Your GP may suspect you have supraventricular tachycardia (SVT) from a description of your symptoms.
If so, you may be asked to have an electrocardiogram (ECG) or be referred to a cardiologist who specialises in heart rhythm disorders and abnormal heart beats (an electrophysiologist).
An ECG is a test that records the heart's rhythm and electrical activity. It's a painless procedure that's usually carried out in hospital or in your GP surgery, and takes about five minutes to complete.
Small stickers (electrodes) are stuck to your arms, legs and chest and connected via wires to an ECG machine. Every time your heart beats, it produces tiny electrical signals. An ECG machine traces these signals onto paper.
During an episode of SVT, your heart rate will usually be between 140 and 250 beats per minute (bpm), compared to a normal heartbeat of 60-100bpm.
If the test is carried out while you're having an episode of SVT, the ECG will record your abnormal heart rate. This will confirm SVT and rule out other conditions.
However, it may be difficult to capture an episode, so your doctor may ask you to wear a small, portable ECG monitor to record your heart rate, either continuously over 24 or 48 hours, or from when you switch it on at the start of an episode. Some monitors can be worn for a week or longer.
Further tests may be carried out once SVT has been confirmed. These will help to determine the exact location of the problem in heart.
For example, you may be asked to take part in an electrophysiology study carried out under sedation, where soft, flexible wires are passed up a vein in your leg and into your heart. The wires measure the electrical signals in your heart and allow doctors to locate the problem.