PROGRAMMING OF IMPLANTABLE CARDIOVERTER-DEFIBRILLATORS ON THE BASIS OF THE UPPER LIMIT OF VULNERABILITY

Background A patient-specific measure of defibrillation efficacy that requires a minimum number of ventricular fibrillation (VF) episodes wo uld be valuable for programming implantable cardioverter-defibrillator s (ICDs). The upper limit of vulnerability (ULV) is the weakest shock strength at or above which VF is not induced when a stimulus is delive red during the vulnerable phase of the cardiac cycle. It correlates wi th the defibrillation threshold (DFT) and can be determined with a sin gle episode of VF. The objective of this study was to test the hypothe sis that ICDs programmed on the basis of the ULV convert spontaneous I CD-detected VF reliably. Methods and Results We studied 100 consecutiv e patients at I(SD implantation and during follow-up of 20 +/- 7 month s. At implantation, the ULV and DFT were determined, and the ICD syste m was tested at a shock strength equal to the ULV + 3 J. During follow -up, the strength of the first shock was programmed to the ULV + 5 J f or arrhythmias detected in the VF zone (cycle length < 292 +/- 17 ms). We reviewed stored detection intervals and electrograms from spontane ous episodes of ICD-detected VF to determine the success rate for appr opriate first shocks. The programmed first-shock strength was 17.5 +/- 5.2 J. During follow-up, there were 120 appropriate first shocks in 3 7 patients. The arrhythmia was rapid monomorphic ventricular tachycard ia (VT) in 70% of episodes (31 patients), VF in 11% (13 patients), pol ymorphic VT in 1%, and unclassified in 17% (15 patients). The first sh ock was successful in 119 of 120 episodes (99%; 95% CI, 93% to 100%). One unclassified episode required two shocks. No patient had syncope a ssociated with an ICD shock or arrhythmic death. Conclusions ICD shock s can be programmed on the basis of the ULV, a measurement made in reg ular rhythm, without a direct measure of defibrillation efficacy.