Optical mapping of pacemaker interactions

Under normal conditions the sinoatrial node serves as the pacemaker of the heart. However, under disease states other pacemaker sites can emerge which compete with the sinoatrial node or with each other. This thesis describes theoretical and experimental studies of pacemaker initiation and interaction. The first aspect of this thesis deals with an arrhythmia called modulated parasystole, which is generated by the interaction between the sinus pacemaker and an ectopic pacemaking focus. A mathematical model was developed to study the dynamics of modulated parasystole using discontinuous circle maps. The mathematical model displayed banded chaos, characterized by zero rotation interval width in the presence of a positive Lyapunov exponent. Banded chaos in the parasystole map produces rhythms characteristic of those found clinically. The second aspect of the thesis deals with spontaneous pacemaker activity and interaction using optical mapping techniques and mathematical models. A macroscopic imaging system was designed and constructed that records fluorescent signals from thin preparations over large areas (1 cm2 ) for long time periods (>30 minutes). Rotating waves (rotors) of cellular activity were observed by mapping calcium in nonconfluent cultures of embryonic chick heart cells. Unlike previous observations of rotors or spiral waves in other systems, the rotors did not persist but exhibited a repetitive pattern of spontaneous onset and offset leading to a bursting rhythm. Similar dynamics were observed in simple excitable media models that incorporated spontaneous initiation of activity and a decrease of excitability as a consequence of rapid activity (fatigue). These results provide a mechanism for bursting dynamics in normal and pathological processes. Activation maps were also obtained from the rabbit atrioventricular (AV) node, a small region of the heart with specialized pacemaking and conducting properties. This work determined sites of delay and spontane