Ventricular Septal Defect

Defect in the septum between the two ventricles is analogous to atrial septal defect in that it causes a unidirectional shunting of blood (fig. 36). An important difference between the two, however, makes ventricular septal defect a rather unique cardiac lesion. The two circulations, systemic and pulmonary, are closed systems: the former operates under high pressure determined by the resistance of small arteries, the latter under low pressure related to the minimal resistance of small branches of the pulmonary artery. Communication between the two circulations through the atria, where pressure in both chambers is close to zero, permits shunting of blood from one side to the other without significant consequences other than overload. However, communication between them in areas of high pressure—the ventricles or the arteries—may have disastrous consequences. A large opening between the two ventricles, with a systolic pressure of 120 mm Hg on the left side and 20 mm Hg on the right side produces a pressure gradient, so that most of the blood might escape to the right ventricle instead of entering the aorta. Clearly, a person could not survive were this allowed to happen; hence in compensation the systolic pressure in the right ventricle rises to match that in the left. Right ventricular systolic pressure can be elevated by one of two mechanisms—abnormal constriction of the small pulmonary arteries, increasing resistance

to a level comparable to that of the systemic arteries and thereby producing pulmonary hypertension; or pulmonary valve stenosis.

A small ventricular septal defect is characterized by a left-to-right shunt through the opening because it offers enough resistance to blood flow to maintain the normal difference in pressures between the two ventricles. The magnitude of the shunt entirely depends on the size of the opening, which ranges from that of a pinhead to that of a dime. For the average adult heart the largest diameter consistent with a normal pressure relationship between the two ventricles is about 1.5 cm, the approximate dividing line between small and large ventricular septal defects. The blood flow through a ventricular septal defect produces turbulence and generates loud heart murmurs audible on examination of a child; hence the diagnosis of such a defect is usually made at birth or at least during infancy. The hemodynamic consequence of a small ventricular septal defect varies with the magnitude of the shunt. Small defects shunting no more than half of the left ventricular output into the right ventricle have no effect on circulatory dynamics. Although the heart murmur is still detectable, no other abnormalities are usually discovered. The prognosis is favorable, with a normal life expectancy. The only risk is the possibility of infective endocarditis. Surgical correction is usually considered unnecessary. A significant number of such patients experience spontaneous closure of the defect—in essence, self-cure. Larger shunts through small ventricular septal defects (greater than twice the left ventricular output) have effects similar to those of atrial septal defect, namely overload of the heart and overfilling of the pulmonary blood vessels. Since ventricular septal defect with large pulmonary flow but normal right ventricular pressure takes a similar course as atrial septal defect, surgical closure (by means of open-heart surgery) is generally performed in early childhood.

Large ventricular septal defect is a more serious problem. The normal function of the left ventricular pump has to be protected by pressure elevation in the right ventricle from birth; hence pulmonary hypertension is already present in infancy. The left-to-right shunting of blood is usually considerable. Persistent high pressure in the pulmonary arterial system, together with large blood flow in the pulmonary circulation, may gradually increase the resistance to the point of reversing the shunt from right to left. This complication

happens occasionally in late childhood but most frequently in adulthood. Large ventricular septal defect complicated by shunt reversal and resulting in cyanosis is called Eisenmenger's syndrome . Despite pulmonary hypertension the growth and development of children with large ventricular septal defect are usually normal, and disability is rare. Even a fully developed cyanotic stage due to shunt reversal may be well tolerated, although in that case life expectancy is greatly reduced.

The treatment for large ventricular septal defect is surgical closure. The goal is to prevent progressive disease of small pulmonary arteries; thus surgical closure is often performed on small children, even infants. In the case of a large left-to-right shunt, closure of the defect may significantly reduce the pressure in the right ventricle and lower or even eliminate pulmonary hypertension. Large ventricular septal defect also occurs in combination with pulmonary stenosis, in which case pulmonary hypertension is absent (see discussion of tetralogy of Fallot below).