Upper limit of vulnerability during defibrillator implantations predicts the occurrence of appropriate shock therapy for ventricular fibrillation.

Background: The utility of the upper limit of vulnerability (ULV) test in patients undergoing defibrillator implantation has been reported, so the purpose of this study was to evaluate the difference in the clinical outcomes between patients with ULV ≤15 J or >15 J.
Methods and Results: A total of 165 patients receiving an implantable cardioverter-defibrillator underwent a vulnerability test. At the time of the implantation, we delivered a 15-J shock on the T-peak and ±20 ms later to cover the most vulnerable part of the cardiac cycle. The clinical outcomes were prospectively analyzed. A 15-J shock induced ventricular fibrillation (VF) in 30 patients (ULV >15 J) and did not in 135 (ULV ≤15 J). The characteristics of the 2 groups were comparable. After a mean follow-up of 757 days, Kaplan-Meier curve analysis showed that the ULV ≤15 J group experienced less VF than the ULV >15 J group (log-rank P=0.003). The occurrence of ventricular tachycardia was similar between the 2 groups (P=0.140). Furthermore, the effectiveness of ATP was comparable. After adjusting for other known predictors of shock therapy, a ULV >15 J was independently associated with the occurrence of VF (hazard ratio: 6.25; 95% confidence interval: 1.913-20.40; P<0.01).
Conclusions: A high ULV value was associated with a high incidence of VF, which suggests that cardiac vulnerability to electrical shock may be linked to electrical instability.