When the heart suddenly starts racing

Paroxysmal supraventricular tachycardia is caused by an electrical glitch in the heart’s upper chamber that causes the heart rate to suddenly soar for no apparent reason—sometimes as high as 200 beats per minute. The condition, which people describe as palpitations or a fluttering sensation in the chest, is usually not dangerous. But long-lasting bouts can lead to lightheadedness, breathlessness, and fainting.