UPPER LIMIT OF VULNERABILITY RELIABLY PREDICTS THE DEFIBRILLATION THRESHOLD IN HUMANS

Background The upper limit of vulnerability is the stimulus strength a bove which electrical stimulation cannot induce ventricular fibrillati on even when the stimulus occurs during the vulnerable period of the c ardiac cycle. The purpose of this study was to test the hypothesis tha t the upper limit of vulnerability can accurately predict the defibril lation threshold in patients undergoing implantable cardioverter-defib rillator (ICD) implantation using nonthoracotomy lead systems. Methods and Results We studied 77 patients at the time of ICD implantation. M ultiple endocardial-endocardial and endocardial-subcutaneous shock pat hways were used. Two different protocols were used to test the upper l imit of vulnerability. In protocol 1 (n=17), the upper limit of vulner ability was tested with two shocks on the peak or the up-slope of the T wave of paced rhythm. The shocks were given randomly either at the p eak and 20 milliseconds before the peak of T wave (n=7) or at 20 and 4 0 milliseconds before the peak of T wave (n=10). In protocol 2 (n=60), the upper limit of vulnerability was tested with three shocks deliver ed at 0, 20, and 40 milliseconds before the peak of the T wave. The we akest shock that failed to induce ventricular fibrillation by a 5-J st ep-down or step-up method was defined as the upper limit of vulnerabil ity. The defibrillation threshold was also determined by a 5-J step-do wn or step-up method. In protocol 1, the upper limit of vulnerability (9+/-6 J) was significantly lower than the defibrillation threshold (1 3+/-7 J) with a correlation coefficient of .87 and P<.001. In protocol 2, the upper limit of vulnerability (13+/-6 J) was not significantly different from the defibrillation threshold (13+/-6 J) with a correlat ion coefficient of .85 and P<.001. In 45 of the 60 patients, the upper limit of vulnerability was less than or equal to 15 J; all had a defi brillation threshold of 20 J. In 51 of the 60 patients, the upper limi t of vulnerability was within 5 J of the defibrillation threshold. The upper limit of vulnerability overestimated the defibrillation thresho ld by >10 J in 8 patients and underestimated the defibrillation thresh old by >10 J in only 1 patient. The overestimation and underestimation occurred only in patients with the upper limit of vulnerability >15 J . Conclusions When tested with three shocks on and before the peak of the T wave, the upper limit of vulnerability accurately predicted the defibrillation threshold in patients undergoing ICD implantation using nonthoracotomy lead systems. This method required either one or no ep isodes of ventricular fibrillation in most patients.