Effects of the Pacing Site, Procainamide, and Lead Configuration on the Relationship Between the Upper Limit of Vulnerability and the Defibrillation Threshold

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Abstract
FAN, W., et.al.: Effects of the Pacing Site, Procainamide, and the Lead Configuration on the Relationship Between the Upper Limit of Vulnerability and the Defibrillation Threshold. In six open chest dogs, we determined the upper limit of vulnerability (ULV) and defibrillation threshold (DFT) by an up‐down algorithm when the pacing site was at the right atrium, at the left ventricular apex, and at the left ventricular base. Monophasic shocks (6 ms) were given to epicardial patches at 20 and 40 ms before the peak of the T wave to bracket the mid‐upslope. In an additional six closed‐chest dogs, we determined the ULV and the DFT with transvenous leads with an 8‐ms biphasic waveform. The S1 pacing site was at the right ventricular apex and the right atrium, and the shocks were given at 20 ms and 40 ms before the peak of the T wave, and on the peak of T wave. The same test was repeated after intravenous procainamide infusion (20 mg/Kg loading, then 2 mg/min maintenance). In the first six dogs, the ULV determined when pacing was given to the left ventricular apex, the left ventricular base, and the right atrium was 4.2 ± 1.7 J, 4.4 ± 2.1 J, and 3.9 ± 1.5 J, respectively; values that were not significantly different from the DFT of 4.8 ± 1.9 J, 4.5 ± 1.9 J, and 4.2 ± 1.3 J, respectively. In the latter six dogs, the ULV versus the DFT was 13.5 ± 5.2 J versus 18.2 ± 6.2 J (right ventricular apex) and 12.8 ± 6.0 J versus 15.4 ± 6.0 J (right atrium) at baseline; 14.6 ± 4.6 J versus 19.5 ± 6.7 J (right ventricular apex) and 14.3 ± 5.5 J versus 18.7 ± 6.4 J (right atrium) during procainamide infusion (P = NS for all). We conclude that, when tested with 2–3 shocks on or before the peak of the T wave, the ULV can be used to estimate the DFT with both epicardial patch and transvenous lead configurations. Different S1 pacing sites and procainamide did not change the relationship between the ULV and the DFT.