Assessing the Excitable Gap in Reentry by Resetting

Background The shortest excitable gap during reentry may determine responses to pacing and antiarrhythmic drugs. The resetting response has been used clinically to assess the excitable gap, but it cannot directly indicate the shortest excitable gap. Methods and Results We studied resetting in the in vitro canine atrial tricuspid ring using an adjustable reentry preparation in which the ring was cut and reconnected electronically with an adjustable delay to vary the cycle length and excitable gap. We reset the tachycardias using 31 delays in 12 experiments. Tachycardias were terminated by premature stimuli in 16 delays. The reset window overestimated the shortest excitable gap by 25±14 ms, and the maximum degree of advancement of tachycardia underestimated the shortest excitable gap by 22±11 ms. The slope of the increasing portion of the resetting response curve was steeper in tachycardias terminated by premature stimuli than in those not terminated (−0.69±0.2 versus −0.37±0.2, P <.01). The effective refractory period difference between the sites of pacing and of block correlated with the slope of the resetting response curve. Damped cycle length oscillation after a long return cycle during resetting was always present when there was a partially excitable gap. Conclusions The reset window during pacing within the circuit and the maximum degree of advancement provided equally good estimates bracketing the shortest excitable gap. The slope of the resetting response curve predicted the likelihood of termination by premature stimuli. Damped cycle length oscillation after resetting detected a partially excitable gap.