Congenital diseases of the dog and cat (Proceedings)
Congenital cardiac defects are those present at birth, and are most often from a genetic cause. In addition to genetic causes, they can also be from infectious, environmental, pharmaceutical, or a variety of causes. Most congenital murmurs present as a mid to high range murmur ausculted on physical exam, but others can be harder to detect. Evaluating these congenital defects is important, as some can be corrected or treated, giving the patient a longer lifespan.
The prevalence of congenital cardiac disease in dogs is thought to be about 0.68%. In cats, this number is about 0.2%.The most common congenital defect in dogs is patent ductus arteriosus (PDA). PDA is detected by ausculting a loud continuous murmur in the left axilla. This murmur is frequently loud enough to be felt as a palpable thrill. The examiner must listen far forward on the thorax to auscult the murmur, as the location of the murmur can be quite focal. Occasionally, the continuous nature of the murmur is missed and only a systolic mitral murmur is detected until chest radiographs or an echocardiogram is performed. Hyperkinetic, or "water-hammer," pulses are another feature of PDA. A variety of breeds, including Border Collies, Shetland Sheepdogs, German Shepherd Dogs, Chihuahuas, Yorkshire Terriers, etc, are reportedly affected.
PDA is a failure of the ductus arteriosus to close after birth. During fetal circulation, oxygenated blood in the pulmonary artery is shunted to the descending aorta to the body, bypassing the lungs. At birth, several factors work to close the ductus, and separate pulmonary from systemic circulation. If the ductus remains open, or patent, blood continually shunts from the higher pressure systemic circulation to the lower pressure pulmonary circulation. The result is an overload of the pulmonary arteries and veins, and subsequent overload of the left side of the heart. Congestive heart failure and occasionally arrhythmias, including atrial fibrillation and ventricular premature contractions, will result if left untreated.
Radiographs and echocardiography are required to evaluate the patient for correction. Typical radiographic features of PDA are a "ductal bump", as well as evidence of left ventricular and left atrial enlargement. Pulmonary overcirculation is often evident on radiographs, as well. Echocardiography generally reveals similar left atrial and ventricular enlargement. The diagnosis of PDA, however, is based on continuous turbulent flow within the pulmonary artery. If electrocardiography is utilized, the most common change is an increase in R-wave amplitude, indicating left ventricular enlargement.
The treatment of PDA involves occluding or ligating the patent ductus. Historically, this has involved open chest surgery to ligate the vessel. Recent advancements in catheter procedures in dogs have allowed veterinary cardiologists to treat PDA more easily and with less peri-operative risk and pain. These include a variety of different devices, including transcatheter embolic coils and nitinol devices. The Amplatz Canine Ductal Occluder, designed by AGA and tested at the University of Minnesota, has become the standard of therapy for PDA. This involves passing a catheter through the femoral artery to the level of the ductus. Angiography allows for visualization of the ductus, and it can be measured to determine appropriate ACDO device size. The device is then delivered at the site of the patent ductus, also through a catheter. ACDO placement has been reported to be more successful at fully occluding the ductus than ligation and coil placement. Patients as small as 3 kg have been treated with the ACDO.