THE EFFECTS OF NORMOTHERMIC AND HYPOTHERMIC CARDIOPULMONARY BYPASS ON DEFIBRILLATION ENERGY-REQUIREMENTS AND TRANSMYOCARDIAL IMPEDANCE - IMPLICATIONS FOR IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR IMPLANTATION

The influence of normothermic and hypothermic cardiopulmonary bypass on defibrillation energy requirements and transcardiac impedance is not well characterized. However, this relationship is of clinical importance during automatic defibrillator implantation done with concomitant cardiac surgery, and there is anecdotal information that criteria for successful implantation are harder to achieve after such operations. We studied the effect of controlled hypothermia on defibrillation energy requirements and transcardiac impedance in a canine model of cardiopulmonary bypass in which 26 animals underwent right atrial and femoral arterial cannulation, as well as continuous hemodynamic and intramyocardial temperature monitoring. The defibrillation energy requirements were evaluated at 60-minute intervals with an epicardial patch system, and transcardiac impedance was measured before and after the multiple inductions and terminations of ventricular fibrillation. In group 1 (n = 10) defibrillation energy requirements were evaluated immediately after initiation of cardiopulmonary bypass at 37 degrees C (T-0), after gradual cooling to 28 degrees C (T-1), and after rewarming to 37 degrees C (T-2), Group 2 (n = 16) comprised time controls that were identically instrumented and studied, but maintained at 37 degrees C throughout. Percent successful defibrillation was plotted against delivered energy, and the raw data fit by logistic regression. The energy at which 50% of shocks were successful (E(50)) was 3.23 +/- 0.89 joules at T-0, 5.12 +/- 1.85 joules at T-1, and 4.42 +/- 1.22 joules at T-2 in group 1; this was not significantly different from the corresponding group 2 E(50) values, which were 3.11 +/- 1.39 joules, 4.95 +/- 2.47 joules, and 5.59 +/- 3.18 joules, respectively. Both groups demonstrated a significant increase in E(50), during the first hour of cardiopulmonary bypass (mean increase from T-0 to T-1 was 1.89 joules in group 1 and 1.84 joules in group 2, p < 0.05). Transmyocardial impedance fell progressively during the group 2 experiments from 73.6 +/- 12.9 Omega at the beginning of the T-0 shock series to 61.4 +/- 8.9 Omega at the end of the T-2 shock series. A similar reduction in transmyocardial impedance was observed during the course of all the group 1 experiments; however, at the beginning of the T-1 shock series impedance was significantly elevated to 77.4 +/- 12.3 Omega (p < 0.05 compared with group 2 and with end T-1 in group 1). There was no relationship between defibrillation energy requirements and transcardiac impedance; there was also no correlation between either of these parameters and intramyocardial extracellular pH or left ventricular end diastolic pressure. We conclude that defibrillation energy requirements significantly and persistently increase during the first hour of cardiopulmonary bypass and the effect is temperature independent. Systemic hypothermia significantly and reversibly elevates transmyocardial impedance but multiple shocks: reduce this effect at 28 degrees C as they do at 37 degrees C.