The ventricular septum separates the left ventricle from the right ventricle and, to a small extent, from the right atrium. It consists of a membranous and muscular portion and is subdivided into inflow, trabecular, and outflow regions. Defects in the septum may occur in each region. They range in size from pinhole defects of 1 mm or less to virtual absence of the septum.

The location of a defect within the ventricular septum is not of great hemodynamic consequence, but it is a critical surgical consideration and an important determinant of natural history. Perimembranous defects are bounded by a portion of the membranous septum and a portion of muscular septum. Muscular defects, bounded entirely by muscle, frequently are multiple and tend to close spontaneously. Doubly committed subarterial defects (formerly designated supracristal or type I defects) lie in the outflow portion of the muscular septum beneath the pulmonary and aortic valves. They often are associated with development of aortic regurgitation and, although relatively rare among white children constitute up to 30% of VSDs in Asian children. Malalignment defects, in which the crest of the septum lies in a plane different from the anterior portion of the aortic root, are found in many complex lesions but also occasionally constitute an isolated VSD. Progressive left ventricular outflow tract obstruction is a common finding with such defects.

VSD allows a communication to exist between the systemic and pulmonary circulations. The hemodynamic effect of this communication depends on the size of the left-to-right shunt, which, in turn, is a function of the anatomic size of the defect and the relative pulmonary and systemic vascular resistances. Left-to-right shunting at the ventricular level results in increased pulmonary blood flow and increased volume work of the left ventricle. Consequently, left atrial and pulmonary venous pressures are increased. The combination of increased pulmonary blood flow and elevated pulmonary venous pressure produces increased oncotic pressure within the pulmonary capillary bed and accumulation of pulmonary interstitial fluid. Decreased pulmonary compliance with increased work of breathing accounts for the early manifestations of congestive heart failure (CHF). More profound heart failure, causing alveolar fluid collection, also can interfere with pulmonary gas exchange.

Pulmonary vascular resistance is elevated in the fetus and newborn. Normally, resistance falls during the infant’s first several days of life, but it may remain high for several months in the presence of large interventricular communication. Hence, the hemodynamic manifestations of a left-to-right shunt are not evident at birth and may not appear until later in infancy. Elevated pulmonary artery pressure that invariably is present with large or “unrestrictive” VSD produces characteristic changes in the pulmonary arteriolar bed. Pulmonary vascular obstructive disease (PVOD) with marked, irreversibly elevated resistance may develop when a child is as young as 2 years old. It usually occurs after a period of low resistance (hence, high pulmonary blood flow with CHF), but it may develop progressively in children whose pulmonary vascular resistance never declines postnatally. The later stages of progressive PVOD in these patients, in which cyanosis is caused by reversed shunting through a large interventricular communication, is termed Eisenmenger syndrome. This well-known late chapter in the natural history of VSD with pulmonary hypertension has made customary the surgical repair of large defects before the child reaches 6 months of age, even in the absence of severe symptoms.

Small VSDs seldom cause significant symptoms and usually come to the attention of a physician because of the associated heart murmur. Whereas the murmur is not present in the immediate newborn period, it may be audible as early as the second day of life and usually is heard at the routine 2-week checkup. It characteristically is a high-pitched, harsh, holosystolic murmur, well localized along the left sternal border. A small VSD may produce a murmur of low pitch, but a high-pitched murmur strongly suggests that the defect is not large. The precordium is quiet, but a localized thrill may be palpable. The first and second heart sounds are normal, and seldom is a diastolic murmur present. Except for mild tachypnea in small infants, other physical findings are normal. Small defects, sometimes called maladie de Roger, do not interfere with normal growth. A significant number, estimated to be in the range of 30% to 70%, will undergo spontaneous closure, usually in the child’s first 2 years of life. Certain types of defects, regardless of size, however, may predispose to development of secondary conditions, especially aortic regurgitation and left ventricular outflow tract obstruction. For this reason, children with physical findings of small VSD should undergo echocardiographic examination to confirm the diagnosis and to localize the defect.

Large defects may come to the attention of a physician later than do small defects because elevated pulmonary vascular resistance may delay the appearance of a murmur. When present, symptoms are those of CHF. They include irritability, increased respiratory effort, poor feeding, and poor weight gain. Recurrent respiratory infections are common occurrences, and pneumonia often is the preliminary diagnosis.

Signs of CHF include tachycardia, tachypnea, increased work of breathing, pallor, diaphoresis, and failure to thrive.
Pulmonary rales are a late finding. The precordium is hyperactive, and a thrill often is palpable. The second heart sound is single or narrowly split. When audible, it usually is accentuated, but often it is obscured by a loud, low-pitched, harsh, holosystolic murmur. The murmur is loudest along the left sternal border but is much less well localized than is the murmur of a small VSD. It may radiate to the right of the sternum but radiates poorly to the back. A diastolic murmur or rumble heard at the lower left sternal border is related to increased mitral flow. Its presence implies a pulmonary–to–systemic flow ratio exceeding 2:1. Pulses may be diminished with severe CHF, but they are symmetric. The liver and sometimes the spleen are enlarged.

Moderate defects may produce physical findings suggestive of small defects, although rarely does one detect a murmur of high pitch. Usually, some degree of tachypnea and increased respiratory effort is present. Because of increased volume load of the left ventricle, the precordial impulse is hyperactive, and often a thrill is present. When associated with low pulmonary vascular resistance and high pulmonary blood flow, a moderate defect may produce findings similar to those of a large defect, including CHF.