A fast but regular beating of the HEART, occurring for periods that may last several hours or days. In most people with this abnormality the heart rate is between 140 and 180 beats a minute; rarely, the rate may rise as high as 250–300 beats. The condition occurs when abnormal electrical impulses that arise in the upper chambers (atria) of the heart override the normal control centre – the SINOATRIAL NODE – for the heartbeat. Symptoms usually include breathlessness, PALPITATION, pain in the chest and fainting. An ELECTROCARDIOGRAM (ECG) is taken to help make the diagnosis. An acute episode can sometimes be stopped by VALSALVA'S MANOEUVRE or by drinking cold water. Anti-arrhythmic drugs (see ARRHYTHMIA) such as adenosine and verapamil are used intravenously if these simple methods are ineffective. Occasionally, a severe attack may need to be treated with an electric shock to the heart: this is known as DEFIBRILLATION.