Mechanisms for discordant alternans

INTRODUCTION: Discordant alternans has the potential to produce larger alternans of the ECG T wave than concordant alternans, but its mechanism is unknown. METHODS AND RESULTS: We demonstrate by one- and two-dimensional simulation of action potential propagation models that discordant alternans can form spontaneously in spatially homogeneous tissue through one of two mechanisms, due to the interaction of conduction velocity and action potential duration restitution at high pacing frequencies or through the dispersion of diastolic interval produced by ectopic foci. In discordant alternans due to the first mechanism, the boundaries marking regions of alternans with opposite phase arise far from the stimulus site, move toward the stimulus site, and stabilize. Dynamic splitting of action potential duration restitution curves due to electrotonic coupling plays a crucial role in this stability. Larger tissues and faster pacing rates are conducive to multiple boundaries, and inhomogeneities of tissue properties facilitate or inhibit formation of boundaries. CONCLUSION: Spatial inhomogeneities of electrical restitution properties are not required to produce discordant alternans.