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Ventricular Septal Defect (VSD) in dogs and cats

The normal heart has two pumping chambers known as ventricles. The left and right ventricles are separated by a muscular wall called the ventricular septum. A defect in this septum (ventricular septal defect, or VSD), sometimes referred to as a "hole in the heart," allows communication between the two pumping chambers. When a VSD is present, blood can flow from the left ventricle through the defect and into the right ventricle. From there, this extra blood is added to the blood that moves to the lungs and returns to the left side of the heart. If a VSD causes problems for the patient, it usually does so because it overworks the left side of the heart.

A defect allowing blood flow between the left and right ventricle. The defect on the right side is usually beneath the septal leaflet of the tricuspid valve. The defect on the left side is usually beneath the aortic valve.

Additional information

An anomalous communication between the two ventricles (figure). The defect may be in the inlet, outlet, muscular, or membranous septum. Most ventricular septal defects (VSD) in small animals are perimembranous and subcristal.

The presence of a VSD results in a pulmonary systemic shunt. The direction and volume of the shunt is determined by the size of the defect, the relationship of the pulmonary and systemic vascular resistances, and the presence of other anomalies. Most VSD in veterinary patients are small and therefore restrictive--i.e., the difference between left and right ventricular pressures is maintained. VSD of moderate size have an area that exceeds 40% of the area of the open aortic valve. They are only partially restrictive and result in various degrees of right ventricular hypertension. Large VSD have an area that is as great or greater than the aorta. They are nonrestrictive, and right ventricular pressure is necessarily systemic. Only moderate and large defects impose a pressure load upon the right ventricle. In a patient with normal resistance to right ventricular ejection, the direction of the shunt is left-to-right. This increases pulmonary venous return and imposes a volume overload on the left atrium and ventricle. With large shunts, left ventricular congestive failure can develop. Generally, the left ventricle unloads into the pulmonary arterial system during systole. Unless the defect is of moderate or large size, the right ventricle is spared.

Systems Affected
Pulmonary If congestive heart failure (CHF) develops. Cardiovascular Theoretically, a large shunt could result in pulmonary vascular disease, pulmonary hypertension, and shunt reversal (i.e., Eisenmenger's syndrome). This is uncommon in small animals. If shunt reversal occurs, it usually does so early in life.

Breed predispositions are recognized. Genetic transmission has not been established.

VSD is one of the most common congenital cardiac malformations in cats; less common in dogs.
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