1026-83 Prospective Comparison of the Biphasic Waveform Upper Limit of Vulnerability to the Defibrillation Threshold in Man

BackgroundThe upper limit of vulnerability (ULV) is defined as the upper limit of shock strength, above which ventricular fibrillation will not be induced when the shock is delivered during the vulnerable period. ULV is postulated to correlate with the defibrillation threshold (DFT) and, if true, should streamline implantable cardioverter-defibrillator (ICD) surgery and followup.Methods and ResultsWe sought to determine whether the biphasic ULV, measured with an easily implemented clinical protocol via the T-shock method available in the 7219D Medtronic ICD using 65% tilt, 120 μF asymmetric pulses, would correlate with the biphasic DFT assessed during follow up electrophysiologic (EP) evaluation of ICD function. Twelve consecutive patients were evaluated. The average age was 67±3.4 years, LV ejection fraction was 0.45±0.04, and 58% had underlying CAD. The index arrhythmia prompting ICD therapy was VF in 83% and VT in 17%. At the time of the follow-up EP study, all patients had VF induced with T-shocks at 310 ms following 3 ventricular paced beats at 400 ms starting at 0.2 Joules and stepping up until the ULV was found as follows: 0.6, 1.0, 2.0, 3.0, 40, 5.0, 7.0,10, 14, and 18 Joules. The DFT was determined using the exact same waveform, polarity and shock steps as was the ULV determination.ResultsULVDFTEnergy (J)68±44102±5.5Correlationr = 0.49, p = 0.11ConclusionWe found a poor correlation between the biphasic ULV and the DFT using this clinically feasible followup technique. The ULV appears to underestimate the DFT using this technique for evaluating ICD defibrillation efficacy during follow-up EP evaluation