UPPER LIMIT OF VULNERABILITY PREDICTS CHRONIC DEFIBRILLATION THRESHOLD FOR TRANSVENOUS IMPLANTABLE DEFIBRILLATORS

ULV Predicts Chronic DFT, Introduction: The upper limit of vulnerabili ty (ULV) is the shock strength at or above which ventricular fibrillat ion cannot be induced when delivered in the vulnerable period. It corr elates acutely with the acute defibrillation threshold (DFT) and can b e determined with a single episode of fibrillation. The goal of this p rospective study was to determine the relationship between the ULV and the chronic DFT. Methods and Results: We studied 40 patients at, and 3 months after, implantation of transvenous cardioverter defibrillator s. The ULV was defined as the weakest biphasic shock that failed to in duce fibrillation when delivered 0, 20, and 40 msec before the peak of the T wave. Patients were classified as clinically stable or unstable based on prospectively defined criteria, There were no significant di fferences between the group means for the acute and chronic determinat ions of ULV (13.5 +/- 5.3 J vs 12.4 +/- 6.8 J, P = 0.25) and DFT (10.1 +/- 5.0 J vs 9.9 +/- 5.7 J, P = 0.74), Five patients (15%) were class ified as unstable. The strength of the correlation between acute ULV a nd acute DFT (r = 0.74, P < 0.001) was similar to that between the chr onic ULV and chronic DFT (r = 0.82, P < 0.001), There was a correlatio n between the change in ULV from acute to chronic and the correspondin g change in DFT (r = 0.67, P < 0.001). The chronic DFT was less than t he acute ULV + 3 J in all 35 stable patients, but it was greater in 2 of 5 unstable patients (P = 0.04). Conclusions: The strength of the co rrelation between the chronic ULV and the chronic DFT is comparable to that between the acute ULV and the acute DFT. Temporal changes in the ULV predict temporal changes in the DFT, In clinically stable patient s, a defibrillation safety margin of 3 J above the acute ULV proved an adequate chronic safety margin.