Atrial Septal Defect

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Also known as: ASD - Persistent Foramen Ovale


ASD is very rare in horses and is also rarely seen as a single defect in dogs and cats. Predisposed breeds include Old English Sheepdog, Doberman Pincher, Boxer, Samoyed. 

Atrial septal defects occur when the interatrial septum is not completely formed or the foramen ovale has failed to close. The resulting patency between the atria allows shunting of blood. Normally small atrial septal defects cause blood to travel from the higher pressure conditions in the left side to the lower pressure conditions in the right side. However, larger defects cause pulmonary hypertension with or without pulmonic stenosis causing shunting to occur in either both directions or from right to left. Left to Right shunting can cause blood volume overload of the right atrium, right ventricle, and pulmonary circulation. Right to left shunting can cause blood volume overload of the left atrium. Eisenmenger's physiology is when left to right shunts create pulmonary overcirculation. Pulmonary overcirculation leads to gross changes in the pulmonary vasculature causing an increased pulmonary vascular resistance and the development of pulmonary hypertension. Right ventricular pressures are elevated as a result making shunts reverse when the pressure on the right side of the heart exceeds that of the left. Deoxygenated blood is then able to access the peripheral circulation. Cyanosis is a direct effect of this phenomenon. Atrial septal defects are normally located higher in the septum with dogs and lower in the septum with cats. Cats with low atrial septal defects are likely to have an endocardial cushion defect complex.

Clinical Signs

Often no clinical signs are detected. There may be signs of exercise intolerance, dyspnea, syncope, cyanosis (right to left shunting). Signs of heart failure may be present such as ascites (right sided heart failure), dyspnea, peripheral oedema and pleural effusion.


History and clinical signs are indicative of heart failure.

Upon physical examination a soft systolic murmur around left heart base (due to pulmonic stenosis) may be heard as well as a soft systolic murmur around pulmonic and tricuspid valves (usually heard due to increased blood flow) and a soft diastolic murmur on right side (tricuspid stenosis; RARE).

On radiographic examination, often there will be no findings. Occasionally you may see right atrial and ventricular enlargement. There may or may not be pulmonary artery enlargement and pulmonary overcirculation.

Echocardiographic findings may show right atrial and ventricular dilation and a septal defect. Doppler exam will document abnormal flow through defect.

Electrocardiograph (ECG) will show a normal result or pattern of right ventricular enlargement.


Usually treatment is not necessary with small atrial septal defects. You can treat right sided congestive heart failure when present. Severe atrial septal defects may need pulmonary artery banding (decreases pulmonary blood flow and therefore volume overload), but this procedure is often only palliative.


Depends on the severity of the atrial septal defect and any other additional defects.

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