Mechanisms in the interruption of reentrant tachycardia by pacing

The mechanisms by which pacing interrupts reentrant tachycardia associated with a structural obstacle were investigated using a computer model of propagated excitation. The model simulated cycle length-dependent refractoriness and slow propagation during incomplete recovery of excitability. Previously established features of the mechanism consisting of collision of reentrant with paced antidromic propagation and block of orthodromic propagation were demonstrated in the model, and factors affecting the mechanism were defined. Arrival time of paced orthodromic excitation at a potential block site and the duration of refractoriness at that site were major factors. Arrival time was determined by pacing stimulus time and propagation velocity. Slow propagation of a particular response acted to prevent the required block during that response, but enhanced the likelihood of a block of a subsequent response by affects on the onset time and duration of refractoriness at the block site at fast rates. In some conditions, responses to later stimuli resulted in block and interruption of tachycardia, while earlier stimuli with slower propagation during the same cycle failed to. Tachycardia rate affected its interruption by pacing by means of the shorter refractory period of the potential block site at fast rates, so that a paced response with a particular arrival time might fail to block. A greater number of successive paced responses were then required to terminate rapid tachycardia.