Atrial septal defects (ASD) in cats

© Rand, J (2006) Problem-based feline medicine. Elsevier Saunders. Toronto

Classically, ASD shows as a soft systolic murmur heard over the left heart base and may radiate to the apex, with associated signs of dyspnoea and collapse. Left to right shunting occurs through the atrial septal defect. In the absence of significant mitral regurgitation, the shunt fraction is small and animals may not have any symptoms. With increased amounts of mitral regurgitation, the shunt fraction increases and if significant, volume overload of the right and left heart is present.

Mitral regurgitation may occur secondary to left heart volume overload or congenital abnormalities of the formation of the valve. Kittens with severe disease develop left-sided congestive heart failure by 6 weeks of age.

Clinical signs

Signs depend on the degree of shunt fraction. Most cats are asymptomatic. At the left heart base, a soft systolic ejection murmur is heard. It is often also heard on the right side of the chest. Signs of increased respiratory rate, dyspnoea or collapse may be seen while playing. Stunted growth is seen in severe cases. Some cases present with ascites, suggesting right-side heart failure.

Diagnosis

Congenital ASD defects present as a murmur found over the left heart base. The murmur is systolic and usually soft. On radiographic analysis, pulmonary over-circulation is demonstrated by a prominent and enlarged vasculature of the lungs. Areas of focal alveolar densities representing pulmonary oedema may be found. There is generalised cardiomegaly.

ECG studies reveal a sinus tachycardia (HR> 220 bpm) if heart failure is present. Evidence of left ventricular hypertrophy with a tall R wave in lead H (> 0.9 mV). In cases where the defect may interrupt or deviate the His bundle, notching and prolongation of the QRS may be seen.

On echocardiography, an echo drop out at the regions of the interatrial septum is present. It is important to note that due to the thickness of the septum in a normal heart, many times there is the illusion of an echo drop out. Multiple views must be obtained to reach a diagnosis.

Doppler confirmation of left-to-right shunting is documented across the atrial septal defect. Doppler studies may show variable degrees of mitral regurgitation. Relative pulmonic stenosis (blood flow velocity > 2.0 m/s) secondary to increased volume of blood may be seen with significant shunt fractions. In severe cases, biatrial and bi-ventricular enlargement can be seen.

Differential diagnosis

Cardiomyopathy is uncommon in young cats, the murmurs are not as loud and may sound different with serial auscultations.

Ventricular septal defects usually have a louder murmur on the right sternal border and the character is harsher.

Mitral valve dysplasia occurs with a murmur louder at the apex, in some cases a diastolic murmur may be present if there is significant mitral stenosis.

Pulmonic or aortic stenosis present with louder and harsher murmurs than atrial septal defects.

Treatment

Many cases are asymptomatic and there is no need for intervention therapy. Definitive therapy requires surgical repair, which is very difficult to accomplish. If the patient is in congestive heart failure, relevant therapy is warranted.