EFFECTS OF PACING ON STATIONARY REENTRANT ACTIVITY - THEORETICAL AND EXPERIMENTAL-STUDY

It is well known that electrical pacing may either terminate or change the rate and/or ECG appearance of reentrant ventricular tachycardia. However, the dynamics of interaction of reentrant waves with waves ini tiated by external pacing are poorly understood. Prevailing concepts a re based on simplistic models in which propagation occurs in one-dimen sional rings of cardiac tissue. Since reentrant activation in the vent ricles occurs in two or three dimensions, such concepts might be insuf ficient to explain the mechanisms of pacing-induced effects. We used n umerical and biological models of cardiac excitation to explore the ph enomena, which may take place as a result of electrical pacing during functionally determined reentry. Computer simulations of a two-dimensi onal array of electrically coupled FitzHugh-Nagumo cells were used to predict the response patterns expected from thin slices of sheep ventr icular epicardial muscle, in which self-sustaining reentrant activity in the form of spiral waves was consistently initiated by premature st imulation and monitored by means of video mapping techniques. The resu lts show that depending on their timing and shape, externally induced waves may collide with the self-sustaining spiral and result in one of three possible outcomes: (1) direct annihilation of the spiral, (2) m ultiplication of the spiral, or (3) shift of the spiral center (ie, co re). Multiplication and shift of the spiral core were attended by chan ges in rate and morphology of the arrhythmia as seen by ''pseudo-ECGs. '' Furthermore. delayed termination (ie, termination of the activity o ne to three cycles after the stimulus) occurred after both multiplicat ion and shift of the spiral center. Bath numerical predictions and exp erimental results support the hypothesis that whether a pacing stimulu s will terminate a reentrant arrhythmia or modify its ECG appearance d epends on whether the interactions between the externally induced wave and the spiral wave result in the de novo formation of one or more '' wavebreaks.'' The final outcome depends on the stimulus parameters (ie , position and size of the electrodes and timing of the stimulus) as w ell as on the position of the newly formed wavebreak(s) in relation to that of the original wave.