Chaos of Cardiac Tissue under Periodic Stimuli and Fibrillation: Experiments and Control

Several cellular cardiac voltage models exhibit chaotic dynamics suggesting that the irregular heart rhythms of cardiac disease may have a nonlinear mechanism of control. However, little experimental evidence has documented the chaotic behavior of cardiac tissue excitations. This study aims to both quantify and qualify the chaotic nature of cardiac tissue from the system's arrhythmic electrical response to fast periodic forcing. Leading Lyapunov Exponents were estimated from action potential duration (APD) time series from single cells of bullfrogs yielding negative exponents for frequencies near period doubling cascades and positive exponents for arrhythmic responses to periodic forcing. Additionally, several stable period-three orbits and unstable periodic orbits were identified. Further, a biphasic perturbation around a forcing frequency resulting in arrhythmic behavior appears to be able to stabilize an unstable periodic orbit of the response. On a multicellular scale, unstable periodic behavior is also observed in the ventricle fibrillation of pigs and humans in isolated regions across the heart. These findings indicate cardiac tissue is governed in part by chaotic factors and nonlinear control can be employed to terminate arrhythmias.