Alternative Mechanisms for Alternans in Cardiac Tissue
Elizabeth M. Cherry*
Department of Biomedical Sciences, Cornell
University, Ithaca, NY 14853, USA
Rapidly paced cardiac tissue displays a period-doubling bifurcation in electrical response known as alternans, in which alternate beats have long and short responses despite a constant pacing period. Traditionally, this bifurcation has been explained in terms of the slope of the restitution function, which relates the response duration to the duration of the interval preceding it. However, a number of other mechanisms also can give rise to alternans. We have made the novel observation that the restitution curve in the alternans regime is not a single-valued function under a wide range of physiological conditions, but instead splits into two branches representing the long and short responses, with no restrictions on slope. In another mechanism, electrotonic effects arising from diffusive currents can produce alternans in an intervening region between a region that can keep up with rapid pacing and another region that cannot and that blocks every other beat. Spiral wave trajectories with petals also can result in alternans at locations within the petals. In this talk we will describe the nonlinear dynamics of these other mechanisms for alternans and detail the conditions under which they are likely to be important in initiating more dangerous cardiac arrhythmias.