In the developing embryo, certain organs are not required to assume their normal function. That's why some blood vessels in the embryo serve to bypass these organs entirely. The lungs are among the organs not necessary to the embryo and are circumvented by a vessel called the "ductus arteriosus," which shunts blood from the pulmonary artery (which normally supplies blood to the lungs) to the aorta (which supplies the entire body).
In the neonate, this structure is expected to completely close within the first week of life so that the blood can move directly to the lungs and receive the oxygen the body needs to carry on normal functions. When this vessel remains open, the resulting disease is called "patent ductus arteriosus" or "PDA."
In affected animals, the pressure change in the heart after birth results in a reverse flow of blood through the still open vessel (from the aorta to the pulmonary artery), which allows for an increased blood flow to the lungs (though, less commonly, the blood will flow in the opposite direction through the vessel). Too much blood in the lungs means fluid build-up. Too much blood in the left side of the heart means the heart has to work harder. Both problems mean this disease will end in heart failure, usually within the first year or two of life, depending on its severity--that is, if the disease remains untreated.
Symptoms and Identification
In this most common of congenital heart defects in dogs, puppies will usually have a loud, continuous heart murmur detectable on the very first veterinary visit. It's at this point that a PDA will be suspected and the pet referred to a cardiologist, internist and/or veterinary surgeon.
Definitive diagnosis at this point involves an echocardiogram (heart ultrasound) with doppler (to demonstrate the direction of blood flow in the abnormal vessel, EKG (electrocardiogram) and X-rays (which may show an enlarged left side of the heart).
Should a delay in the diagnosis and treatment occur, the heart may begin to fail. In these cases, fluid in the lungs causes coughing, labored breathing, exercise intolerance and sometimes collapse. Left untreated, most affected animals will eventually succumb to heart failure, usually before 1-2 years of age, though some less severe cases (where the vessel is partially or almost completely closed) may do well for long periods of time.
Breeds most at risk for this disease are the Maltese, Pomeranian, Shetland Sheepdog and Kerry Blue Terrier. An increased risk has been demonstrated in the Keeshond, Miniature and Toy Poodle, Bichon Frise, Yorkshire Terrier, English Springer Spaniel, Collie, Cocker Spaniel, German Shepherd, Irish Setter and Chihuahua.
This is a highly treatable heart condition as long as early intervention is undertaken (between 8 to 16 weeks). Surgery is the mainstay of correction and it's considered a fairly simple surgery, despite the need to enter the chest cavity to get it done. Cardiologists and board certified surgeons are both usually well-equipped to handle this procedure. Dogs so treated usually have a normal life expectancy.
Owners of dogs for whom the expense of surgery is too high will find that medical management of the disease through drugs and exercise restriction will usually be insufficient, though this depends on the degree of patency of the abnormal vessel.
Dogs who already have signs of heart failure by the time treatment is undertaken may not fare so well, even with surgery, and will likely require a lifetime of medication to compensate for some irreversible changes to the heart as a result of the condition.
The typical cost for diagnosis and treatment of a PDA when accomplished early on in the disease process runs at about $2,500 to $5,000. Drug management is considered relatively inexpensive due to the short lifespans of these patients ($30-$100 a month).
The mode of inheritance is considered complex. Affected dogs and their parents and siblings should not be bred. This is the only known means of prevention at the current time.
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. 1989. Hereditary congenital heart defects in dogs. Journal of Small Animal Practice 30:153-165. Information on inheritance.
Patterson, D.F. 1996. The genetics of canine congenital heart disease. ACVIM - Proceedings of the 14th Annual Veterinary Medical Forum: 225-226. This reference has good information for breeders and veterinarians regarding screening and genetic counseling for congenital heart defects.