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 Standard Poodles 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

Investigation typically begins with auscultation during a routine examination, when a heart murmur prompts closer attention to the structure and function of the heart. The pattern of the murmur—its timing, location, and quality—can suggest where an abnormality may lie, though it cannot confirm the presence or size of a septal opening. From there, imaging and sometimes blood work help to characterise the defect, measure its haemodynamic consequences, and assess how the heart chambers and pulmonary circulation have responded over time.

Physical examination and auscultation

Purpose: A murmur heard over the left heart base, often accompanied by a widely split second heart sound, may point toward increased flow through the pulmonary valve. The character of the pulse, respiratory effort at rest, and any signs of reduced stamina during handling can offer clues to the severity of the shunt.
Considerations: A murmur indicates turbulent flow but does not reveal the underlying anatomy or the direction of blood movement. Small defects may produce audible murmurs, whilst very large openings can sometimes be quiet if pressures have equalised.

Echocardiography

Purpose: Ultrasound imaging of the heart allows direct visualisation of the septal defect, measurement of chamber dimensions, and assessment of blood flow direction and velocity across the opening using Doppler techniques. It can also reveal right-sided enlargement and estimate pulmonary artery pressure.
Considerations: Image quality depends on the animal's body shape, coat, and cooperation during the scan. Subtle defects or those located high in the septum can be harder to visualise, and pressure estimates are indirect calculations rather than direct measurements.

Thoracic radiography

Purpose: Chest radiographs show the overall size and shape of the heart silhouette and the appearance of the pulmonary vessels. Enlargement of the right atrium and ventricle, or prominence of the pulmonary arteries, may be visible when the shunt volume is substantial.
Considerations: Radiographs provide a static two-dimensional outline and cannot define the defect itself or measure flow. They are more useful for detecting secondary changes than for diagnosing the septal opening directly.

Electrocardiography

Purpose: An electrocardiogram records the electrical activity of the heart and may show patterns consistent with right atrial or ventricular enlargement, such as tall P waves or a rightward axis shift. It can also identify rhythm disturbances that sometimes accompany chamber remodelling.
Considerations: The ECG reflects electrical rather than structural changes, and many animals with atrial septal defects have entirely normal tracings. It does not confirm the presence of a defect or quantify its size.

Blood oxygen measurement

Purpose: Pulse oximetry or arterial blood gas analysis can detect reduced oxygen saturation in animals with reversed shunts, when deoxygenated blood bypasses the lungs and mixes with the systemic circulation. This finding suggests that pulmonary pressures have risen high enough to alter the direction of flow.
Considerations: Oxygen levels remain normal in most animals with left-to-right shunts, so a normal reading does not rule out a defect. Reduced saturation appears late in the disease process and indicates that compensatory mechanisms have been exhausted.

Options & trade-offs

Management is shaped by the size of the defect, the degree of right-sided heart remodelling, the presence or absence of symptoms, and the likelihood that the shunt direction will reverse over time. Some animals require no intervention beyond periodic monitoring, whilst others benefit from medical support to ease the heart's workload or, in selected cases, from procedures to close the opening. The approach chosen often reflects a combination of what the animal's physiology allows and what the household can sustain over months to years.

Periodic monitoring without treatment

Animals with small defects and no clinical signs may be observed at intervals with repeat echocardiography to track chamber dimensions and pulmonary pressures. This allows early detection of progression whilst avoiding treatment when the heart is compensating well. Monitoring schedules vary, often starting at six to twelve months and adjusting based on stability.

Trade-offs: This approach avoids medication costs and potential side effects, but it depends on access to imaging and the owner's ability to attend follow-up appointments. It does not alter the defect itself and cannot prevent gradual changes in pulmonary pressures if they begin to rise.

Medical management with diuretics and cardiac drugs

Diuretics can reduce fluid accumulation in the lungs when right-sided heart failure begins to develop, easing respiratory effort and improving comfort. Other medications, such as ACE inhibitors or pimobendan, may be used to reduce afterload or support contractility, though their role in isolated atrial septal defects is less well defined than in other forms of heart disease. Doses are adjusted based on response and tolerance.

Trade-offs: Medical therapy can improve quality of life and manage symptoms, but it does not close the defect or reverse structural changes already present. Some animals develop side effects such as increased thirst, electrolyte shifts, or gastrointestinal upset, and ongoing medication requires routine administration and cost.

Surgical or interventional closure

Closure of the defect can be attempted via open-heart surgery with cardiopulmonary bypass, or in some cases through minimally invasive catheter-based techniques that deploy an occluding device across the opening. Success depends on the defect's size, location, and the presence of sufficient tissue rims to anchor a device or suture line. These procedures are typically considered before irreversible pulmonary vascular changes have occurred.

Trade-offs: Closure can eliminate the shunt and prevent progression, but it carries the risks inherent in cardiac surgery or catheterisation, including anaesthesia, bleeding, infection, and device migration. Not all defects are anatomically suitable, and access to facilities and specialists with experience in these techniques is limited. Cost is substantial, and recovery requires close monitoring.

Activity modification and environmental adjustments

For animals showing reduced stamina or mild respiratory signs, adjusting exercise intensity and duration can ease the heart's workload and maintain comfort. Avoiding extreme heat, high humidity, and overexertion allows the animal to set its own pace. Weight management can also reduce circulatory demands.

Trade-offs: Lifestyle changes can improve day-to-day wellbeing without the risks of medication or surgery, but they do not alter the underlying haemodynamics. Their effectiveness depends on the severity of the defect and the owner's ability to observe and respond to subtle shifts in tolerance.

Observation and palliative care in late-stage disease

When Eisenmenger physiology has developed and the shunt has reversed, oxygen delivery is compromised and further intervention carries high risk. In these cases, management focuses on maintaining comfort through gentle oxygen supplementation, minimal stress, and medications to ease breathing or manage arrhythmias as they arise. The goal shifts to quality of life rather than correction.

Trade-offs: Palliative care respects the limits of what the circulation can tolerate and prioritises the animal's immediate comfort. It does not extend lifespan or restore function, and it requires close attention to subtle changes in breathing, behaviour, and colour of the mucous membranes.

Common misconceptions

Misconception:

"A heart murmur always means the animal will become unwell quickly."

Reality:

The presence of a murmur indicates turbulent blood flow but does not predict the timeline or severity of clinical signs. Many dogs and cats with atrial septal defects live for years without symptoms, and the murmur may be the only finding throughout their lives. Progression depends on the size of the opening and how the heart and lungs adapt over time.

Misconception:

"Surgery can fix the defect at any stage of the disease."

Reality:

Closure procedures are most effective when performed before irreversible changes in the pulmonary vessels have occurred. Once pressures in the lungs have risen high enough to reverse the direction of the shunt, closing the defect can worsen outcomes by forcing the right ventricle to work against a resistance it can no longer overcome. Timing and careful assessment of pulmonary pressures are central to deciding whether intervention is appropriate.

Misconception:

"If the animal is not coughing or struggling to breathe, the heart is fine."

Reality:

Atrial septal defects can produce subtle signs that appear long before overt respiratory distress, such as tiring more quickly on walks, sleeping more, or appearing less keen to play. Right-sided heart enlargement and rising pulmonary pressures can progress silently for months or years, and some animals maintain near-normal activity levels until compensation begins to fail. Regular monitoring can detect changes before they become clinically obvious.

Related conditions

Ventricular Septal Defect

Ventricular septal defect involves an opening in the wall separating the lower chambers of the heart, whereas atrial septal defect affects the upper chambers. Both conditions share a similar mechanism—abnormal blood flow between chambers—and may present with comparable signs such as murmurs, exercise intolerance, and the potential for progressive heart changes over time.

Patent Ductus Arteriosus

Patent ductus arteriosus is another congenital heart defect in which abnormal blood flow occurs, in this case through a vessel that should close after birth. Like atrial septal defect, it can cause volume overload on parts of the heart and lungs, and both conditions may be discovered incidentally during examination or when signs of reduced stamina emerge.

Pulmonary Hypertension

Pulmonary hypertension can develop as a consequence of atrial septal defect when prolonged abnormal blood flow through the defect leads to increased pressure in the vessels of the lungs. This elevated pressure places additional strain on the right side of the heart and may influence the pattern of signs and the approach to management.

Congestive Heart Failure in Dogs

Congestive heart failure may develop in dogs with larger atrial septal defects if the volume of blood crossing between chambers over time leads to changes in heart structure and function that compromise the heart's ability to meet the body's needs. The accumulation of fluid in the lungs or abdomen reflects the heart's reduced efficiency, a pattern that can emerge in various underlying heart conditions.

Tricuspid Valve Dysplasia

Tricuspid valve dysplasia is a structural abnormality of the valve on the right side of the heart, and it can sometimes occur alongside atrial septal defect as part of a broader pattern of congenital heart disease. Both conditions affect right-sided heart structures and can contribute to similar clinical signs when the right heart becomes volume-overloaded.

Understanding how an atrial septal defect fits into the broader picture of your dog or cat's cardiovascular health can be helpful when planning long-term care. Other congenital heart conditions occasionally occur alongside septal defects, and the Metabolic Health pillar offers context on how circulation, oxygen delivery, and organ perfusion interact across the body's systems. If your animal has been diagnosed recently, questions about monitoring intervals, what changes to watch for at home, and how activity levels might be adjusted can form a useful conversation at the next appointment.