The ventricular septum is the partition between the two muscular parts of the heart, the left and right ventricles.  The left ventricle normally functions to pump blood throughout the whole body.  The right ventricle normally pumps blood through the lungs then back to the left ventricle.  The ventricular septum is formed from a variety of fetal tissues, and a disturbance in any of the components can cause a VSD.

Aside from the known genetic basis in Keeshonden dogs, the cause of most VSD’s in other dogs and cats is unknown.  There are several varieties of VSD’s, the most typical being subcristal (or membranous) in location.  This lesion is located just under the tricuspid valve (the valve that separates the right atrium from the right ventricle).  Supracristal (subpulmonic) defects are located just under the pulmonary valve (the valve that directs blood to the lungs).  Defects in this area are more likely to allow the aortic valve (the valve that directs blood to the body) to prolapse or leak.

The pathophysiology of a VSD is influenced by many factors.  Typically, blood “shunts abnormally” across the defect allowing a higher amount of blood than normal to pass through the lungs.  This increased blood flow then returns back to the left side of the heart and can dilate the left ventricle and left atrium and eventually lead to heart failure.  The higher blood flow in the lungs can potentially lead to high pulmonary pressures  (high blood pressure localized to the lungs) and resultant respiratory failure.

Potential outcomes to a ventricular septal defect are many and include:

1) patient tolerance of the lesion (no clinical signs);

2) partial or complete closure of the defect by the tricuspid valve;

3) progressive aortic valve prolapse and resultant congestive heart failure;

4) pulmonary hypertension and resultant respiratory failure.

5) volume overload and subsequent congestive heart failure

Treatments vary according to location and severity of the defect.  Open heart surgery and primary closure of the defect is rarely attempted in dogs.  Sometimes medication can slow progression of the heart enlargement.  If congestive heart failure develops (cough, breathing hard, fluid accumulation), immediate attention is needed to attempt stabilization.