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How common is atrial fibrillation and what causes it?

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About 1 in 20 people over the age of 65 have AF. It becomes more common with increasing age. It is uncommon in younger people unless you have certain heart conditions. Causes of AF include the following:

High blood pressure is the most common cause. (High blood pressure puts a strain on the heart muscle.)
AF is a common complication of various heart conditions. For example, AF is a complication of ischaemic heart disease. This is the condition that causes angina and heart attacks and is common in older people. Various other heart problems may also trigger AF to develop. For example, AF occurs in some people with heart valve problems, cardiomyopathy, and pericardial disease.
Other conditions and situations that may trigger AF to develop include: an overactive thyroid gland (hyperthyroidism); pneumonia; pulmonary embolus; drinking a lot of alcohol; drinking a lot of caffeine (tea, coffee, etc).
In about 1 in 6 cases of AF there is no apparent cause. The heart is otherwise fine and there are no other diseases to account for it. This is called 'lone AF'.

What are the symptoms of atrial fibrillation?

Symptoms often develop quickly soon after the AF develops. Possible symptoms include:

Palpitations. This means that you become aware of your heart. You may feel it beating in a fast and irregular way.
Dizziness.
Angina (chest pains) may develop. In particular, the pains tend to occur when you exert yourself, but they may occur even when you are resting.
Breathlessness is often the first symptom that develops. It may occur all the time, but you may become breathless just when you exert yourself such as when you walk up stairs.

The reason why breathlessness, dizziness, and angina may develop is because when the heart beats too fast, it becomes less efficient. Small amounts of blood pumped faster by the heart are not as good as larger amounts that are pumped at the slower normal rate. This can lead to a pooling of blood in the veins of the lungs, and a reduced output of blood from the heart which can lead to these symptoms.

Some people with AF have no symptoms, particularly if their heart rate is not very fast. The AF may then be diagnosed by chance when a doctor feels your pulse.