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(Circulation. 2000;101:1337.)
© 2000 American Heart Association, Inc.

Basic Science Reports

Pacing After Shocks Stronger Than the Upper Limit of Vulnerability

Impact on Fibrillation Induction

Nipon Chattipakorn, MD, PhD; Parwis C. Fotuhi, MD; Catherine M. Sreenan, BS; James B. White, PhD; Raymond E. Ideker, MD, PhD

From the Departments of Medicine (N.C., P.C.F., C.M.S., R.E.I.) and Physiology (N.C., J.B.W., R.E.I.), University of Alabama at Birmingham.

Correspondence to Nipon Chattipakorn, MD, PhD, 1670 University Blvd, B140, Birmingham, AL 35294-0019. E-mail toon{at}crml.uab.edu

Background—After upper-limit-of-vulnerability (ULV) shocks of the same strength and coupling interval (CI) during the T wave, (1) the epicardial activation pattern (EAP) for the first postshock cycle is indistinguishable between shocks that do (VF) and do not (NoVF) induce ventricular fibrillation (VF) and (2) 3 cycles in rapid succession always occur during VF but not during NoVF episodes. To study the role of these rapid cycles, rapid pacing was performed after a shock stronger than the ULV that 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 not 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 captured 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 were 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 initiate VF.

Key Words: mapping • fibrillation • shock

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