Information for Patients and Caregivers

What is atrial fibrillation – and is it serious?

Atrial fibrillation (AF) is an irregular heartbeat. It is caused by a failure of the heart’s natural pacemaker that is responsible for orchestrating the coordinated contractions of the four chambers of the heart. Under normal circumstances the pacemaker first makes the two chambers at the top of the heart (the atria) contract, squeezing blood into the two lower chambers (the ventricles). These in turn are triggered to contract, squeezing blood out into the arteries.

In atrial fibrillation, a malfunction of the pacemaker interrupts this coordinated sequence of contractions causing the atria to twitch erratically – known as fibrillation – and the ventricles to contract more often and irregularly. Not surprisingly the heart works less effectively as a pump in atrial fibrillation, with the result that there are areas within the heart where blood flow is slow or stagnant. Where this happens blood clots can form and it is these that account for the increased risk of stroke associated with atrial fibrillation.

Why atrial fibrillation increases your stroke risk

The problem with a blood clot in the heart is that some or all of it can break away and travel along major blood vessels directly into the brain. As these very large blood vessels branch and reduce in size, at some stage the clot or fragment of clot is going to be too large to progress any further and will block the vessel in which it is travelling. This means that the area of the brain downstream can no longer receive the oxygen and nutrients that the blood normally delivers, causing the nerve cells to stop working and ultimately die.

This is what happens in a stroke – or more specifically in an ischaemic stroke (meaning one caused by an interruption in the blood supply). Because clots forming in the heart may be quite big they can block the larger branch vessels that supply extensive areas of the brain. Consequently, strokes arising from atrial fibrillation can be very serious, which is why it is so important to prevent them.

How to avoid a stroke from atrial fibrillation

As you have seen in the preceding pages, if you have atrial fibrillation you are at an increased risk of stroke due to the formation of blood clots in the heart. So what can be done to reduce this risk?

Clots are formed in a process known as coagulation. This is a complex series of steps that must occur in a specific sequence. Hence, by interrupting one or more of these steps it is possible to reduce the likelihood of a clot forming and thereby decrease the risk of stroke. The drugs that block the coagulation process in this way are known as anticoagulants.

For many years the most commonly used anticoagulants for stroke prevention in patients with atrial fibrillation have been the vitamin K antagonists. Drugs in this class, such as warfarin, are affected by interactions with a number of common foods and with other drugs. As a result, frequent blood tests are needed to find the appropriate dosage – a process known as coagulation monitoring.

Developments have resulted in new oral anticoagulants which can be used as alternatives to vitamin K antagonists. These include a direct thrombin inhibitor and oral, direct Factor Xa inhibitors. The new oral anticoagulants are recommended in preference to VKAs. These new drugs have a low risk for interactions and consequently do not require routine coagulation monitoring.