What is an atrial septal defect (ASD)?
As the name implies, an atrial septal defect is a defect or hole in the muscular wall (the septum) that separates the right and left atria- 2 of the 4 chambers of the heart.
Before birth, the heart starts out as a single tube which differentiates into 4 chambers during embryological development. Abnormalities can arise at several steps in the process, resulting in a hole (defect) in the final septa that normally separate the heart into right and left atria and ventricles. This causes abnormal blood flow in the heart with varying effects on the dog, depending on the size and location of the defect.
The mode of inheritance is not known for ASDs, but many cardiac defects are believed to have a polygenic mode of inheritance, with variable expression. However recent studies suggest a series of single major gene defects may be responsible for inherited heart defects in both dogs and people.
What breeds are affected by ASDs?
Atrial septal defects are fairly uncommon in the dog. The boxer, Doberman pinscher, samoyed, and old English sheepdog are considered to have an increased risk for ASD.
For many breeds and many disorders, the studies to determine the mode of inheritance or the frequency in the breed have not been carried out, or are inconclusive. We have listed breeds for which there is a general consensus among those investigating in this field and among veterinary practitioners, that the condition is significant in this breed.
What does an atrial septal defect mean to your dog & you?
A small defect will be of no significance to your dog. Dogs with no or mild clinical signs generally have a normal lifespan. Your dog may be more susceptible to respiratory tract infections.
With larger defects however, there will be abnormal blood flow from the higher pressure left side of the heart across the defect to the right side. This causes more work for the right side of the heart , which can eventually lead to right-sided heart failure. Signs may include respiratory difficulties, fainting, tiring with exercise, abnormal cardiac rhythms, abdominal swelling, or sudden death.
How is an atrial septal defect diagnosed?
Often, as with most heart defects, the first indication of a problem is when your veterinarian hears a heart murmur on your pup's first physical examination. There may be exercise intolerance or respiratory difficulties. This usually occurs in an older dog or a young pup with a large defect where congestive heart failure has already developed.
Some low-grade murmurs are "innocent" and disappear by 6 months of age, but if the murmur is significant, your veterinarian will suggest a diagnostic workup to determine the cause. He or she will listen very carefully to your dog's heart to determine the point of maximal intensity of the murmur and when the murmur occurs during the cardiac cycle. Other diagnostic aids include chest x-rays and an electrocardiogram (ECG), which typically show enlargement of the right side of the heart. Further tests such as an ultrasound can be performed to determine the size of the defect.
FOR THE VETERINARIAN:
- MURMUR: soft, mid-systolic ejection murmur; loudest in pulmonic area - left cranial thorax; audible splitting of the second heart sound.
- ELECTROCARDIOGRAM: may be normal , or show right ventricular and possibly right atrial enlargement; partial or complete right bundle branch block is common.
- RADIOGRAPHS: may be normal with small shunts; with larger shunts you will commonly see left and right atrial enlargement, right ventricular hypertrophy, and pulmonary vasculature enlargement.
- Echocardiography (best) and angiocardiography can be used to evaluate the magnitude of the shunt.
How is an atrial septal defect treated?
Many dogs with ASDs do very well. Signs associated with heart disease are treated when and if they develop. Treatment includes medications to support the heart and to reduce congestion in the lungs, a sodium-restricted diet, and exercise restriction.
Surgical closure of the defect is possible in dogs with severe signs, but is not often considered due to the risks and expense associated with open heart surgery.
Affected individuals and their parents should not be used for breeding. Siblings should only be used after careful screening.
FOR MORE INFORMATION ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.
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 counselling for congenital heart defects.
Copyright © 1998 Canine Inherited Disorders Database. All rights reserved.
Revised: August 18, 2003.This database is funded jointly by the Animal Welfare Unit at the Atlantic Veterinary College, University of Prince Edward Island, and the Canadian Veterinary Medical Association.