..is characterized by attacks of abnormally high (2 - 4 or more times normal speed) heart rate, which can last for hours or even days. It's caused by spontaneous impulses in the upper chamber of the heart that play havoc with your body's "natural pacemaker". It can cause chest pain and heart palpitations, and fainting spells and shortness of breath. Attacks can be stopped with drugs or by straining (as when constipated) with mouth and nose blocked, though this is dangerous and can cause a stroke, but sometimes the only effective method is electric cardioversion. (done with the defibrillator (the thing with paddles) you always see on TV)