Patent Foramen Ovale (PFO)
A patent foramen ovale (PFO) and an atrial septal defect (ASD) are two different types of hole in the heart, both connecting the left and right upper chambers (atria). They have different causes.
Everyone has a foramen ovale in the womb, which allows blood to bypass the lungs while oxygen is delivered from the mother via the placenta. When a baby is born and starts breathing, the foramen ovale (which is more of a flap than a hole) snaps shut. In most people it then heals completely. In around 25% of the population the flap remains open — this is a patent foramen ovale (PFO).
An atrial septal defect (ASD) is a true gap in the wall (septum) between the two upper chambers. ASDs are generally larger than PFOs and have different implications.
How are PFOs and ASDs diagnosed?
Both can be undiagnosed for many years, often coming to light because of another problem such as palpitations or stroke. The investigations:
- Transthoracic echocardiogram (TTE) — the first-line test. May identify an enlarged right heart suggesting an ASD, and forms the basis for the bubble study.
- Bubble study — a TTE performed while saline (mixed with a few air bubbles) is injected into a vein. If bubbles are seen crossing from the right side of the heart to the left, that proves there is a hole. This is the standard way of diagnosing a PFO.
- Transoesophageal echocardiogram (TOE) — gives detailed images of the inter-atrial septum. The definitive test where intervention is being considered, particularly to size and characterise an ASD.
- Electrocardiogram (ECG) and ambulatory monitoring — to exclude atrial fibrillation, which can cause stroke independently of a PFO and may change the treatment.
- Cardiac MRI — occasionally used to quantify shunt size and assess right ventricular impact in larger ASDs.
- Brain MRI — performed in patients who have had a stroke, to characterise the brain changes.
What does stroke have to do with PFOs and ASDs?
Strokes in older patients are usually caused by cerebrovascular disease — narrowings in the blood vessels supplying the brain caused by, for example, high blood pressure, high cholesterol and diabetes.
Strokes in younger patients are much rarer and tend to have a different cause. One mechanism is paradoxical embolism: we all make tiny blood clots in our veins day to day, which are normally carried back to the lungs and reabsorbed. If there is a hole between the right and left upper chambers, those clots can cross over and travel to the brain, causing a stroke (or, more rarely, to the heart causing a heart attack, or to the gut). This is more likely if pressure on the right side of the heart is briefly higher than the left — for example during heavy lifting or straining, especially when dehydrated.
This is why young patients who have had a stroke generally have a number of additional tests to rule out unusual causes — including blood tests for clotting abnormalities and a bubble study to look for a hole in the heart.
When do they need to be treated?
ASDs usually do need to be closed. The hole increases blood flow on the right side of the heart, which over time can cause the right heart to enlarge, leading to rhythm disturbances and breathlessness. Some patients have an ASD identified at the time of stroke.
PFOs are different. They are very common and in most patients require no treatment. In specific situations — most often after a stroke in a younger patient, where alternative causes have been excluded — closure is recommended.
What experience does Dr Dawkins have in the management of PFOs and ASDs?
Dr Dawkins runs the structural intervention programme in Oxford and is responsible for closing all of the PFOs and ASDs in the region. He runs the regular PFO multidisciplinary meeting where every new referral is discussed, and the brain and cardiac imaging is reviewed jointly by a cardiologist and a neurologist.