TIMING OF THE UPPER LIMIT OF VULNERABILITY IS DIFFERENT FOR MONOPHASIC AND BIPHASIC SHOCKS - IMPLICATIONS FOR THE DETERMINATION OF THE DEFIBRILLATION THRESHOLD

The upper limit of vulnerability (ULV) has been used in clinical studi es to predict the DFT in patients with ICDs. Despite the ULV-DFT corre lation, uncertainties about the optimal timing of the ULV determinatio n remain. Previous studies using monophasic or biphasic shock waveform s reported differences in the ULV timing with respect to the electroca rdiographic T wave. The purpose of this study was to directly compare the ULV timing for mono-versus biphasic T wave shocks. In ten isolated rabbit hearts, mono-and biphasic shocks were delivered randomly durin g the vulnerable window and at varying shock strengths to determine th e ULV. The ULV timing was ex-pressed as the coupling interval at the U LV, the myocardial repolarization state at the ULV measured by monopha sic action potential recordings, and the relation between the ULV and the peak of the simultaneously recorded volume conducted T wave. The U LV for biphasic shocks occurred at longer coupling intervals than for monophasic shocks (188.0 +/- 9.5 ms vs 173.5 +/- 8.8 ms, P < 0.001). T his resulted in a more repolarized myocardial state at the ULV for bip hasic than for monophasic shocks (81.1% +/- 7.5% vs 66.9% +/- 9.0%, P = 0.002). The ULV for monophasic shocks occurred predominantly during the upslope of the T wave (8.0 +/- 9.7 ms before the peak of the T wav e) whereas the ULV for biphasic shocks occurred at or after the peak o f the T wave (5.9 +/- 9.3 ms after the peak of the T wave) (P < 0.001) . Biphasic shocks delay the timing of the ULV as compared to monophasi c shocks. This is important for the prediction of the DFT by ULV measu rements.