UPPER LIMIT OF VULNERABILITY IS A GOOD ESTIMATOR OF SHOCK STRENGTH ASSOCIATED WITH 90-PERCENT PROBABILITY OF SUCCESSFUL DEFIBRILLATION IN HUMANS WITH TRANSVENOUS IMPLANTABLE CARDIOVERTER-DEFIBRILLATORS

Objectives. The goals of this study were to determine the probability of successful defibrillation at the upper limit of vulnerability and t o evaluate a minimal safety margin for implantable cardioverter-defibr illator first shocks based solely on the upper limit of vulnerability, Background. The upper limit of vulnerability is the strength at or ab ove which ventricular fibrillation is not induced when a stimulus is d elivered during the vulnerable phase of the cardiac cycle. It has been proposed as an estimate of defibrillation efficacy because it correla tes with the defibrillation threshold and can be determined with a sin gle episode of fibrillation. Methods. We studied 40 patients prospecti vely at implantation of transvenous cardioverter-defibrillators. Defib rillation threshold was defined as the weakest biphasic shock that def ibrillated after 10 s of ventricular fibrillation. The upper limit of vulnerability was defined as the weakest biphasic shock that did not i nduce ventricular fibrillation when given at 0, 20 and 40 ms before th e peak of the T wave in ventricular paced rhythm at cycle length 500 m s. After determination of the upper limit of vulnerability and defibri llation threshold, patients underwent six additional fibrillation-defi brillation episodes. The strength of five of the defibrillation shocks was equal to the upper limit of vulnerability; the strength of one of the six shocks was randomly selected implantable cardioverter-defibri llator was tested at the upper limit of vulnerability plus 3 J in 28 p atients, Results. The defibrillation threshold was 8.8 +/- 5.0 J (mean +/- SD), and the upper limit of vulnerability was 11.3 +/- 4.6 J; the defibrillation threshold and upper limit of vulnerability were highly correlated (r = 0.89, p < 0.001). The success rate for the 200 defibr illation shocks with strength equal to the upper limit of vulnerabilit y was 90% (95% confidence intervals based on proportion of successes i n 40 patients: 86% to 94%). All five defibrillation test shocks at the upper limit of vulnerability were successful in 24 patients (60%); fo ur of five were successful in 12 patients (30%); and three of five wer e successful in 4 patients (10%). All 30 test shocks and 28 implantabl e cardioverter defibrillator shocks with a strength equal to the upper limit of vulnerability plus 3 J were successful. Conclusions. The upp er limit of vulnerability is a good estimator of the shock strength as sociated with 90% probability of successful defibrillation (DFT90). A strength of 3 J above the upper limit of vulnerability is a good estim ate of the minimal acute safety margin for implantable cardioverter de fibrillator first shocks.