A premature contraction of one or more of the chambers of the heart. A beat of the heart occurs sooner than it should do in the ordinary rhythm, and is followed by a longer rest than usual before the next beat. In an extrasystole, the stimulus to contraction arises in a part of the heart other than the usual. Extrasystoles often give rise to an unpleasant sensation as of the heart stumbling over a beat, but their occurrence is not usually serious. (See HEART.)