The effect of prior hypothermia on the physiological response to norepinephrine

Abstract

Objective: this study determines the effect of prior hypothermia on the cardiovascular responses to norepinephrine (NE) after rewarming. Methods: the experiment was a 2×2 controlled design with four groups of feline animals. The two variables were the presence or absence of previous cooling, and the use or non-use of NE after rewarming. During the ‘cooling’ phase, animals were either cooled using an external arterial–venous femoral shunt to 30°C or maintained at 37°C. After ‘rewarming’ animals were stratified to receive either NE at rates to deliver 0.2, 1.0 or 5 μg/kg per h or normal saline infusions. Animals were instrumented to measure mean arterial pressure (MAP) and cardiac output (CO) and systemic vascular resistance (SVR) was calculated. Results: there were no differences between groups at baseline and low dose NE (0.2 μg/kg per min). At 1.0 μg/kg per min, NE caused a significant increase in CO ( P<0.01) and no effect of MAP or SVR in the rewarmed animals when compared with normothermic controls. In rewarmed animals 5.0 μg/kg per min NE caused a significant increase in CO ( P<0.01) and no effect on MAP or SVR. In normothermic controls there was a significant increase in SVR ( P=0.02) and MAP ( P=0.05) and no effect on CO. Conclusion: this study shows that the effect of prior hypothermia on cardiovascular responses to moderate and high doses of NE is an improved CO with no affect on SVR and MAP. This could alter the clinical utility of NE in this situation.