Ventricular septal defect (VSD)
A ventricular septal defect is a hole ("defect") in the muscular wall inside the heart (the septum) that separates the two main pumping chambers of the heart: the right ventricle and the left ventricle.
Before birth, the heart starts out as a single tube which gradually differentiates into 4 chambers during normal growth of the fetus in the dam (mother). Abnormalities can arise at several steps in the process, resulting in defects in the muscular walls that normally separate the heart into the right and left atria, and the right and left ventricles. The result is abnormal blood flow in the heart with varying effects in a dog, depending on the size and location of the defect.
In the English bulldog and keeshond, inheritance is autosomal recessive, with variable expression. This means that either one of the parents can pass along the gene to its offspring; one normal parent and one parent with the genetic defect that causes VSD is enough to transmit the condition to offspring, usually to varying degrees. That means that a dog with a small, insignificant VSD may produce puppies with a similarly small VSD, a large and severe VSD, or sometimes no VSD at all (a type of carrier state where the problem may then arise in subsequent generations).
The extent to which a dog will be affected by this defect depends on the size and location of the VSD inside the heart. Many dogs with VSDs have small defects that may not cause any problems and are entirely compatible with a normal lifespan.
Interestingly with VSD, the dogs with the smallest, least harmful defects are often the dogs with the loudest -not softest- heart murmurs. This means that having a loud heart murmur turn up during a veterinary exam, when the heart is checked (ausculted) using a stethoscope, is not automatically a serious problem. Rather, a heart murmur should be investigated further to determine whether a VSD is present (see below), and if so, how severe the condition is.
With larger VSDs, the circulation is disrupted in a way that forces the heart to work much harder. In some individuals this can be tolerated without causing symptoms for a long time (months to years) but in others, life-threatening symptoms are possible even at an early age.
Signs or symptoms associated with this disorder may develop within months or years, depending on the size of the defect, and they include shortness of breath and exercise intolerance. Rarely, collapse and cardiac arrest may occur in a dog with a VSD, especially if the defect is large and the dog is physically overactive. A veterinarian can monitor the condition of the VSD and the heart in an affected dog and recommend treatment, or consultation with a veterinary cardiologist, as required. Treatment may include medications to support the heart and to reduce congestion in the lungs, a special diet, and exercise restriction, but unlike in humans, repair of the defect is rarely performed because it would usually require open-heart surgery.
Among puppies with large VSDs, it is probable that many die early, before 8 weeks of age or before they are examined by a veterinarian. For this reason, stillborn puppies or pups that die very young (before weaning) should be autopsied, because the assumption of "fading puppy" or "runt of the litter" as the cause of death often misses the point: the cause of death may have been a severe heart malformation like a large VSD, and knowing whether or not this is the case can be vital for future breeding decisions.
Often, as with most heart defects, the first indication of a problem is when the veterinarian hears a heart murmur during a puppy's routine physical examination. Sometimes there is exercise intolerance or respiratory difficulty, but this is usually in an older dog or a young pup with a large defect. It is important to note that normal healthy puppies without VSDs may sometimes have heart murmurs too, for a few weeks as they are growing up. Therefore, if a veterinarian identifies a heart murmur, the best course of action may be to wait 2-3 weeks and recheck (because "innocent" murmurs that puppies outgrow should be gone after 2-3 weeks, whereas the murmur of a VSD would persist) or simply to have tests to confirm whether a VSD is present.
Diagnostic tests that are very helpful in this regard include thoracic radiographs (chest x-rays) and a cardiac ulstrosund (sonogram of the heart, echocardiogram).
If a VSD is identified, then the degree of severity and impact can be pinpointed, and the long-term implications discussed. A VSD may produce no strain on the heart at all (many people have VSDs and live normal lives with them) whereas other VSDs may become life-shortening, depending on the extent of the defect identified with tests.
Overt symptoms caused by VSDs are treated when and if they develop. Treatments include medications to support the heart and to reduce pulmonary congestion, a special diet if medications are being given, and exercise restriction. None of these forms of treatment is useful (and some may be detrimental) if overt symptoms/signs referable to the VSD have not yet occurred.
Two surgical options exist but are used very seldom. Where a large VSD has been identified, but before right-to-left shunting has developed, pulmonary artery banding can be done to decrease the blood flow across the defect, thereby reducing the circulatory overload on the lungs and the left heart. Another procedure involves actual repair of the defect, via open-heart surgery. This procedure is performed in fewer than 10 veterinary centers worldwide and carries a high risk; a veterinarian can help determine whether an individual dog is a suitable candidate for either of these procedures.
- MURMUR: harsh regurgitant holosystolic murmur, loudest in *right* intercostal spaces 2 to 4; also can often be heard cranially on left thorax. The differential diagnosis for systolic murmurs that are loudest on the right side of the thorax is very limited (VSD, tricuspid regurgitation, subaortic stenosis in some cases).
- ELECTROCARDIOGRAM: normal or suggestive of LV enlargement (with larger shunts; R waves > 2.5 mV in lead II); may suggest right-sided enlargement if right-to-left shunt develops (rare). Not definitive for diagnosis of VSD but may identify some secondary changes that are consistent.
- RADIOGRAPHS: normal or left-sided enlargement; with larger shunts, increased prominence of pulmonary vasculature ("overcirculation"); right-sided enlargement if right-to-left shunt develops.
- ECHOCARDIOGRAM: Diagnostic test of choice for confirming VSD and identifying secondary changes (if any) and concurrent defects (if any).
- OTHER: Clinical signs usually only occur if the pulmonary vascular volume is more than 2 to 3 times normal, which requires a defect of medium to large dimensions. Most commonly, if signs occur, they are associated with a left-to-right shunt and signs of left-sided congestive heart failure are seen. Where secondary obstructive pulmonary vascular disease develops, so that pulmonary resistance exceeds that of the systemic vasculature, a right-to-left shunt develops (Eisenmenger's physiology). This is associated with cyanosis and polycythemia and generally carries a guarded prognosis, although dogs that are quiet housepets may have an acceptable quality of life in a calm household.
Affected pups and their parents (assumed to be carriers) should not be bred. Siblings that appear normal after careful physical examination may be used for breeding with caution. Their offspring should be carefully examined and, if affected, the breeding of the parents should be discontinued.
Oyama MA, Sisson DD, Thomas WP, Bonagura JD. Congenital heart disease. In Ettinger SJ, Feldman EC, eds. Textbook of Veterinary Internal Medicine, 7th ed (St. Louis, MO: Elsevier Saunders, 2010) pp. 1250-1298.
O'Sullivan ML. Ventricular septal defect. In Cote E, ed. Clinical Veterinary Advisor: Dogs and , 2nd ed (St. Louis, MO: Elsevier Mosby, 2011) pp. 1168-1169.
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