Simulated sinoatrial exit blocks explained by circle map analysis

In an accompanying study, it was seen that most cardiac arrhythmias that we re simulated during poor intercellular coupling in the sinus node, were the same as those obtained in a two-element system in which one element suffer ed from a strong leakage current. This element corresponds to the sinus nod e periphery and is thus the one which feeds the atrium. In this paper, the interior element was replaced by a periodic stimulator. The dynamics of the peripheral element is then determined by its phase response curve. Phase r esponse curves for sinus node elements subject to leakage were simulated fo r many different amplitudes of depolarizing stimuli. Simulations with circl e maps based on these curves produced the same sequence of progressing leve ls of exit block as stimulus strength decreased, as did the two-element sys tem when coupling strength was reduced. The bifurcations of the circle maps leading to the observed rhythms were identified. We found that the essenti al qualities of the phase response curves were determined by generally acce pted properties of membrane currents. This suggests that the observed rhyth ms and bifurcations are generic. (C) 2001 Academic Press.