An atrial septal defect is a defect or hole in the muscular wall (the septum) that separates the right and left atria or chambers of the heart.
As with most heart defects the first indication of a problem is when your veterinarian hears a heart murmur during 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.
During gestation, the heart starts out as a single tube which differentiates into four 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.
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.
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