Alternans Goes Subcellular

n 1872 Traube 1 first described pulsus alternans, a regular beat-to-beat alternation of the strength of the heartbeat.Since then, cardiologists and physiologists have learned that cardiac alternans can come in many flavors: as mechanical, electrical, or [Ca 2ϩ ] i transient alternans (Ca 2ϩ alternans). 2 They also had to realize that alternans is a life-threatening condition, less so because of impaired cardiac output but because it can lead to ventricular fibrillation (VF) and sudden cardiac death.How exactly electromechanical alternations of the heartbeat can cause VF has long been an open question.A leap forward came recently with an elegant study on electrical (T-wave) alternans in guinea-pig hearts.3 It was shown that during alternans, neighboring regions within the heart started to alternate out-of-phase with each other (discordant alternans).Such discordant electrical alternans, if sufficient in magnitude, led to unidirectional block and reentry, thereby causing VF.