The Multiple Mechanisms of Spatially Discordant Alternans in the Heart.

Cardiac alternans, referring to beat-to-beat alternations to the action potential (AP) duration (APD) of cardiomyocytes (Fig. 1), are associated with contractile dysfunction and arrhythmogenesis in multiple disease conditions. In tissue, these cellular phenomena can manifest as spatially concordant alternans (SCA), wherein all regions alternate with the same phase, or spatially discordant alternans (SDA), wherein different regions alternate with opposite (period-2; Fig. 1) or otherwise offset (higher period) phase. Whereas SCA can directly affect cardiac output (mechanical force can alternate strong/weak associated with long/short APD), SDA present also the possibility for transition to potentially fatal arrhythmias (1).