Pacing after shocks stronger than the upper limit of vulnerability - Impact on fibrillation induction

Background-After upper-limit-of-vulnerability (ULV) shocks of the same stre ngth and coupling interval (CI) during the T wave, (1) the epicardial activ ation pattern (EAP) for the first postshock cycle is indistinguishable betw een shocks that do (VF) and do not (NoVF) induce ventricular fibrillation ( VF) and (2) greater than or equal to 3 cycles in rapid succession always oc cur during VF but not during NoVF episodes. To study the role of these rapi d cycles, rapid pacing was performed after a shock stronger than the ULV th at by itself did not induce rapid cycles and VF. Methods and Results-A 504-electrode sock was sutured to the heart in 6 pigs to map EAPs. The S2 shock strength and S1-S2 CI at the ULV were determined by T-wave scanning with an up/down protocol. Ten shocks 50 to 100 V above the ULV (aULV) were delivered at the same S1-S2 CI to confirm that VF was n ot induced. Then, the postshock interval after aULV shocks was scanned with an S3 pacing stimulus from the LV apex until the shortest S2-S3 CI that ca ptured was reached. This was repeated for S4, S5, etc, until VF was induced : To induce VF, 3 pacing stimuli (S3-S5) with progressively shorter CIs wer e required; S3 or S3, S4 never induced VF. After cycle S5, which induced VF , 2 EAP types occurred: focal (74%) and reentrant (26%). Conclusions-At least 3 cycles with short CIs are necessary for VF induction after aULV shocks. Cycles S3-S4 may create the substrate for cycle S5 to i nitiate VF.