Cardiology and Cardiothoracic Surgery/Surgical Anatomy of the Cardiovascular System

Basic Cardiac Anatomy

Mediastinum

General.One third of the cardiac mass lies to the right of the midline, and two thirds to the left. The short axis of the heart corresponds to the atrioventricular groove and lies on an angle closer to the vertical than to the horizontal.The heart is covered by the sternum and the costal cartilages of the third, fourth, and fifth ribs. Posteriorly, the heart abuts the pulmonary hila, the trachea, bronchi, and esophagus.

Pericardium.The pericardium has the visceral and parietal layers. There are two identifiable recesses within the pericardium lined by serous layer: the transverse sinus (anteriorly bound by the aorta and pulmonary trunk and posteriorly the interatrial groove), and the oblique sinus (a cul-de-sac behind the left atrium).

Nerves. The phrenic nerve is anterior, on the anterior scalene muscle, and just posterior to the internal thoracic artery at the thoracic inlet. On the right side, the phrenic nerve courses on the lateral surface of the superior vena cava (SVC). The phrenic nerve runs anterior to the pulmonary hilus. The vagus nerves, on the other hand, enter the thoracic cavity posterior to the phrenic nerves and course along the carotid arteries. The right vagus nerve sends the right recurrent laryngeal nerve around the right subclavian artery; the left recurrent laryngeal courses around the arch of the aorta, anterior to the ligamentum arteriosum, to the tracheoesophageal groove. The vagus nerves then course behind the pulmonary hila, giving rise to the left pulmonary plexus. The vagus nerves move out of the thorax, but not before giving off a branch called the subclavian loop from the stellate ganglion to the eye and head.

The Heart

If you study the heart from the perspective of the short axis in situ you will come to realize three things: (a) the respective atria lie to the right of the ventricles, (b) the right side of the heart lies anterior to the left, and (c) since the aortic valve (AoV) is wedged between the other valves, the AoV is related to all other chambers. The AoV position minimizes the septum opposing the tricuspid valve (TV) and mitral valve (MV). The TV is attached to septal tissue closer to the apex compared to the MV. The central fibrous body, where the leaflets of AoV, TV, and MV meet lies cephalad and anterior to the muscular atrioventricular septum. This is the main component of the fibrous skeleton of the heart.

Right Atrium and Tricuspid Valve

The right atrium (RA) contains three basic parts, the appendage, the vestibule, and the venous component. The vestibule is visible only after cardiotomy. The junction of the appendage and the venous component is known as the terminal groove (adjacent to the internal crista terminalis). The vestibule is smooth walled myocardium continuous with the leaflets of the tricuspid valve. The pectinate muscles within the RA appendage extend around the entire parietal margin of the atrioventricular junction. These pectinate muscles originate at right angles from the terminal crest. The venous portion of the RA extends from the terminal groove and the interatrial groove; it receives blood from the SVC, inferior vena cava (IVC), and coronary sinus (CS).

The sinus node is located at the juxtaposition of the SVC and RA appendage (at the antero-superior aspect of the terminal groove). The blood supply to the sinus node comes from the nodal artery (a branch of the right coronary artery) in approximately 55% of the population, and from the left circumflex artery in the remainder. This artery is usually buried within the atrial myocardium.

The inter-atrial septum contains the fossa ovalis. The area around the CS is where the RA wall overlies the muscular septum. If you remove the floor of the CS, then you will see the anterior extension of the AV groove in this region. When surgeons need to access the left heart, they must go through the oval fossa because outside of it’s boundaries and they will pierce the heart to the outside.

The entire atrial component of atrioventricular conduction is located within the triangle of Koch. The atrioventricular bundle of His penetrates directly at the apex of the triangle of Koch. To avoid atrial arrhythmias you must avoid damaging the nodes and/or their blood supplies.

The vestibule of the RA converges into the TV. The three leaflets of the TV are the (a) septal, (b) anterosuperior, and (c) inferior aka mural. The anteroseptal commisure is supported by the medial papillary muscle. The anteroinferior commmisure is usually supported by the prominent anterior papillary muscle. The TV does not have a true collagenous annulus; rather, the fibrofatty tissue of the atrioventricular groove folds in on itself to create a makeshift annulus for the TV.

Left Atrium and Mitral Valve

The left atrium (LA), like the RA, possesses three main components: the appendage, the vestibule, and the venous component. On the left side, however, the venous component is much larger than the appendage. In the LA the pectinate muscles are located only within the LA appendage. The LA appendage has only limited contact with the vestibule. The large LA vestibule is connected to the smooth area of the venous component and these insert into the mitral valve leaflets.

The MV is supported by the anterolateral and posteromedial papillary muscles. The anterior (aortic) leaflet is in direct continuity with the AoV. The posterior (mural) valve is the more parietal of the two, dominating 2/3 of the mitral annular circumference. The MV is supported by a dense fibrous annulus. The left circumflex artery is adjacent to the left half of the posterior leaflet annulus.

The Right Ventricle and Pulmonary Valve

The right ventricle (RV) is divided into the inlet portion (which surrounds the TV apparatus), apical trabeculae, and outlet portions. The outlet portion contains the infundibulum, which is a muscular structure supporting the leaflets of the pulmonary valve PV. This valve does not quite have a round annulus, but rather one that is contoured to the leaflet cusps. The RV contains a sharp muscular division known as the supraventricular crest that separates the tricuspid and pulmonary valves. This ridge is actually the posterior aspect of the pulmonary infundibulum. The supraventricular crest is often considered to be the RV outflow tract.

The Left Ventricle and the Aortic Valve

The left ventricle (LV), like the RV, can be separated into three components: the inlet portion (surrounding the mitral valve), apical trabeculae, and outlet portions. The LV outflow tract supports the AoV and has both muscular and fibrous components (compared to the RV infundibulum, which is entirely muscular). This makes sense, since the MV is closer to the upper portion of the heart than is the TV; remember that the TV is anchored closer to the apex than is the MV.

The AoV is a semilunar valve, similar to the PV. It does not have a discrete annulus. The central location of the AoV relates it to every valve and chamber of the heart. The leaflet attachments cross the aortoventricular junction and thus attaches to both. The center of coaptation between the leaflets is called the nodule of Arantius. The line of coaptation between the noncoronary and left cusps is along the line of aortomitral continuity: the fibrous subaortic curtain is beneath this commisure. At the non-coronary cusp, the AoV is related to the RA wall. The coaptation between the non-coronary and right cusps is directly above the portion of the atrial septum containing the atrioventricular node. The commissure itself is right above the penetrating AV bundle and membranous interventricular septum. Beneath the coaptation between the right cusp and left cusp, the wall of the aorta forms the uppermost part of the LV outflow tract.

The Coronary Circulation

The orifices of the coronaries are usually in the upper third of the coronary sinuses. The three main arteries are the left circumflex (LCx), left-anterior descending (LAD), and right coronary artery (RCA). Right dominance occurs in 85-90% of normal individuals. Left dominance usually occurs more frequently in males than in females.

The left main artery courses between the pulmonary trunk and the LA appendage. It is typically 10-20mm in length. Sometimes the left main artery can be absent and there will be two orifices for the LAD and LCx on the L sinus of Valsalva (1% pts). The LAD branches include 2-6 diagonals, 3-5 septal perforators, and RV branches. The diagonals supply the anterolateral wall of the LV. The septal perforators supply the anterior two-thirds of the septum. In 4% of hearts, the LAD bifurcates proximally and descends as two parallel vessels. Occasionally, the LAD wraps around the apex to feed the distal portion of the posterior interventricular groove.

The LCx wraps around the left atrioventricular groove and in 85%-90% of people terminates at the obtuse margin. In others, it reaches the crux of the heart and gives rise to the posterior descending artery (hence the term left dominant). The LCx’s primary branches are the obtuse marginals. It supplies the posteromedial papillary muscle. In 40-50% of hearts, the LCx sends branches to the sinus node. In left dominant hearts, it supplies the atrioventricular node.

In 85-90% of people, the RCA crosses the crux and provides the PDA (right dominant circulation).

The CS predominantly drains the LV and receives approximately 85% of coronary venous blood. It drains into the RA on the lateral border of the triangle of Koch. The anterior interventricular vein runs parallel to the LAD and then courses with the LCx as the great cardiac vein. It receives blood from the marginal and posterior left ventricular branches before becoming the coronary sinus.

Conduits used in Coronary Artery Bypass Grafting

The Left Internal Mammary Artery

The left internal mammary artery (LIMA; also known as the left internal thoracic artery) is a branch of the left subclavian artery, arising in its early portions directly across from the thyrocervical trunk. The LIMA passes inferiorly, posterior to the subclavian vein and lateral to the origin of the left brachiocephalic vein. In this region (the proximal 3cm of the LIMA), the phrenic nerve traverses from lateral to medial: it may cross the LIMA on its anterior or posterior surface.

Surgical Pearl

During left internal mammary artery harvesting for anastomosis to the left anterior descending coronary artery, special care must be taken in the proximal 3cm of the left internal mammary artery due to the phrenic nerve's passage through this region. Furthermore, the pericardiophrenic artery may branch from the left internal mammary artery in this region. Severing this artery may result in devascularization of the phrenic nerve.

The LIMA passes almost vertically in orientation posterior to costal cartilage and intercostal spaces for approximately 20cm (15-25cm) until it bifurcates at the sixth rib to become the musculophrenic artery and the superior epigastric arteries. The left internal mammary veins course with the LIMA: superiorly as a single vessel located medial to the LIMA and inferiorly as vessels located on either side of the LIMA. Proximally, the LIMA is covered only by endothoracic fascia and parietal pleura until it reaches the transversus thoracis muscles which cover the LIMA's posterior surface.

During its course inferiorly, the LIMA bifurcates at each intercostal space in close proximity to the superior border of each costal cartilage. The bifurcations occur in an organized fashion, with the intercostal branch laterally, the thoracic perforator anteriorly, and the sternal branch medially.

Internal Mammary Artery Target organs
Pericardium Sternum Phrenic nerve Diaphragm Anterior thoracic wall Mammary glands Pectoralis major Anterior abdominal wall