The atrial septum is the muscular wall that separates the two upper chambers of the heart (the right atrium and the left atrium). It acts as a partition preventing the mixing of oxygenated blood returning from the lungs on the left side with deoxygenated blood arriving from the body on the right. Maintaining this separation is essential for efficient oxygenation of the blood.
During fetal development, the atrial septum has an opening called the foramen ovale, which allows blood to bypass the lungs (which are not yet functioning in the womb). In most people, this closes shortly after birth when the lungs expand and pressure changes seal the opening. In approximately 25 to 30 percent of the population, the foramen ovale does not fully fuse, creating a patent foramen ovale (PFO). Most PFOs cause no symptoms, but in some people they can allow blood clots to pass from the venous to the arterial circulation, causing a paradoxical embolism and stroke. PFO closure (by catheter-based device or surgically) may be offered to selected patients with cryptogenic stroke.
A true defect in the atrial septum, larger than a PFO, is called an atrial septal defect (ASD). ASDs allow a continuous left-to-right shunt (blood crosses from the higher-pressure left atrium to the right atrium) that, if large, increases the blood flow through the right heart and pulmonary circulation. Over time, untreated large ASDs can cause right heart enlargement, atrial fibrillation and, rarely, pulmonary hypertension. ASD closure is recommended for haemodynamically significant defects.
For survivors of sudden cardiac arrest in whom no obvious cause is found, a PFO or ASD may be investigated as a possible contributing factor, particularly if there are other features suggesting a paradoxical embolism. Transoesophageal echocardiography (TOE) with bubble contrast is the most sensitive imaging technique for detecting atrial septal abnormalities.
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