COMPARATIVE REPRODUCIBILITY OF DEFIBRILLATION THRESHOLD AND UPPER LIMIT OF VULNERABILITY

The upper limit of vulnerability (ULV) is the strength at or above whi ch VF is not induced when a stimulus is delivered during the vulnerabl e phase of the cardiac cycle. Previous studies have demonstrated a sta tistically significant correlation between the ULV and the defibrillat ion threshold (DFT) in groups of patients. However, the correlation be tween ULV and DFT may not be close in individual patients. This imperf ect correlation may be due to physiological factors or to limitations of the measurement methods. The reproducibility of either DFT or ULV h as not been studied critically. The purpose of this study was to compa re the reproducibility of clinically applicable methods for determinat ion of DFT and ULV. We prospectively studied 25 patients with a transv enous implantable cardioverter defibrillator (Medtronic 7219D) at post operative electrophysiological study. DFT was defined as the lowest en ergy that defibrillated after 10 seconds of VF. The ULV was defined as the lowest energy that did not in du ce VF with three shocks at 0, 20 , and 40 ms before the peak of the T wave in ventricular paced rhythm at a cycle length of 500 ms. Both the DFT and the ULV were determined twice for biphasic pulses using a three-step, midpoint protocol. There was no significant difference between the two determinations of DFT ( 10.1 +/- 5.9 J vs 10.4 +/- 5.8 J), the two determinations of ULV (13.4 +/- 6.8 J vs 13.8 +/- 6.6) or the DFT-ULV Pearson correlation coeffic ients for each determination (0.8, P < 0.001, vs < 0.001). to analyze reproducibility, Lin concordance coefficients for second determination versus first determination were constructed for both ULV and DFT This coefficient is similar to the Pearson correlation coefficient, but me asures closeness to the line of identity rather than the line of regre ssion. The Lin concordance coefficient for ULV was higher than that fo r DFT (0.93, 95% CI 0.85-0.47 vs. 0.64, 95% CI 0.33-0.82; P < 0.01) Fo r paired comparison of defibrillation efficacy under different experim ental conditions, the sample sizes required to detect differences of 2 J, 3 J, and 4 J (80% power, P < 0.05) were 52, 24, and 15 for DFT ver sus 25, 8, and 6 for ULV. We conclude that a simple, clinically applic able method for determination of ULV is more reproducible than the sin gle point DFT. Measured correlations between the ULV and single point are limited by the reproducibility of the DFT measurement.