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Summary
In the mammalian embryo, the heart develops as a tube which then separates into the four chambers that comprise the pumping mechanism for the body's blood (the right and left atria and right and left ventricles). With a ventricular septal defect (VSD), the wall (or "septum") between the two ventricles fails to fuse normally, leaving a hole (or "defect") in the heart's inner wall.
Because the blood needs to move in a one-directional fashion, the presence of this defect means that the some of the blood that enters the left side of the heart (after receiving oxygen from the lungs) gets shunted to the right side instead of to the rest of the body, as it's meant to do. Ultimately, this defect means that the left side of the heart has to work harder to pump out the amount of blood the body requires for normal functions...and that by doing so it might tend to break down under the weight of this "overload."
Depending on the size and exact location of the defect, the pressure in the right side of the heart may be so high that the blood moves from the right side of the heart to the left, thus bypassing the lungs. This is called a "reverse shunt" and similarly leads to an overtaxed heart--though its effects now affect the right side of the organ instead of the left.
This disease is a hereditary one that is acquired congenitally by pets, primarily dogs. That is to say that pups are born with the problem. It's one that also varies in its degree of severity depending on the size of the hole between the two ventricles.
Symptoms and Identification
Some puppies will be so insignificantly affected that a small defect in the wall between the ventricles spontaneously closes relatively early in life. These pups will not show any symptoms, though they'll typically have an audible heart murmur until the abnormal opening closes.
Others will suffer severe disease, in which case a loud murmur will be detected by a veterinarian early on in puppyhood, though symptoms may not ensue until later.
The most common symptoms are the result of the left side of the heart's "work overload." This will cause an enlargement of this side of the heart, which can lead (within months or years) to shortness of breath, exercise intolerance, sudden death (the result of an abnormal heart rhythm), or, in the case of a less common reverse shunt, cyanosis (grayish mucous membranes).
Diagnosis of VSD is achieved through X-rays to demonstrate the enlargement of the left side of the heart (or the right with a reverse shunt), EKG (electrocardiogram) to pick up any possible electrical changes as a result of the enlargement, and, most crucially, an echocardiogram (heart ultrasound) with doppler to identify the defect in the ventricular wall and the abnormal flow of blood there.
Affected Breeds
Unfortunately, VSD affects many breeds sporadically and its method of inheritance has not been determined. English Bulldogs and Keeshonds are an exception. The greater predisposition of these breeds has led to research demonstrating that the mode of inheritance is autosomal recessive with variable expression.
Treatment
Treating VSD is typically accomplished through medical means, meaning that drugs are the primary approach for those dogs who show symptoms (remember, some may not). For more severe cases, however, medical therapy may be insufficient.
In these instances, a surgical technique called "pulmonary artery banding" may be somewhat effective. In this approach, reducing the diameter of the pulmonary artery can reduce the pressure differential in the heart, thereby reversing some or most of the abnormal blood flow.
Surgery to repair the most severe defects has been undertaken in some cases, albeit very expensively, to the tune of a very high risk and only in university settings where the equipment for "open heart" techniques is available.
Exercise restriction, special (low sodium) diets and weight management are commonly recommended for all symptomatic VSD patients.
Veterinary Cost
Medical management of VSD through cardiac drugs and diuretics is considered minimal compared to the cost of open heart surgery, for example. Nonetheless, a lifetime of drugs can prove an onerous financial burden for larger dogs or those who require special formulations of drugs.
Surgery is the most expensive approach. Pulmonary artery banding can be had non-invasively now in some referral facilities, though normally for not less than $3,000-$4,000 and, depending on the surgeon's experience, the facility and the geographic location, for as much as $5,000-$10,000. Open heart techniques may surpass even these large figures.
Prevention
Preventing VSD requires removing affected dogs as well as their littermates and parents from the breeding pool. Otherwise, there is no known preventative approach. This is especially critical for breeds like the English bulldog and keeshond in which the disease's genetic basis has been well established.
References
Bonagura, J.D. and Darke, P.G.G. 1995. Congenital heart disease. In S.J. Ettinger and E.C. Feldman (eds.)Textbook of Veterinary Internal Medicine, p. 892-943. W.B. Saunders, Toronto.
Patterson, D.F. 1996. The genetics of canine congenital heart disease. ACVIM-Proceedings of the 14th Annual Veterinary Medical Forum: 225-226.