Effects of lidocaine on relation between defibrillation threshold and upper limit of vulnerability in open-chest dogs

Background. The purpose of the present study was to test the effects of lidocaine on the relation between the defibrillation threshold and the upper limit of vulnerability. Methods and Results. The shock strength associated with a 50% probability of successful defibrillation (DFT₅₀) and the shock strength associated with a 50% probability of reaching the upper limit of vulnerability (ULV₅₀) were determined in 11 open-chest dogs by using the delayed up-down method before and during lidocaine (seven dogs) or normal saline (four dogs) infusion. The ventricles were paced at a cycle length of 300 msec. Shocks of various strengths were then given via a patch-patch electrode configuration on the anterior and posterior surfaces of the ventricle to determine the ULV₅₀. Once ventricular fibrillation was induced, shocks were given 15-20 seconds later via the same electrode configuration to determine the DFT₅₀. Lidocaine infusion resulted in a serum level of 15±4 μg/ml. This was associated with a lengthening of the QT interval but not with the widening of the QRS complex. In all dogs, both the ULV₅₀ and the DFT₅₀ increased significantly when tested during lidocaine infusion. Mean ULV₅₀ during lidocaine infusion was 496±70 V or 13.1 ±4.3 J, which were significantly higher than the baseline values of 333±67 V or 5.3±2.2 J (p<0.001 for both voltage and energy). Mean DFT₅₀ during lidocaine infusion was 407±41 V or 8.7±1.7 J, which were significantly higher than the baseline values of 300±38 V and 4.4±1.1 J (p=0.004 for voltage and P>=0.013 for energy). The r values between the ULV₅₀ and the DFT₅₀ were 0.79 (P=0.037) for voltage and 0.80 (P=0.030) for energy at baseline and 0.85 (p=0.016) for voltage and 0.88 (p=0.009) for energy during the lidocaine infusion. However, the increments of the ULV₅₀ (163±88 V or 7.8±4.6 J) were significantly greater than the increments of the DFT₅₀ (107±51 V or 4.4±1.9 J, P=0.035 for voltage and P=0.023 for energy). Normal saline infusion did not alter DFT₅₀ or ULV₅₀. Conclusions. Lidocaine infusion significantly increases both ULV₅₀ and DFT₅₀. These results are compatible with the upper limit of vulnerability hypothesis of defibrillation. However, the greater increase of the upper limit of vulnerability than the defibrillation threshold with lidocaine infusion indicates that other factors may also need to be considered to explain the results. (Circulation 1992;85:1146-1151).