The physiologic responses to epinephrine during cooling and after rewarming in vivo

INTRODUCTION: The purpose of our study was to determine whether hypothermia has any effects on physiological hemodynamic responses to epinephrine (Epi), and whether rewarming reverses these effects. METHODS: Sprague-Dawley rats were instrumented to measure mean arterial pressure (MAP), and left ventricular (LV) pressure-volume changes were recorded by using a Millar pressure-volume conductance catheter. Core temperature was reduced from 37°C to 28°C and returned to 37°C by using both internal and external heat exchangers. Two groups of rats were infused with either saline (n = 7), or Epi 0.125 μg/min continuously (n = 7). At 33°C, 30°C, and 28°C, the Epi infusion was temporarily increased from 0.125 to 1.25 μg/min. RESULTS: Before cooling, Epi infusion in both groups resulted in a significant, dose-dependent increase in heart rate (HR), stroke volume (SV), cardiac output (CO), LV dP/dt(max )(maximum derivative of systolic pressure over time), but only Epi infusion at 1.25 μg/min caused elevation of MAP. During cooling to 30°C, Epi infusion at 0.125 μg/min caused a significant elevation of central hemodynamic variables, whereas MAP remained unchanged. In contrast, Epi infusions at 1.25 μg/min caused a significant elevation of MAP during cooling to 28°C but no increases in central hemodynamics. After rewarming, all hemodynamic variables returned to baseline in both groups, but only the saline-treated animals displayed the prehypothermic hemodynamic dose responses to Epi infusions. CONCLUSIONS: This study shows that hypothermia causes a change in the physiological hemodynamic response to Epi, which is not reversed by rewarming.