The direction of an intracardiac shunt at the atrial level is controlled by:

B. The compliance of the right and left ventricles

DISCUSSION: The direction of an intracardiac shunt is governed by the compliance of the downstream chambers. For an atrial level shunt, the compliance of the right and left ventricles and their ability to distend with increased volume during diastolic filling dictates the direction of the shunt flow. Since the right ventricle is usually a more compliant—and therefore more distensible—chamber than the left ventricle, flow across an ASD occurs from left to right across the open tricuspid valve during diastole. The size of an ASD does not correspond to the degree of shunt as long as the defect is large enough to be unrestrictive to flow. A large shunt can occur through a relatively small defect if the ventricular compliance is favorable.