Limitations of the Countershock Dose Response: A Study of Transthoracic Current

Dose response assessment of Countershock efficacy has been widely determined with respect to energy but not current. The purpose of this study was to examine the utility of the dose response method in a current-based model of trunsthoracic defibrillalion (penlobarbital anesthetized dogs, n = 8). Ventricular fibrillation induction lasting 15 seconds was separated by 5-minute intervals. Current defibrillation threshold (DFT: the lowest current that successfully defibrillated) was determined by decreasing current on successive trials. Energy DFT equaled the energy value of the corresponding current DFT. Subsequent data were expressed in normalized terms with each DFT assigned a normalized value of 1.00. Three shocks were delivered in random order at each of seven normalized current nodes (total of 21 shocks); 0.55, 0.70, 0.85, 1.00, 1.15, 1.30, and 2.00 × DFT (early testing). Randomization was repeated, and a second set of 21 trials were performed (late testing). Composite plots were made relating normalized current and energy to the percent successful defibrillation. The dose response expressed in normalized energy demonstrated an overall shift to the left compared to current. The difference was significant at every node value below the estimated DFT. Ninety percent of successful trials with respect to current and energy occurred at or above 0.85 DFT and 0.55 DFT, respectively. Significant changes in impedance occurred between early testing (60 ± 6 ohms) and late testing (47 ± 5 ohms), n = 8, mean ± SD. Current, compared to energy, is a more accurate parameter in the dose response assessment of transfhoracic defibrillation. This phenomenon is largely secondary to a time-dependent fall in impedance that results in lower delivered voltage and energy for a fixed delivered current. Energy should not be used as a parameter of Countershock efficacy.