ventricles. The anterior interventricular groove marks the pathway of the left anterior descending artery, one of the two main branches of the left main coronary artery. The left anterior descending artery supplies septal branches to the interventricular septum, and diagonal branches to the left ventricle. The acute margin is the sharp angle between the sternocostal and diaphragmatic surfaces where the right ventricle lies and will be closest to you on the surgeon’s side (patient’s right). The right coronary artery extends along this angle, and its branches are thus called the "acute marginal" branches. On your assistant's side (patient's left) is the obtuse margin; it does not form as sharp an angle. The left main coronary artery branches into the left anterior descending artery and the left circumflex artery. The left circumflex artery traverses toward the obtuse margin; its branches are called "obtuse marginal" branches as they wrap around the obtuse margin of the heart.
The posterior surface of the heart contains parts of all four chambers. They meet at the “posterior cross of the heart” with the posterior interventricular groove, interatrial groove, and atrioventricular groove. The posterior descending artery (a branch of either the right coronary artery or the left circumflex artery) runs within the posterior interventricular groove, while the coronary sinus courses along the atrioventricular groove.
For further details on coronary anatomy view the "Coronary Artery Anatomy" chapter.
Right Atrium & Tricuspid Valve
The right atrium includes several key structures. The right atrial appendage is visible both externally and from within the right atrium as a short pouch of trabeculated muscle superior on the atrium. Internally, the medial wall of the right atrium is comprised of the interatrial septum, which divides the right and left atria. The fossa ovalis is an oval shaped indentation in the interatrial septum, a remnant of the closed foramen ovale. This is also where a patent foramen ovale (PFO) would be found. The right atrium also includes openings for the SVC superiorly and IVC inferiorly for return of systemic venous blood, a muscle bundle called the crista terminalis running between the caval openings containing important conduction tissue, and the orifice of the coronary sinus most inferiorly for return of venous blood from the coronary veins. The presence of important conduction tissues further distinguishes the right atrium from the other cardiac chambers. Since cardiac conduction tissue is comprised of specialized cardiac myocytes that are not markedly visually distinct in appearance, the location of the sinoatrial (SA) node, atrioventricular (AV) node, Bundle of His, and other structures of the signal conduction pathway are correlated with their common landmarks.
The SA node’s electrical signal is the origin of the heartbeat; it is located at the junction of the SVC and right atrial appendage. Its blood supply is an SA nodal branch that arises from the right coronary artery (55% of the time) or from the left circumflex artery (45% of the time). This artery supply may transversely cross the right atrium (in the path of your proposed atriotomy incision) or travel along the superior dome of the atrium. Therefore, it is important to avoid the SA node itself when you are cannulating the SVC for bicaval cardiopulmonary bypass cannulation and to identify and avoid the path of its blood supply when opening the right atrium.
The next step in signal propagation is from the SA node to the AV node. The AV node lies within an anatomical triangle of notable landmarks, known as the “triangle of Koch.” The borders of the triangle are the septal leaflet of the tricuspid valve, the coronary sinus at its base, and the “tendon of Todaro” on the atrial side. The tendon of Todaro is an anatomical structure within the right atrium created by the intersection of the two flap-like valves that drape the inlet of the IVC (Eustachian valve) and the coronary sinus (Thebesian valve). Together, these valves join and form an apparatus known as the “tendon of Todaro”. At the apex of the triangle of Koch (towards the ventricle), the conduction system continues as the “Bundle of His” within the interventricular septum. This bundle travels in close proximity to the septal and anterior leaflet of the tricuspid valve. Therefore, tricuspid annuloplasty procedures use incomplete tricuspid rings with a “gap” halfway along the septal leaflet where no sutures are placed in order to avoid damaging the AV node and Bundle of His.
The tricuspid valve has three leaflets: the septal leaflet is most medial and forms one border of the triangle of Koch at the interventricular septum, the anterior leaflet is the largest and near the left ventricle, and the smaller posterior leaflet is near the free wall of the right atrium. The leaflets are anchored to the right ventricle by chordae tendinae connected to papillary muscles.
Tricuspid regurgitation is caused by failure of the leaflets to achieve proper coaptation. This is attributed to one of three primary etiologies: right ventricular dilation, tricuspid annulus dilation, or loss of valvular leaflet integrity. Right ventricular dilation spreads the papillary muscles apart, thereby tethering the leaflets and preventing full valvular closure. Tricuspid annular dilation leads to preferential enlargement on the side of the anterior and posterior leaflets since the septal leaflet is protected by its anchored position to the interventricular septum. Finally, leaflet pathology may be seen with primary connective tissue disorders such as Marfan’s or secondary to destruction such as by endocarditis.
Left Atrium & Mitral Valve
The left atrium is accessed via atriotomy at the interatrial groove (Sondergaard’s groove) as described earlier (Interactive 1.4). Other options for access include superior atriotomy via the dome of the left atrium or trans-septally through the interatrial septum from the right atrium.
Once inside the left atrium, the more ventral aspect of the incision is retracted. Oxygenated blood return from the lungs enters the left atrium via the right and left superior/inferior pulmonary veins. During operations requiring clear visualization of the left sided heart structures and unobstructed access to the mitral valve, a vent can be placed into the left superior pulmonary vein (through the atriotomy) to remove this pulmonary blood return from the surgical field. Another internal structure to note is the orifice of the left atrial appendage (LAA), which is longer than the right atrial appendage. The narrow space within the LAA allows potential for thrombus formation during states of turbulent or stagnated blood flow, such as atrial arrhythmias. For this reason, patients with atrial fibrillation can undergo procedures to clip or ligate the LAA to close this space.
With correct retractor positioning, the mitral valve will come into view. It has two leaflets: anterior and posterior. Each leaflet is divided into three components: Anterior (A1, A2, A3) and Posterior (P1, P2, P3). These number sequentially from lateral to medial (left to right). At A1 and P1 is the anterolateral commissure, which is connected via chordae tendinae to the anterolateral papillary muscle. At A3 and P3, the posteromedial commissure is connected via chordae tendinae to the posteromedial papillary muscle.
The posterior leaflet is closest to the free wall of the left atrium in the atrio-ventricular groove. The left circumflex artery travels in this same groove, as does the coronary sinus, and both are in danger of injury when placing sutures in the posterior annulus.
The anterior leaflet of the mitral valve is in continuity with the fibrous skeleton of the heart, which directly connects it to the aortic valve. The pillar of the commissure between the left and non-coronary cusps of the aortic valve is located halfway between the trigones. Thus, sutures at the anterior leaflet annulus are at risk for catching the aortic valve leaflets (see mitral chapter for videos).
The central fibrous body of the heart with its left and right trigones can be felt as a cartilaginous structure behind the anterior leaflet at A2. The coronary sinus travels around the right trigone. The triangle of Koch with its corresponding conduction tissue is near the coronary sinus in this area (as seen in the tricuspid valve). Hence, the components of the atrioventricular conduction axis are also located near this aspect of the mitral valve and are at risk of injury during mitral valve procedures.
The mitral valve functions properly when the corresponding anterior and posterior mitral leaflets meet to create a zone of apposition with the leaflets tucked into the ventricle. Mitral regurgitation can be caused by any element of the mitral valve apparatus that disrupts this surface of coaptation at the level of the ventricular chamber, papillary muscle, or chordae tendinae, or leaflet overgrowth or damage.
This anatomy as well as classification system for mitral regurgitation will be covered in further detail in the mitral valve chapter.
Aortic Valve
After blood is delivered to the left ventricle by way of the mitral valve, it exits to the body through the aortic valve. The aortic valve is a tri-leaflet structure that is suspended in the ascending aorta along three commissures. Above the aortic valve are a set of “sinuses” known as the sinuses of Valsalva. During diastole, the aortic valve leaflets are closed. This allows blood to pool in the sinuses of Valsalva, facilitating blood flow into the coronary arteries, which originate from their respective sinuses. Each leaflet’s portion of the sinus is positioned at the midpoint of the respective leaflet: the left coronary sinus is at the middle of the left aortic valve leaflet; the right coronary sinus is at the middle of the right aortic valve leaflet. At this level, the right and left main coronary arteries branch off to deliver blood to the heart. The third leaflet, called the “non-coronary” leaflet, also has a sinus, but no corresponding artery. Importantly, the tributaries of the atrio-ventricular conduction system run behind the noncoronary leaflet, especially at the commissure between the non-coronary and right leaflets.
Knowledge of the aortic valve and its relational anatomy is extremely important not only for aortic valve replacement, but for instances where aortic root enlargement is required or when addressing aortic root abscesses. Certain surgical techniques take advantage of the commissural positions to achieve access or enlargement of the aortic root. Detailed descriptions can be found in the following aortic valve chapter.
Pulmonary Valve
The main pulmonary trunk carries blood from the right ventricle to the lungs for oxygenation. Like the aortic valve, the pulmonary valve is a tri-leaflet semilunar valve with anterior, right, and left leaflets. The aortic valve is posterior to the pulmonary valve, with the commissure between the right and left aortic valve leaflets aligning with the commissure between the right and left pulmonary valve leaflets. The aorta becomes anterior to the pulmonary trunk as it ascends, where the now posterior pulmonary trunk extends superiorly before splitting into the right and left main pulmonary arteries.
References:
Zhu, Xiaodong. Surgical Atlas of Cardiac Anatomy. Springer, 2015.