alternans

:  alternation in the pattern of activity (such as that produced by the heartbeat) Her electrocardiogram showed small spikes that alternated in shape—a classic sign of cardiac tamponade called electrical alternans.—Tony Dajer, Discover, March 2002 The noninvasive detection of an increased risk of ventricular fibrillation may be made possible by the identification of T-wave alternans, a beat-to-beat fluctuation in the area under the T wave that is associated with an increased risk of lethal arrhythmia.—James E. Muller and Richard L. Verrier, The New England Journal of Medicine, 15 Feb. 1996—see pulsus alternans