Atrial Septal Defects (ASDs) are a result of failure of closure of the atrial septum during fetal development resulting in a communication between the left and right atria. The hemodynamic consequences associated with ASDs vary upon the size of the defect. Small ASDs may go undetected and have no clinical significance. Moderate to large defects usually result in left to right shunting of blood during diastole between the atria. This shunting occurs during diastole due to the differences in compliance between the left and right ventricles rather than pressure differences as atria are generally low pressured chambers during systole and diastole. This scenario results in volume overload of the left ventricle with eccentric hypertrophy. Soft left basilar systolic heart murmurs and split second heart sounds may be heard in animals with a large left-to-right shunting ASD.
The murmur is heard as a result of turbulent flow across the pulmonic valve as the right ventricular outflow tract is now dealing with three times the normal amount of blood. A split S2 can be heard as there is delay for pulmonic valve closure after aortic valve closure because the right heart is dealing with this tremendous increase in blood flow when compared to the left heart.
Clinical signs of a small ASD are often not present as these defects rarely have hemodynamic consequences. Patients with moderate to large left-to-right shunting ASDs may show signs of congestive right-sided heart failure secondary to right sided volume overload. Signs of right-sided heart failure include ascites, abdominal distension, difficulty breathing, inappetance, lethargy or exercise intolerance. Syncope can also occur with moderate to large ASDs.
Treatment for moderate to large ASDs includes management of right-sided heart failure with diuretics and angiotensin converting enzyme inhibitors. Other medications may be needed with disease progression.
Animals with moderate to large ASDs should be evaluated with thoracic radiographs, echocardiogram and ECG every 6-12 months if stable.