Acyanotic heart disease - Atrial septal defect
Atrial septal defect
The foramen ovale remains patent in 26% of adults, but the prevalence of atrial septal defects is approximately 30 per 100 000 births. Atrial septal defect is more common in women than in men.
Due to the anatomical valve-like construction, a foramen ovale may be patent but adopt a sealed position keeping both atria separate. Atrial septal defects are abnormal openings between the two atria. The ostium secundum defect is the most common site of an atrial septal defect, occurring at the foramen ovale. The ostium primum defect is situated close to the atrioventricular valves and may be associated with an atrioventricular septal defect (endocardial cushion defect). Least common is the sinus venosus defect that is situated high up in the atria and is associated with abnormal drainage of the pulmonary veins into the right atrium.
An atrial septal defect allows blood to divert from the left to the more compliant right atrium (left to right shunt) with a subsequent increase in pulmonary blood flow. Normally, the pulmonary circulation is able to accommodate the increase in blood flow; however, under chronic conditions, increased pulmonary resistance develops and eventually leads to irreversible pulmonary hypertension. The blood pressure in the pulmonary circulation can be high enough to reverse the flow of blood from the right atrium to the left atrium (Eisenmenger's syndrome).
Scope of disease
Under pathological conditions, blood (and emboli) may flow from the right to the left atrium and enter the systemic circulation. A patent foramen ovale is widely held to be an important cause of cryptogenic stroke, especially in young adults. Atrial septal defects also predispose patients to infective endocarditis.
The patent foramen ovale is currently under investigation as a risk factor for migraine due to ischaemic stroke from paradoxical emboli.
The clinical status and age of presentation depend on the size and position of the atrial septal defect. Small defects will cause no symptoms at all, whereas larger defects can give rise to shortness of breath, palpitations and fatigue. Clinical examination reveals a wide and fixed splitting of the second heart sound and a pulmonary flow murmur (increased flow) in some. The defect itself does not produce a murmur.
An ECG will reveal a partial right bundle branch block with right axis deviation for an ostium secundum defect and left axis deviation for an ostium primum defect.
Chest X-ray may reveal cardiomegaly from right atrial enlargement with prominent pulmonary arteries from increased flow.
Transthoracic echocardiography is sufficient to screen for an atrial septal defect, but transoesophageal imaging provides the best anatomical visualization. Cardiac catheterization to calculate pulmonary-systemic flow ratio is reserved as an investigation prior to intervention.
Asymptomatic patients with small defects do not require intervention and should be kept under regular surveillance.
Operation is advised for patients when pulmonary-systemic flow ratios reach 1.5:1.0, in the absence of complications such as pulmonary hypertension.
Percutaneous devices are currently being evaluated for the closure of anatomically favourable atrial septal defects . The device is introduced via the femoral vein into the right atrium, across the defect and into the left atrium using fluoroscopy. It is then positioned across the defect, straddling the septum and closing it.
The natural history is associated with progressive disease and diminished exercise tolerance
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