If you notice a sudden change in your heartbeat and you have chest pain, see your GP straight away.
Feeling your pulse can give a strong indication of whether you have atrial fibrillation, but a complete diagnosis requires a full medical investigation.
If your GP thinks you have atrial fibrillation after assessing your symptoms, they may ask you to have an electrocardiogram and refer you to a cardiologist (heart specialist).
An electrocardiogram (ECG) is a test that records the rhythm and electrical activity of your heart.
Small stickers, called electrodes, are attached to your arms, legs and chest and connected by wires to an ECG machine.
Every time your heart beats, it produces tiny electrical signals. An ECG machine traces these signals on paper. During atrial fibrillation, your heart rate is irregular and may be over 140 beats a minute.
An ECG is usually carried out in a hospital or GP surgery. It takes about five minutes and is painless.
If you have the test during an attack of atrial fibrillation, the ECG will record your abnormal heart rate and a diagnosis of atrial fibrillation can be confirmed (and other conditions ruled out).
However, it may be difficult to capture an attack, so your GP may ask you to wear a small, portable electrocardiogram recorder that will either trace your heart rate continuously over 24 hours, or when you switch it on (at the start of an attack).
An echocardiogram is an ultrasound scan of the heart. It can help identify any other heart problems and assess the structure and function of the heart and valves.
A chest X-ray will identify any lung problems that may have caused the atrial fibrillation.
Blood tests can also be useful in the diagnosis. They may show anaemia, which may be complicating the situation, problems with kidney function or hyperthyroidism (overactive thyroid gland).