Measurement of the Upper Limit of Vulnerability during Defibrillator Implantation can Substitute Defibrillation Threshold Measurement

We investigated whether defibrillation thresholds (DFTs) could be measured more safely during defibrillator implantation by measuring the upper limit of vulnerability (ULV) without using any special equipment. Nonthoracotomy ICD implantation with endocardial leads was performed in 13 patients, and through the use of the ICD function itself, ULV and DFT were measured using the delayed four-episode up-down algorithm. Myocardial injures caused by high-energy current were assessed by electrocardiograms and serial CPK-MB. ULV was confirmed in all cases, and it strongly correlated with DFT. The average ULV was 5.9 ± 3.3J, while the average DFT was 7.9 ± 4.3J (r = 0.89, p < 0.0001, DFT = 1.20+1.14x ULV). The average ULV was thus significantly lower (p < 0.01). Although six patients were on amiodarone therapy, the strong correlation between ULV and DFT was also maintained (r = 0,97), p < 0.01) in these patients. In all cases, the CPK-MB failed to increase, and no myocardial injuries were detectable on electrocardiograms. We confirmed that ULV could be easily and safety measured during ICD implantation, and that ULV could be used instead of DFT.