Prevalence of rate-dependent behaviors in cardiac muscle

We explore the rate-dependent dynamic response of periodically paced bullfr og (Rana catesbeiana) cardiac muscle. Alternans (2:2 behavior) occur in 35% of animals and 2:1 <----> 1:1 bistability in 74% of animals. In addition, we observe 2:2 <----> 2:1 bistablility. We discuss the implications of thes e 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 dynam ics. [S0031-9007(99)08837-7].