Early Afterdepolarizations with Growing Amplitudes do not Require Stable Limit Cycles in the Fast Subsystem of Cardiac Action Potential Models

Early afterdepolarizations (EADs) are pathological oscillations in cardiac action potentials during the repolarization phase and may be caused by drug side effects, ion channel disease or oxidative stress. The most widely observed EAD pattern is characterized by oscillations with growing amplitudes. So far, its occurence is explained in terms of a supercritical Hopf bifurcation in the fast subsystem of the action potential dynamics from which stable limit cycles with growing amplitudes emerge. In this article we demonstrate that EAD patterns with growing amplitudes do not require supercritical Hopf bifurcations in the fast subsystem but can also arise from loss of stability at subcritical Hopf or limit point bifurcations.