THE ZONE OF VULNERABILITY TO T-WAVE SHOCKS IN HUMANS

Introduction: Shocks during the vulnerable period of the cardiac cycle induce ventricular fibrillation (VF) if their strength is above the V F threshold (VFT) and less than the upper limit of vulnerability (ULV) . However, the range of shock strengths that constitutes the vulnerabl e zone and the corresponding range of coupling intervals have not been defined in humans. The ULV has been proposed as a measure of defibril lation because it correlates with the defibrillation threshold (DFT), but the optimal coupling interval for identifying it is unknown. Metho ds and Results: We studied 14 patients at implants of transvenous card ioverter defibrillators. The DFT was defined as the weakest shock that defibrillated after 10 seconds of VF. The ULV was defined as the weak est shock that did not induce VF when given at 0, 20, and 40 msec befo re the peak of the T wave or 20 msec after the peak in ventricular pac ed rhythm at a cycle length of 500 msec. The VFT was defined as the we akest shock that induced VF at any of the same four intervals. To iden tify the upper and lower boundaries of the vulnerable zone, we determi ned the shock strengths required to induce VF at all four intervals fo r weak shocks near the VFT and strong shocks near the ULV. The VFT was 72 +/- 42 V, and the ULV was 411 +/- 88 V. In all patients, a shock s trength of 200 V exceeded the VFT and was less than the ULV. The coupl ing interval at the ULV was 19 +/- 11 msec shorter than the coupling i nterval at the VFT (P < 0.001). The vulnerable zone showed a sharp pea k at the ULV and a less distinct nadir at the VFT. A 20-msec error in the interval at which the ULV was measured could have resulted in unde restimating it by a maximum of 95 +/- 31 V. The weakest shock that did not induce VF was greater for the shortest interval tested than for t he longest interval at both the upper boundary (356 +/- 108 V vs 280 /- 78 V; P < 0.01) and lower boundary (136 +/- 68 msec vs 100 +/- 65 m sec; P < 0.05). Conclusions: The human vulnerable zone is not symmetri c with respect to a single coupling interval, but slants from the uppe r left to lower right. Small differences in the coupling interval at w hich the ULV is determined or use of the coupling interval at the VFT to determine the ULV may result in significant variations in its measu red value. An efficient strategy for inducing VF would begin by delive ring a 200-V shock at a coupling interval 10 msec before the peak of t he T wave.