SamDawkins Consultant Cardiologist

Atrial Septal Defect (ASD)

An atrial septal defect (ASD) is a hole in the wall (septum) between the two upper chambers of the heart, the left and right atria. Unlike a patent foramen ovale (PFO) — which is a small flap that failed to seal at birth — an ASD is a true gap in the septum. ASDs are usually larger than PFOs, and the long-term consequences are different: they cause sustained left-to-right shunting that gradually enlarges the right side of the heart.

What types of ASD are there?

There are four main types, defined by their location in the septum:

  • Ostium secundum ASD — by far the most common (around 70-80% of all ASDs). Sits in the middle of the septum and is usually amenable to transcatheter closure.
  • Ostium primum ASD — lower in the septum, often associated with abnormalities of the mitral valve. Typically requires open-heart surgery rather than transcatheter closure.
  • Sinus venosus ASD — high in the septum, often associated with anomalous pulmonary venous drainage. Usually requires surgery.
  • Coronary sinus ASD — rare, involves the coronary sinus.

The type, size and location of the defect determine which treatment is appropriate.

What are the symptoms?

Many ASDs cause no symptoms in childhood and are picked up only in adulthood — sometimes during the workup for unrelated chest symptoms, sometimes as an incidental finding on echocardiography. As the right heart enlarges, patients may develop:

  • Breathlessness on exertion
  • Reduced exercise tolerance
  • Fatigue
  • Palpitations, often from atrial fibrillation or atrial flutter (more common with longstanding ASDs in older patients)
  • Recurrent chest infections (uncommon)
  • Stroke from paradoxical embolism (less common than with PFO)
  • Right-sided heart failure if the defect is left untreated for many years

A heart murmur is often present on examination, sometimes with a fixed split second heart sound — the classic clinical sign of an ASD.

How is an ASD diagnosed?

The diagnosis is usually made on echocardiography, supported by additional imaging where intervention is being considered.

  • Transthoracic echocardiogram (TTE) — typically the first test. May identify an enlarged right heart suggestive of an ASD, sometimes visualising the defect directly.
  • Bubble study — a TTE performed while saline (mixed with a few air bubbles) is injected into a vein. Bubbles crossing from right to left confirm a shunt.
  • Transoesophageal echocardiogram (TOE) — the definitive test for ASD anatomy. Sizes the defect, identifies the type, and confirms suitability for transcatheter closure (looking for adequate “rims” of tissue around the defect for the device to anchor against).
  • Cardiac MRI — used selectively to quantify the shunt (Qp:Qs ratio) and assess right ventricular size and function.
  • Right heart catheterisation — occasionally used to measure pulmonary artery pressures, particularly in older patients or where pulmonary hypertension is suspected.
  • Electrocardiogram (ECG) and ambulatory monitoring — to identify any associated atrial arrhythmia.

When does an ASD need to be closed?

Most ASDs benefit from closure once identified, particularly when the defect is significant (i.e. there is a measurable left-to-right shunt and right heart enlargement). Closing the ASD:

  • Reverses right heart enlargement — usually within months to a year.
  • Reduces the long-term risk of atrial fibrillation, right heart failure, and pulmonary hypertension.
  • Reduces the risk of paradoxical embolism (and therefore stroke), where present.
  • Improves exercise tolerance in symptomatic patients.

Some small ASDs without measurable shunting can be observed rather than closed. The decision is always made in a structural multidisciplinary team meeting.

How is an ASD closed?

  • Transcatheter closure — the standard treatment for most ostium secundum ASDs. A device (an Amplatzer or similar septal occluder) is delivered through the femoral vein under x-ray and TOE guidance, and deployed across the defect. It is performed as a day-case or overnight procedure, without open-heart surgery. See PFO and ASD Closure for the full procedure description.
  • Open-heart surgery — required for ostium primum, sinus venosus, and coronary sinus ASDs, and for very large secundum defects without adequate rims for a device to anchor against.

What experience does Dr Dawkins have in the management of ASDs?

Dr Dawkins runs the structural intervention programme in Oxford and is responsible for the transcatheter closure of ASDs (and PFOs) in the region. He chairs the regular structural multidisciplinary meeting where every ASD referral is reviewed, with imaging discussed jointly by interventional cardiologists, congenital cardiologists, surgeons and imaging specialists.