The heart

• Trachea; • Oesophagus; • Thoracic duct; • Lymph nodes; and • Nerves.

The anterior mediastinum contains the:

• Thymus; • Mammary vessels; and • Lymph nodes.

The posterior mediastinum contains the: • Descending aorta; • Oesophagus; • Azygos venous system; • Thoracic duct; and • Para-aortic, oesophageal and paraspinal nodes.

The middle mediastinum contains the: • Heart and pericardium: • Nerves; • Lymph nodes; and • Great vessels.

THE HEART

Gross anatomy and orientation

(Figs 4. 20-4. 22; also Figs 4. 6 and 4. 7) The heart is pyramidal in shape and lies obliquely in the chest. Its square-shaped base points posteriorly and the elongated apex to the left and inferiorly. The left atrium forms the base or posterior part, w i th the superior and inferior pulmonary veins draining into its four corners. The right atrium forms the right border, w i th superior and inferior venae cavae draining into its upper and lower parts. The apex and left border are formed by the left ventricle. The right ventricle forms the anterior part. The inferior (diaphragmatic) part of the heart is formed by both ventricles anteriorly and a small part of right atrium posteriorly where the IVC enters this chamber.

The oblique orientation of the heart causes the ventricles to lie anterior and inferior to the atria. The heart is also rotated in a clockwise fashion about its axis, so that the right atrium and ventricle are at a slightly higher level than their left counterparts. The interatrial and interventricular septa are said to lie in the left anterior oblique plane. This means that the long axis of the septa runs anteriorly to the left. The tricuspid and mitral valves, which separate the right and left atria and ventricles respectively, are roughly vertically oriented. The plane of the valves is also inclined inferiorly and to the left. This means that the transverse axis of the pair of valves runs to the right and anteriorly, and they are said to lie in the right anterior oblique plane.

Pericardium (Fig. 4. 23) This is a closed sac consisting of parietal and visceral layers that enclose a potential space which contains 20-25 mL of serous fluid. It is draped over the heart and great vessels. The visceral layer adheres to the myocardium and is also known as the epicardium. The parietal layer is free, except inferiorly, where it is bound to the central tendon of the diaphragm, and superiorly where it fuses w i th the covering of the great vessels. The pericardial reflections, which are really the boundaries of the closed sac, are found posteriorly

Fig. 4. 22 Heart and mediastinum viewed from left. (Courtesy of Professor J. B. Coakley. )

around the IVC and pulmonary veins where the space between the veins forms the oblique sinus of the pericardium. The serous pericardial layers extend anterosuperi¬ orly over the superior vena cava (SVC) and separately over the aorta and pulmonary artery. The space between the aorta and pulmonary artery is known as the transverse sinus of pericardium.

The pericardium extends superiorly for 2-3 cm over the ascending aorta and over the pulmonary artery almost to its bifurcation. It also extends for a short distance over the venae cavae and pulmonary veins. Some fat is present between the epicardium and myocardium. This increases w i th age. Fat is also present between the pericardium and mediastinal pleura, and it may be extensive in the anterior and lateral cardiophrenic angles, where it is known as the pericardial fat pad.

Radiological features of the pericardium

CT and MRI (Fig. 4. 23) The pericardium may be visible on CT as a thin dense line separated from the myocardium by a thin layer of epi¬ cardial fat. Pericardial sinuses and recesses w i th a little normal fluid may be visible around and between the great vessels on CT or MRI and should not be misinterpreted as lymphadenopathy or other mediastinal disease. The superior pericardial recess in particular may extend almost to the right paratracheal region. Cardiac chambers and valves (Figs 4. 20-4. 22 and 4. 24)

Right atrium This has a smooth posterior wall into which the great veins drain. The coronary sinus drains into the posterior wall between the orifice of the IVC and the tricuspid valve. The anterior wall has muscular ridges that are continuous w i th the muscular ridges of the atrial appendage. The interatrial septum bears an oval depression on its lower part, known as the fossa ovalis. This represents the closed foramen ovale through which oxygenated blood from the maternal circulation reached the left side of the heart in the fetus. A raised limbus surrounds the fossa ovalis. The right atrial appendage is roughly triangular and projects upward and forward and to the left; it is the only part of the right atrium to contribute to the cardiac outline on the lateral view. The inner wall of the atrial appendage is ridged by musculi pectinati w i th a vertical ridge, the crista terminalis, separating it from the smooth-walled parts of the right atrium.

Right ventricle This chamber is roughly triangular and flattened from front to back as the left ventricle bulges into it. The lower half of the right ventricle normally touches the lower part of the sternum on the lateral view. Viewed from the side it has a muscular inflow tract and a smooth outflow tract, separated by a muscular prominence or conus also known as the

Fig. 4. 23 Pericardium: (a) CT scan showing pericardium.

1. Pericardium 4. Interventricular septum 2. Right ventricle 5. Aorta 3. Left ventricle 6. Oesophagus

(b) CT; and (c) MRI showing pericardial recesses.

1. Aorta 4. Right pulmonary artery 2. Superior aortic recess 5. Aorta of the transverse sinus 6. Oblique sinus 3. Superior vena cava

infundibulum. The outflow tract inclines superiorly, to the left and posterior to the pulmonary valve. The entrance to the right ventricle is the tricuspid valve. This has three leaflets or cusps, each attached to the papillary muscles of the ventricular wall by several tendinous cords - the cordae tendinae. The pulmonary valve has three semilunar cusps right and left anterior and a posterior cusp. It faces to the left and slightly posteriorly. It is the most anterior and superior of all the cardiac valves. Left atrium

This is square-shaped and smooth-walled and forms the upper posterior part of the heart on the lateral view. It receives the four pulmonary veins in its upper part (Fig. 4. 16). It has a long, narrow, trabeculated appendage that projects anteriorly on the left side of the pulmonary trunk, overlapping its origin. This is embedded in fat and is not seen on the frontal view unless enlarged.

Left ventricle This is a thick-walled finely trabeculated cavity that is shaped like an elongated cone, being roughly circular in cross-section. It forms the lower half of the posterior part of the heart on the lateral view. The mitral valve separates it from the left atrium. This valve has two cusps - anterior and posterior - whose free margins are attached to the ventricular wall by chordae tendinae. Instead of a muscular conus as on the right, the larger anterior cusp of the mitral valve separates inflow and outflow tracts, and blood flows over both its surfaces. The mitral and aortic valves are in fibrous continuity. The aortic valve has three semilunar cusps - anterior, and right and left posterior. Above each cusp is a localized dilatation or sinus. These are known as the sinuses of Valsalva.

The right coronary artery arises from the anterior sinus and this is also known as the right coronary sinus. The left coronary artery arises from the left posterior sinus - also known as the left coronary sinus. No artery arises from the right posterior sinus, so this is also called the non-coronary sinus (Fig. 4. 25). The ventricles are separated by the interventricular septum, which is mostly thick and muscular. It has a short membranous part at the top and bulges into the right ventricle, causing this to have a flattened appearance from front to back, and giving the left ventricle a circular shape in cross-section.

The coronary arteries and veins (Figs 4. 25-4. 28)

Right coronary artery (Figs 4. 26 and 4. 28) The right coronary artery (RCA) supplies the right ventricle and inferior wall of the left ventricle. It arises from the anterior (right) sinus of Valsalva and passes to the right between the pulmonary trunk and the right atrium to descend in the right atrioventricular groove as the marginal artery. On the inferior surface of the heart it anastomoses w i th the left coronary artery in the region of the posterior interventricular groove.

Branches

The branches of the right coronary artery are as follows: • Conus artery to the pulmonary outflow tract; • Atrial and ventricular branches; • Branch to sinoatrial node, which curves anticlockwise around the SVC to reach the sinoatrial node; • Acute marginal branches, which run anteriorly from the

RCA to supply the right ventricle; • Branch to atrioventricular node, which arises from the

RCA as it forms a characteristic loop at the region where it continues as the posterior interventricular artery; and • Posterior interventricular artery, which runs anteriorly from the terminal part of the RCA in the posterior interventricular groove, and supplies the inferior surface of the left ventricle and the posterior two-thirds of the interventricular septum.

Left coronary artery (Figs 4. 27 and 4. 28) The left coronary artery (LCA) arises from the left posterior sinus of Valsalva and supplies the remainder of the left ventricle. It arises as the left main coronary artery and passes behind and to the left of the pulmonary trunk to reach the left part of the atrioventricular groove. It bifurcates early into the left circumflex artery, which continues laterally in the atrioventricular groove to anastomose w i th the right coronary artery, and the anterior descending artery, which descends in the interventricular groove.

Branches of the anterior descending artery

These are as follows: • Septal branches; • Diagonal branches that run over the anterolateral wall of the left ventricle supplying it; and • A branch to the right ventricle (occasionally).

1. Sinus artery 4. Right ventricular branch 2. Right coronary artery 5. Posterolateral branch 3. Conus branch 6. Posterior descending artery (b)

1. Right coronary artery 4. Posterolateral branch 2. Right ventricular branch 5. Posterior descending artery 3. Conus branch

(a) 1. Left main coronary artery 2. Left anterior descending artery (LAD) 3. Septal branch 4. First diagonal branch 5. Second diagonal branch 6. Continuation of LAD 7. Circumflex artery 8. First obtuse marginal branch 9. Second obtuse marginal branch (b)

1. Left main coronary artery seen end on 2. Circumflex artery 3. Obtuse marginals 4. Posterolateral branch 5. Left anterior descending artery (LAD) 6. Septal branch 7. First diagonal branch 8. Second diagonal branch 9. Continuation of LAD

Branches of the left circumflex artery

These are as follows:

• Obtuse marginal branches, which supply the lateral wall of the left ventricle; and • Atrial branches.

In general, the RCA supplies the right ventricle and the inferior part of the left ventricle. The left coronary artery supplies the remainder of the left ventricle. The interventricular septum is supplied by the left coronary anteriorly and the right coronary artery posteriorly. The atria have a variable supply. In more than 50% of cases the sinoatrial node is supplied by the right coronary artery, and in 90% of cases the right coronary artery supplies the atrioventricular node. Coronary dominance is determined by the vessel that supplies the inferior and lateral walls of the left ventricle.

In right dominance (the usual situation) the right coronary artery gives rise to the posterior interventricular branch and continues around in the atrioventricular groove, giving branches to the posterolateral wall of the left ventricle. In left-dominant situations the right coronary artery is short, and the left circumflex supplies the posterolateral wall of the left ventricle and gives off the posterior descending artery.

The veins of the heart (Fig. 4.29) Venous drainage of the heart is mainly (approximately 60%) via veins that accompany the coronary arteries and which drain via the coronary sinus. The coronary sinus lies in the posterior atrioventricular groove and drains into the posterior wall of the right atrium to the left of the orifice of the IVC. Its tributaries are:

• The great cardiac vein, which ascends in the anterior interventricular groove and then runs to the left in the atrioventricular groove to become the coronary sinus; • The middle cardiac vein, which ascends in the posterior interventricular groove; • The small cardiac vein, which accompanies the marginal branches of the RCA on the inferior surface of the heart and then runs posteriorly in the right atrioventricular groove to enter the right side of the coronary sinus; and • The left posterior ventricular vein, which accompanies the obtuse marginals of the left coronary artery, running up the posterior aspect of the left ventricle to drain into the coronary sinus. • The anterior cardiac veins drain much of the anterior surface of the heart and drain into the anterior wall of the right atrium directly. Several small veins, the venae cordis minimae, drain directly into the cardiac chambers.

The conducting system of the heart

The sinoatrial node, which is just to the right of the orifice of the SVC, initiates the cardiac impulse. The impulse spreads through the musculature of the right atrium to the atrioventricular node, which lies in the interatrial septum. It is then conducted to the ventricles via the atrioventricular bundle of His (a specialized muscle that can conduct the electrical impulse). The bundle of His divides into right and left branches in the upper part of the interventricular septum which activate the ventricular contraction.

Radiology of the heart

Chest radiography The cardiac contour is seen on the frontal and lateral chest f i l m. Posteroanterior films are preferred to anteroposterior ones as the heart, being anterior, is closer to the film and is not magnified to the same extent as w i th anteroposterior films. The cardiothoracic ratio, usually less than 50%, may be up to 55% in Asian and Afro-Caribbean subjects and up to 60% in infants. (For a description of the cardiac contour, see the section on the mediastinal contour, p. 143.)

The coronary arteries may be calcified in normal people.

The position of the valves may be deduced on PA and lateral films in relation to the cardiac outline or the sternum and ribs (Fig. 4.30). The aortic and mitral valves are the most important to recognize, as they are most often affected by disease. On a PA chest radiograph the valves lie close to a line from the left atrium to the lowest point of the right heart border. On a lateral view the pulmonary and aortic valves lie just above, and the mitral and aortic valves just below, a line drawn from T5 to the apex of the heart. The lowest part of the aortic valve is very close to the anterior part of the mitral valve, where they are anatomically in fibrous continuity.

Fluoroscopy The heart and its valves may be assessed by fluoroscopy. If the valves are calcified (usually pathologically) they may be distinguished by their characteristic motion as well as their location. The aortic valve has a to-and-fro motion in the plane of the ascending aorta, that is, upwards, backwards and to the right. The mitral valve has a circular as well as a to-and-fro motion in the left anterior oblique plane of the left atrium and ventricle.