Ventricular septal defects (VSD) occur most frequently in keeshounds and English bulldogs and are one of the more common cardiac defects in cats.
Most ventricular septal defect involve the upper membranous portion of the interventricular septum and can be classified as high ventricular septal defects. The haemodynamic abnormalities, the clinical signs and progression are determined by the size of the defect. Blood normally flows from the left to the right ventricle only during systole; if pulmonary hypertension develops the direction of blood flow may reverse. An animal with a small defect may show no clinical signs. A large ventricular septal defect will initially produce pulmonary overcirculation and left ventricular volume overload. Increased vascular resistance may lead to pulmonary hypertension, right ventricular pressure overload, and hypertrophy of the right ventricle and right ventricular outflow tract. Hypertrophy of the right ventricular outflow tract results in infundibular pulmonic stenosis which contributes to the right ventricular pressure overload and eventual reversal of the shunt (Eisenmenger syndrome). As with most congenital cardiac defects a ventricular septal defect can occur as an isolated defect or in conjunction with other defects such as atrial septal defect, patent ductus arteriosus, aortic or pulmonic stenosis. Ventricular septal defect is also one of the components of tetralogy of Fallot. In cats most ventricular septal defects are associated with tricuspid valve dysplasia.
A ‘diagonal’ systolic murmur with a point of maximal intensity between the 2nd and 4th right intercostal spaces and the 5th and 6th left intercostal spaces is highly suggestive of a ventricular septal defect. A precordial thrill may be present- Pulmonary hypertension may be associated with splitting of the second heart sound. Shunt reversal results in attenuation of the murmur, cyanosis and ultimately signs of right-sided congestive heart failure.
Changes indicative of left ventricular, right ventricular or biventricular enlargement may be noted with a severe ventricular septal defect. Arrhythmias and conduction disturbances, for example bundle branch block, have also been reported.
Small defects are unlikelv to be associated with radiographic abnormalities. The radiographic changes seen with a larger ventricular septal defect are similar to those associated with a patent ductus arteriosus except that the aortic knuckle is absent and the left ventricular enlargement tends to be less pronounced. There may be evidence of generalized cardiomegaly and pulmonary overcirculation; the left atrium may appear particularly prominent. Right ventricular or biventricular enlargement is more likely to occur if the shunt reverses.
A ventricular septal defect high in the interventricular septum should be demonstrated in several planes. Pulsed Doppler signals from the right ventricle show high systolic blood velocities across the defect with varying degrees of turbulence and can confirm the direction of the shunt. In some dogs there may be evidence of aortic regurgitation. The left ventricle may appear hyper-kinetic if the shunt is large and both the left atrium and left ventricle appear dilated. The right ventricle usually appears normal unless the shunt reverses.
Selective angiocardiography with injection of contrast material into the left ventricle can be used to confirm the presence of a left to right shunting ventricular septal defect and results in simultaneous opacification of the left and right ventricles as well as the aorta and pulmonary artery.
Ventricular septal defect: Treatment
Small defects often require no treatment. Pulmonary banding may be considered in mature dogs with normal or only mildly increased vascular resistence to reduce flow of blood through the shunt. Anatomic repair of the defect is the only treatment likely to be effective in dogs with left to right shunts and high vascular resistance (pulmonary banding could reverse shunt flow in such cases). Dogs with bidirectional or reverse shunting ventricular septal defects are not surgical candidates.