A dog and cat snuggle in the grass.

Ventricular Septal Defect (VSD) in Dogs and Cats

Ventricular septal defect (VSD) is a common congenital abnormality in both dogs and cats. The heart consists of four chambers, the left and right atrium and the left and right ventricles. A VSD means that there is a defect in the muscle wall that separates the left and right ventricles. Normally, these two chambers would not have any communication. The defect is usually in the perimembranous portion of the septum, which is anatomically higher up on the septum. The defect can vary in size and location on the septum. Unfortunately, pets with a VSD may also have other cardiac congenital defects.

When there is a defect between the two ventricles, it causes blood to be shunted from the left side of the heart to the right side, due to the pressure gradient difference. The left side of the heart is a higher pressure system than the right. The size of the defect limits how much blood can be shunted.

A small defect may cause minimal clinical signs, but a large defect causes severe repercussions. When blood is shunted back to the right side, it then recirculates through the lungs and back to the left side of the heart. This shunting can cause pulmonary hypertension (high blood pressure in the lungs), dilation of the right ventricle, and constriction of blood vessels. As the pressure in the lungs rises, it causes the right side of the heart’s blood pressure to rise, ultimately causing blood to reverse it’s shunting direction and move from the right side to the left. This causes unoxygenated blood to shunt into the left ventricle and get pumped out into the body. Ultimately, this leads to cyanosis (lack of oxygen to tissues) and can be fatal.

Clinical Signs of VSD

Clinical signs can vary from mild to severe, depending on the size of the shunt and the progression of disease. The most common clinical finding on a physical exam is a loud heart murmur, heard when auscultating the right thorax. Other clinical signs include exercise intolerance, lethargy, pot-bellied appearance (from right-sided heart failure), and cyanosis.


A definitive diagnosis of a VSD is done by an ultrasound of the heart called an echocardiogram. This allows visualization of the heart chambers and following of blood flow. Shunting of blood can be seen and the defect, chamber enlargement, and pulmonary hypertension can be measured.

Thoracic radiographs (X-rays) can be taken of the heart to increase suspicion of a VSD. When this defect is present, radiographs show an enlarged heart (cardiomegaly) and potential evidence of heart failure.

Bloodwork completed in patients with a VSD may show an elevated red blood cell count, due to the body’s compensation for chronic lack of oxygen to tissues (hypoxia).

VSD Treatment

Animals with small defects and no clinical signs may not need treatment and have a normal course of life. Yearly ultrasounds are recommended to monitor for cardiac changes. For patients with large defects, treatment is recommended to minimize secondary consequences. Surgical repair of the defect has been postulated, but this is not readily available, and requires cardiac bypass and a high level of surgical expertise.

Placement of an occluder has proven to be successful at closing defects in dogs. Pulmonary artery banding has also been documented as a treatment option to increase pressure in the right ventricle and minimize shunting from the left ventricle.

Medical therapy to minimize clinical signs of congestive heart failure and pulmonary hypertension have been utilized in end stage treatment of patients with a VSD.


Prognosis is highly dependent on the severity of the defect. In dogs and cats with a small defect, a normal life expectancy has been reported. In cases with large ventricular defects, clinical signs can limit lifespan and quality of life. A study of 109 dogs and cats with a VSD showed that 71% of patients had minimal clinical signs and a long survival time.

Breed Predilection

There are dog breeds that have shown increased predisposition for development of a VSD. English Springer Spaniels have been known to have a heritable trait for this defect.

Other breeds with increased predisposition include: