Prevalence of Rate-Dependent Behaviors in Cardiac Muscle

We explore the rate-dependent dynamic response of periodically paced bullfrog (Rana catesbeiana) cardiac muscle. Alternans (2:2 behavior) occur in $35\%$ of animals and $2:1\leftrightarrow 1:1$ bistability in $74\%$ of animals. In addition, we observe $2:2\leftrightarrow 2:1$ bistablility. We discuss the implications of these results for two map-based models of cardiac dynamics. The high prevalence of bistability suggests that this dynamical behavior must be accounted for in the design of closed-loop feedback protocols to stabilize cardiac dynamics.