CONDITION
Atrial Septal Defect
An atrial septal defect is an opening in the wall that normally separates the two upper chambers of the heart. In a healthy heart, this wall keeps oxygen-rich blood on the left side separate from oxygen-poor blood returning from the body on the right. When an opening exists, blood can flow between these chambers in ways it ordinarily would not, and over time this can alter how the heart and lungs work. Many dogs and cats with small openings show no outward signs, and the condition may be discovered during a routine examination when a heart murmur is heard. Others may develop reduced stamina, breathe more rapidly than usual during rest, or show a reluctance to exercise as they grow. The pattern depends largely on the size of the opening and how much blood is crossing between the chambers. This page explores the signs that may bring an atrial septal defect to attention, the mechanisms by which an opening affects circulation and heart structure, the investigations used to confirm and characterise it, and the range of approaches that exist for managing it across a dog or cat's life.
Why this matters now
Atrial septal defects are present from birth, arising during the development of the heart in the womb. Certain breeds carry a higher likelihood of this malformation, including Boxers, Dobermanns, Samoyeds, and Old English Sheepdogs in dogs, and in cats it appears less commonly but has been documented across many breeds. The opening is fixed in size once the animal is born, so the degree of impact is determined early, though the consequences may not become apparent until growth places greater demands on the circulation.
In animals with small defects, the heart may compensate effectively for years, and many live without symptoms into middle age or beyond. Larger openings tend to reveal themselves earlier, often within the first few years of life, as the volume of blood crossing between chambers begins to overwork the right side of the heart and increase pressure in the vessels supplying the lungs. Over months to years, this can lead to enlargement of the right atrium and ventricle, and in some cases the pressure changes become significant enough to reverse the direction of blood flow across the defect.
Signals & patterns
Early signals
Heart murmur on examination
A veterinary surgeon may hear an abnormal sound during auscultation, even when the animal appears entirely well. This occurs because turbulent blood flow through the defect or changes in flow patterns within the heart chambers create sounds that differ from the normal rhythmic closures of the valves.
Reduced stamina during play
An owner may notice that their dog or cat tires more quickly than littermates or peers, stopping to rest during games or walks that others continue without difficulty. This can reflect the heart's reduced efficiency in delivering oxygen to muscles when blood is recirculating between the upper chambers rather than moving forward to the body.
Faster breathing at rest
The respiratory rate may be persistently higher than expected when the animal is calm or sleeping. This often signals that the lungs are receiving more blood flow than usual, and the body is compensating by moving air more frequently to maintain adequate oxygen exchange.
Slower weight gain in young animals
Puppies or kittens with moderate to large defects may grow more slowly than their siblings, as the energy cost of maintaining circulation rises and less is available for building body mass. This pattern becomes clearer when compared against expected growth curves for the breed.
Later signals
Reluctance to exercise or move
As the heart's ability to meet demand declines, animals may avoid activities they once enjoyed, preferring to lie down or stopping frequently during walks. This reflects both physical fatigue and the body's instinct to limit exertion when circulation is strained.
Coughing or laboured breathing
Fluid may begin to accumulate in or around the lungs when pressure in the pulmonary vessels rises significantly, leading to a wet-sounding cough or visible effort to draw breath, particularly after activity or when lying flat.
Pale or blue-tinged gums
If blood flow reverses through the defect so that oxygen-poor blood begins to mix into the left side of the heart, the mucous membranes may take on a dusky or greyish-blue colour. This occurs when the lungs can no longer compensate for the altered circulation.
Click to read about the biological mechanisms
How this is usually investigated
Options & trade-offs
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