TACHYCARDIA

Supraventricular tachycardia (SVT) is a heart condition featuring episodes of an abnormally fast heart rate.

The heart will suddenly start racing, then stop or slow down abruptly.

Episodes can last for seconds, minutes, hours or (in rare cases) days. They may occur regularly, several times a day, or very infrequently, once or twice a year.

The heart rate may be as high as 250 beats per minute, but is usually between 140 and 180 (a normal heartbeat should be 60-100 beats per minute at rest).
What happens

When the heart beats normally, its muscular walls contract (tighten and squeeze) to force blood out and around the body. They then relax, so the heart can fill with blood again (see How the heart works). This process is repeated for every heartbeat.

In SVT, the heart muscle is contracting so fast that it cannot relax between contractions. This reduces the amount of blood being pumped around the body, which can make you feel dizzy and short of breath (see Supraventricular tachycardia - symptoms).

You usually feel heart palpitations (noticeable heartbeats) and a fast pulse.
Why it happens

SVT is caused by abnormal electrical impulses that start suddenly in the upper chambers of your heart (the atria). These impulses override your heart's natural rhythm (see Supraventricular tachycardia - causes).

It is often a short circuit in the electrical system of your heart that causes these spontaneous impulses. The short circuit causes an electrical signal to travel continuously around in a circle. Each time the signal completes the circuit, the impulse spreads out to the rest of your heart, forcing it to beat rapidly.

SVT attacks often happen for no obvious reason. However, they may be triggered by a change in posture, exertion, emotional upset, coffee or alcohol.
Who is affected

SVT can occur in anyone at any age, but it often occurs for the first time in children or young adults.
Outlook

In the vast majority of cases, attacks of SVT are harmless, do not last long and settle on their own without treatment.

If necessary, SVT can be treated with drugs that correct the abnormal heart rate. It can also be permanently treated with a very effective surgical procedure called catheter ablation, which interrupts the abnormal electrical circuits (see Supraventricular tachycardia - treatment).