Atrial Septal Defect                                                                               

Anatomy & Physiology

The defect itself may lie within the oval fossa or outside it. If outside the it is termed a sinus venous defect. Those near the origin of the SVC are often associated with anomalous drainage of the right upper pulmonary vein. More rarely they may be near the the inferior vena cava. Occasionally the defect is near the coronary sinus.

The left atrial  pressure is slightly higher than the right so the flow is from left to right but is of low velocity and hence does not generate a murmur.

Depending upon the size of the defect and the resistance to flow the blood flow through the right heart and into the lungs is increased. This decreases lung compliance and may lead to breathlessness, particularly on exertion.

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Presentation

It is the second commonest congenital cardiac defect (after VSD). It usually presents as an asymptomatic murmur in childhood although uncommonly it may present earlier with failure to thrive, breathlessness and recurrent infections. A significant number present in adult life.

On examination there is an ejection systolic murmur in the left upper sternal border due to the increased flow across the pulmonary valve. In those with a significant shunt a tricuspid diastolic flow murmur may be heard.

The second sound is widely split as the L to R shunt at atrial level increases RV filling and thus RV ejection time is increased and pulmonary closure is delayed. The split is fixed as tendency to increase RA filling in inspiration from the caval veins is offset by reduced L to R flow across defect. The converse holds for expiration and hence the split does not vary with respiration. This sign is difficult to elicit in children.

Natural History

There is a high spontaneous closure rate in infancy. Those with persistent, significant defects will develop cardiac failure and arrhythmias in middle life. Campbell et al. British Heart Journal 1970;32:820

Investigations

Electrocardiogram

There are usually signs of mild right ventricular hypertrophy in those with a haemodynamically significant lesion. Partial RBBB lead V₁ is a sensitive but not very specific sign.
 

Chest X-ray

There is often cardiomegaly associated with pulmonary plethora due to the increased pulmonary flow. The central pulmonary artery is usually prominent.

Echocardiogram

The septal defect is usually seen best from the subcostal window. The associated signs of right ventricle volume over load and increased flow across pulmonary valve are also identified. Colour flow Doppler is helpful.

Management

Traditionally significant ASDs have been closed surgically. This was the first operation undertaken on cardiopulmonary bypass by Gibbon in 1953. Whilst very successful there is still a significant morbidity from cardiopulmonary bypass although the mortality is very low.

There has thus been the stimulus for developing a transcatheter approach and a variety of devices are now available. Most have a similar design with a double disc made of nitinol metal which can be squashed into any shape and reforms into original shape on release. The devices can close defects from a few millimeters (PFO) up to 3 cm or so. Considerations for transcatheter closure are the size of the child relative to the defect, its position in the septum and the amount of rim around the defect available for the device to attach to. As a rough rule of thumb the child's weight (kg) should equal the defect size in millimeters.

The technique is usually undertaken under general anaesthetic with transesophageal and fluoroscopic guidance. A sheath (large catheter) is advanced from the femoral vein to the left atrium. The device is attached to a wire by a screw thread, collapsed down and inserted into the sheath. It is advanced through the sheath by pushing the wire. As it emerges from the sheath the first disc springs open. The sheath and device are then pulled back so the device is fitting snugly against the left atrial wall. The sheath is then withdrawn, the right atrial disc springs out and the device clamps onto the atrial septum sealing the defect. The wire is then unscrewed from the device and withdrawn along with the sheath.

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Prognosis

The current operative mortality is less than 1%. The most common significant complication is pericardial effusion which may be life threatening.

The long term results are excellent - Murphy et al NEJM 1990;323;1645 demonstrating 30-year actuarial survival rate among survivors of 74 percent (compared with 85 percent among controls) and those operated in the latter years had a similar survival to controls. Device closure has been undertaken over a much shorter time frame and long term results are not available but the outcome so far is comparable with surgery.

Adult Considerations

As many children with an ASD are asymptomatic they may escape detection and present in adult life with fatigue or dyspnoea on exertion or with arrhythmias, right heart failure, paradoxical embolism or recurrent pulmonary infections.

Whilst results are not as good for surgical closure in adult life (Murphy et al N Engl J Med 1990;323;1645) they are considerably improved on the natural history and unless there is severe irreversible pulmonary vascular disease closure is recommended (Konstantinides S et al N Engl J Med 1995;333:469-473).

Patent Foramen Ovale

These are small (< 5 mm) and as they do not cause a significant left to right shunt there is no indication for closure from the haemodynamic point of view.

However they are associated with a slight increase in paradoxical embolism in young people. Some may warrant closure by the transcatheter route if this event occurs.

This page was last edited 16/2/2004

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