Lesions Resulting in Increased Volume Load

The most common lesions in this group are those that cause left-to-right shunting (see Chapter 420): atrial septal defect, ventricular septal defect (VSD), AV septal defects (AV canal), and patent ductus arteriosus. The pathophysiologic common denominator in this group is communication between the systemic and pulmonary sides of the circulation, which results in shunting of fully oxygenated blood back into the lungs. This shunt can be quantitated by calculating the ratio of pulmonary to systemic blood flow, or Qp : Qs. Thus, a 2 : 1 shunt implies twice the normal pulmonary blood flow.

The direction and magnitude of the shunt across such a communication depend on the size of the defect, the relative pulmonary and systemic pressure and vascular resistances, and the compliances of the 2 chambers connected by the defect. These factors are dynamic and may change dramatically with age: Intracardiac defects may grow smaller with time; pulmonary vascular resistance, which is high in the immediate newborn period, decreases to normal adult levels by several weeks of life; and chronic exposure of the pulmonary circulation to high pressure and blood flow results in a gradual increase in pulmonary vascular resistance (Eisenmenger physiology, Chapter 427.2). Thus, a lesion such as a large VSD may be associated with little shunting and few symptoms during the initial weeks of life. When pulmonary vascular resistance declines in the next several weeks, the volume of the left-to-right shunt increases, and symptoms begin to appear.

The increased volume of blood in the lungs decreases pulmonary compliance and increases the work of breathing. Fluid leaks into the interstitial space and alveoli and causes pulmonary edema. The infant acquires the symptoms we refer to as heart failure, such as tachypnea, chest retractions, nasal flaring, and wheezing. The term heart failure is a misnomer, however; total left ventricular output is actually several times greater than normal, although much of this output is ineffective because it returns directly to the lungs. To maintain this high level of left ventricular output, heart rate and stroke volume are increased, mediated by an increase in sympathetic nervous system activity. The increase in circulating catecholamines, combined with the increased work of breathing, results in an elevation in total body oxygen consumption, often beyond the oxygen transport ability of the circulation. Sympathetic activation leads to the additional symptoms of sweating and irritability and the imbalance between oxygen supply and demand lead to failure to thrive. Remodeling of the heart occurs, with predominantly dilatation and a lesser degree of hypertrophy. If left untreated, pulmonary vascular resistance eventually begins to rise and, by several years of age, the shunt volume will decrease and eventually reverse to right to left (Eisenmenger physiology, Chapter 427.2).

Additional lesions that impose a volume load on the heart include regurgitant lesions (Chapter 422) and the cardiomyopathies (Chapter 433). Regurgitation through the AV valves is most commonly encountered in patients with partial or complete AV septal defects (AV canal, endocardial cushion defects). In these lesions, the combination of a left-to-right shunt with AV valve regurgitation increases the volume load on the heart and often leads to more severe symptoms. Isolated regurgitation through the tricuspid valve is seen in mild to moderate forms of Ebstein anomaly (Chapter 424.7). Regurgitation involving one of the semilunar valves is usually also associated with some degree of stenosis; however, aortic regurgitation may be encountered in patients with a VSD directly under the aortic valve (supracristal VSD) and in patients with membranous subaortic stenosis.

In contrast to left-to-right shunts, in which intrinsic cardiac muscle function is generally either normal or increased, heart muscle function is decreased in the cardiomyopathies. Cardiomyopathies may affect systolic contractility or diastolic relaxation, or both. Decreased cardiac function results in increased atrial and ventricular filling pressure, and pulmonary edema occurs secondary to increased capillary pressure. Poor cardiac output leads to decreased organ blood flow, sympathetic activation, and the symptoms of poor perfusion and decreased urine output. The major causes of cardiomyopathy in infants and children include viral myocarditis, metabolic disorders, and genetic defects (Chapter 433).

Lesions Resulting in Increased Pressure Load

The pathophysiologic common denominator of these lesions is an obstruction to normal blood flow. The most frequent are obstructions to ventricular outflow: valvular pulmonic stenosis, valvular aortic stenosis, and coarctation of the aorta (Chapter 421). Less common are obstruction to ventricular inflow: tricuspid or mitral stenosis, cor triatriatum and obstruction of the pulmonary veins. Ventricular outflow obstruction can occur at the valve, below the valve (double-chambered right ventricle, subaortic membrane), or above it (branch pulmonary stenosis or supravalvular aortic stenosis). Unless the obstruction is severe, cardiac output will be maintained and the clinical symptoms of heart failure will be either subtle or absent. This compensation predominantly involves an increase in cardiac wall thickness (hypertrophy), but in later stages it also involves dilatation and can progress to ventricular dilation and failure.

The clinical picture is different when obstruction to outflow is severe, which is usually encountered in the immediate newborn period. The infant may become critically ill within several hours of birth. Severe pulmonic stenosis in the newborn period (critical pulmonic stenosis) results in signs of right-sided heart failure (hepatomegaly, peripheral edema), as well as cyanosis from right-to-left shunting across the foramen ovale. Severe aortic stenosis in the newborn period (critical aortic stenosis) is characterized by signs of left-sided heart failure (pulmonary edema, poor perfusion) and right-sided failure (hepatomegaly, peripheral edema), and it may progress rapidly to total circulatory collapse. In older children, severe pulmonic stenosis leads to symptoms of right-sided heart failure, but not to cyanosis unless a pathway persists for right-to-left shunting (e.g., patency of the foramen ovale).

Coarctation of the aorta in older children and adolescents is usually manifested as upper body hypertension and diminished pulses in the lower extremities. In the immediate newborn period, however, the clinical presentation of coarctation may be delayed because of the presence of a patent ductus arteriosus. In these patients, the open aortic end of the ductus may serve as a conduit for blood flow to partially bypass the obstruction. These infants then become symptomatic, often dramatically, when the ductus finally closes, usually within the 1st 2 mo of life.

Cyanotic Lesions with Decreased Pulmonary Blood Flow

These lesions must include both an obstruction to pulmonary blood flow (at the tricuspid valve or right ventricular or pulmonary valve level) and a pathway by which systemic venous blood can shunt from right to left and enter the systemic circulation (via a patent foramen ovale, atrial septal defect, or VSD). Common lesions in this group include tricuspid atresia, tetralogy of Fallot, and various forms of single ventricle with pulmonary stenosis (Chapter 424). In these lesions, the degree of cyanosis depends on the degree of obstruction to pulmonary blood flow. If the obstruction is mild, cyanosis may be absent at rest. These patients may have hypercyanotic (“tet”) spells during conditions of stress. In contrast, if the obstruction is severe, pulmonary blood flow may be totally dependent on patency of the ductus arteriosus. When the ductus closes in the 1st few days of life, the neonate experiences profound hypoxemia and shock.

Cyanotic Lesions with Increased Pulmonary Blood Flow

This group of lesions is not associated with obstruction to pulmonary blood flow. Cyanosis is caused by either abnormal ventricular-arterial connections or total mixing of systemic venous and pulmonary venous blood within the heart (Chapter 425). Transposition of the great vessels is the most common of the former group of lesions. In this condition, the aorta arises from the right ventricle and the pulmonary artery arises from the left ventricle. Systemic venous blood returning to the right atrium is pumped directly back to the body, and oxygenated blood returning from the lungs to the left atrium is pumped back into the lungs. The persistence of fetal pathways (foramen ovale and ductus arteriosus) allows for a small degree of mixing in the immediate newborn period; when the ductus begins to close, these infants become extremely cyanotic.

Total mixing lesions include cardiac defects with a common atrium or ventricle, total anomalous pulmonary venous return, and truncus arteriosus (Chapter 425). In this group, deoxygenated systemic venous blood and oxygenated pulmonary venous blood mix completely in the heart and, as a result, oxygen saturation is equal in the pulmonary artery and aorta. If pulmonary blood flow is not obstructed, these infants have a combination of cyanosis and heart failure. In contrast, if pulmonary stenosis is present, these infants may have cyanosis alone, similar to patients with tetralogy of Fallot.