permit blood to flow in only one direction (Figure 7-2). These valves consist of flaps (or cusps) of endocardium that are anchored to the papillary muscles of the ventricles by cordlike structures called the chordae tendineae. The mitral valve (bicuspid valve) (left atrioventricular) between the left atrium and the left ventricle has two cusps, whereas the tricuspid valve (right atrioventricular) between the right atrium and the right ventricle has three cusps. The semilunar valves separate the ventricles from the great vessels leaving the heart. The pulmonary valve lies between the right ventricle and the pulmonary artery, whereas the aortic valve separates the aorta from the left ventricle.

Figure 7-2 Structure of the heart valves.

Deoxygenated venous blood is returned to the heart from the body through the superior and inferior venae cavae, which empty into the right atrium. Blood flows from the right atrium across the tricuspid valve into the right ventricle, which then pumps blood through the pulmonary valve into the pulmonary artery. Within the capillaries of the lungs, the red blood cells take up oxygen and release carbon dioxide. The freshly oxygenated blood then passes through the pulmonary veins into the left atrium, from which it flows across the mitral valve into the left ventricle. Contraction of the left ventricle forces oxygenated blood through the aortic valve into the aorta and the rest of the arterial tree to provide oxygen and nourishment to tissues throughout the body. The general circulation of the body is termed the systemic circulation (Figure 7-3), whereas the circulation of blood through the lungs is the pulmonary circulation. Because greater pressure is needed to pump blood through the systemic circulation than through the pulmonary circulation, the wall of the left ventricle is considerably thicker than that of the right ventricle.

Figure 7-3 Systemic arterial system.

The atria and ventricles alternately contract and relax. The contraction phase is called systole; the heart chambers relax and fill with blood during diastole (the relaxation phase). The normal cardiac impulse that stimulates mechanical contraction of the heart arises in the sinoatrial (SA) node, or pacemaker, which is situated in the right atrial wall near the opening of the superior vena cava. The impulse passes slowly through the atrioventricular (AV) node, which is located in the right atrium along the lower portion of the interatrial septum, and then spreads quickly throughout the ventricles by way of a band of atypical cardiac muscle fibers the bundle of His. The bundle of His terminates in Purkinje fibers, which can conduct impulses throughout the muscle of both ventricles and stimulate them to contract almost simultaneously. Specialized pacemaker cells in the SA node possess an intrinsic rhythm, so that even without any stimulation by the autonomic nervous system the node itself initiates impulses at regular intervals (about 60 to 75 beats per minute).

If the SA node for some reason is unable to generate an impulse, pacemaker activity shifts to another excitable component of the conduction system. These ectopic pacemakers also generate impulses rhythmically, although at a much slower rate. For example, if the AV node were to control pacemaker activity, the heart would beat 40 to 60 times per minute. If the conduction system of the heart is unable to maintain an adequate rhythm, the patient may receive an artificial pacemaker that electrically stimulates the heart, either at a set rhythm or only when the heart rate decreases below a preset minimum.

The heart muscle is supplied with oxygenated arterial blood by way of the right and left coronary arteries. These small vessels arise from the aorta just above the aortic valve. Unoxygenated blood from the myocardium drains into the coronary veins, which lead into the coronary sinus before opening into the right atrium.

The heart is surrounded by a double membranous sac termed the pericardium. The pericardium has a well-lubricated lining that protects against friction and permits the heart to move easily during contraction.

Congenital heart disease

Left-to-Right Shunts

The most common congenital cardiac lesions are left-to-right shunts, which permit mixing of blood in the systemic and pulmonary circulations. Because blood is preferentially shunted from the high-pressure systemic circulation to the relatively low-pressure pulmonary circulation, the lungs become overloaded with blood. The magnitude of the shunt depends on the size of the defect and the differences in pressure on the two sides. An increased load on the heart produces enlargement of specific cardiac chambers, depending on the location of the shunt. The size of the defect determines the hemodynamic consequence for the systemic cardiac output.

The most common congenital cardiac lesion is atrial septal defect, which permits free communication between the two atria as a result of either lack of closure of the foramen ovale after birth or its improper closure during gestation. Because the left atrial pressure is usually higher than the pressure in the right atrium, the resulting shunt is from left to right and causes increased pulmonary blood flow and overloading of the right ventricle. This produces the radiographic appearance of enlargement of the right ventricle, the right atrium, and the pulmonary outflow tract (Figure 7-4).

Figure 7-4 Atrial septal defect. Frontal projection of the chest demonstrates cardiomegaly along with an increase in pulmonary vascularity reflecting a left-to-right shunt. Small aortic knob (white arrow) and descending aorta (small arrows) are dwarfed by enlarged pulmonary outflow tract (open arrow).

In a patient with a ventricular septal defect, the resulting shunt is also from left to right because the left ventricular pressure is usually higher than the pressure in the right ventricle. The shunt causes increased pulmonary blood flow and consequently increased pulmonary venous return (Figure 7-5). This leads to diastolic overloading and enlargement of the left atrium and left ventricle. Because shunting occurs primarily in systole and any blood directed to the right ventricle immediately goes into the pulmonary artery, there is no overloading of the right ventricle and radiographically no right ventricular enlargement is seen.

Figure 7-5 Ventricular septal defect. Heart is enlarged and somewhat triangular, and there is increased pulmonary vascular volume. Pulmonary trunk is very large and overshadows the normal-sized aorta, which seems small by comparison.

The third major type of left-to-right shunt is patent ductus arteriosus. The ductus arteriosus is a vessel that extends from the bifurcation of the pulmonary artery to join the aorta just distal to the left subclavian artery. It serves to shunt blood from the pulmonary artery into the systemic circulation during intrauterine life. Persistence of the ductus arteriosus, which normally closes soon after birth, results in a left-to-right shunt. The flow of blood from the higher-pressure aorta to the lower-pressure pulmonary artery causes increased pulmonary blood flow, and an excess volume of blood is returned to the left atrium and left ventricle. Radiographically, there is enlargement of the left atrium, the left ventricle, and the central pulmonary arteries, along with a diffuse increase in pulmonary vascularity (Figure 7-6). The increased blood flow through the aorta proximal to the shunt produces a prominent aortic knob in contrast to the small- or normal-size aorta seen in atrial and ventricular septal defects.

Figure 7-6 Patent ductus arteriosus. A, Frontal chest radiograph demonstrates cardiomegaly with enlargement of the left atrium, left ventricle, and central pulmonary arteries. There is diffuse increase in the pulmonary vascularity. B, In another patient, an aortogram shows patency of the ductus arteriosus (arrow).

All left-to-right shunts can be complicated by the development of pulmonary hypertension (Eisenmenger’s syndrome). This is caused by increased vascular resistance within the pulmonary arteries related to chronic increased flow through the pulmonary circulation. Pulmonary hypertension appears radiographically as an increased fullness of the central pulmonary arteries with abrupt narrowing and pruning of peripheral vessels (Figure 7-7). The elevation of pulmonary arterial pressure tends to balance or even reverse the left-to-right shunt, thus easing the volume overloading of the heart.

Figure 7-7 Eisenmenger’s syndrome in atrial septal defect. There is slight but definite cardiomegaly and a great increase in the size of the pulmonary trunk. Right and left pulmonary artery branches are huge, but the peripheral pulmonary vasculature is relatively sparse. Long-standing pulmonary hypertension has produced degenerative changes in the walls of the pulmonary arteries, which have become densely calcified.

Radiographic Appearance

Enlargement of the right ventricle causes upward and lateral displacement of the apex of the heart (Figure 7-9). This results in the classic coeur en sabot appearance, in which the heart resembles the curved-toe portion of a wooden shoe. In about one fourth of patients with tetralogy of Fallot, the aorta is on the right side.

Figure 7-9 Tetralogy of Fallot. Plain chest radiograph demonstrates the characteristic lateral displacement and upward tilting of the prominent cardiac apex. There is also decreased pulmonary vascularity and a flat pulmonary outflow tract.

Currently, echocardiography is the modality of choice to demonstrate the four abnormalities constituting tetralogy of Fallot. Spin-echo MRI best demonstrates the ventricular septal defect, right ventricular outflow (pulmonary stenosis), overriding aorta, and right ventricular hypertrophy. Cine MRI shows pulmonary stenosis as a flow void, for demonstrating it better than echocardiography.

Treatment

Without surgical repair, most patients with tetralogy of Fallot die before reaching puberty. Although operative repair provides some chance of recovery, the outcome depends on the severity of the defect and other extenuating circumstances. In some cases, surgery has only a palliative, supportive role.

Coarctation of the Aorta

Coarctation refers to a narrowing, or constriction, of the aorta that most commonly occurs just beyond the branching of the blood vessels to the head and arms. The blood supply and the pressure to the upper extremities are higher than normal. As a result, there is decreased blood flow through the constricted area to the abdomen and legs. Classically the patient has normal blood pressure in the arms, but very low blood pressure in the legs. Coarctation of the aorta is the most frequent cause of hypertension in children.

The relative obstruction of aortic blood flow leads to the progressive development of collateral circulation—the enlargement of normally tiny vessels in an attempt to compensate for the inadequate blood supply to the lower portion of the body.

Radiographic Appearance

Coarctation of the aorta is often seen radiographically as rib notching (usually involving the posterior fourth to eighth ribs) resulting from pressure erosion by dilated and pulsating intercostal collateral vessels, which run along the inferior margins of these ribs (Figure 7-10A). Notching of the posterior border of the sternum may be produced by dilation of mammary artery collaterals.

Figure 7-10 Coarctation of the aorta. A, There is notching of the fourth through eighth posterior ribs (arrows). B, Plain chest radiograph demonstrates a figure-3 sign (arrow pointing to its center). Upper bulge represents prestenotic dilatation; lower bulge represents poststenotic dilatation. C, Esophagram demonstrates reverse figure-3 sign (arrow pointing to its center).

Coarctation of the aorta often causes two bulges in the region of the aortic knob that produce a characteristic figure-3 sign on plain chest radiographs (Figure 7-10B) and a reverse figure-3 (or figure-E) impression on the barium-filled esophagus (Figure 7-10C). The more cephalic bulge represents dilatation of the proximal aorta and the base of the subclavian artery (prestenotic dilatation); the lower bulge reflects poststenotic aortic dilatation.

With use of echocardiography to evaluate the aortic arch, the severity of the coarctation can be determined. Doppler echocardiography measures the gradient flow at the stenosis, and the diastolic runoff can be demonstrated.

Aortography can accurately localize the site of obstruction, determine the length of the coarctation, and identify any associated cardiac malformations. More recently, MRI has been used to demonstrate aortic narrowing (Figure 7-11) and to evaluate the appearance of the aorta after corrective surgery.

Figure 7-11 Coarctation of the aorta. Sagittal MR image demonstrates coarctation (straight arrow) of the thoracic aorta with associated poststenotic dilatation of the proximal descending thoracic aorta (curved arrow). L, Left subclavian artery.

Treatment

Uncorrected aortic coarctation has a dismal prognosis, and surgical repair is required for patient survival. Currently, three surgical repairs are being used: (1) end-to-end anastomosis, (2) patch aortoplasty, and (3) left subclavian flap aortoplasty (surgical reformation of the aorta).

Summary of Findings for Congenital Heart Disease

PA chest, posteroanterior image of the chest; US, ultrasound.

Acquired vascular disease

Coronary Artery Disease

Narrowing of the coronary arteries causes oxygen deprivation of the myocardium and ischemic heart disease. In most patients, narrowing of the lumen of one or more of the coronary arteries is attributable to the deposition of fatty material on the inner arterial wall (atherosclerosis). Factors predisposing to the development of coronary artery disease include hypertension, obesity, smoking, a high-cholesterol diet, and lack of exercise.

The speed and degree of luminal narrowing determine whether an atherosclerotic lesion causes significant and clinically evident ischemia. Temporary oxygen insufficiency causes angina pectoris, a feeling of severe chest pain that may radiate to the neck, jaw, and left arm (sometimes both arms) and that is often associated with the sensation of chest tightness or suffocation. Attacks of angina pectoris are often related to a sudden increase in the demand of the myocardium for oxygen, such as after strenuous exercise or a heavy meal or with emotional stress or exposure to severe cold. The placing of a nitroglycerin tablet under the tongue causes venous dilatation, thus decreasing preload and myocardial oxygen demand.

Occlusion of a coronary artery deprives an area of myocardium of its blood supply and leads to the death of muscle cells (myocardial infarction) in the area of vascular distribution. The size of the coronary artery that is occluded and the myocardium that it supplies determines the extent of heart muscle damage. The greater the area affected, the poorer the prognosis because of the increased loss of pumping function that may result in congestive heart failure (CHF). A favorable prognostic factor is the development of collateral circulation, through which blood from surrounding vessels is channeled into the damaged tissue. If the patient survives, the infarcted region heals with fibrosis. Long-term complications include the development of thrombi on the surface of the damaged area and the production of a local bulge (ventricular aneurysm) at the site of the weakness of the myocardial wall.