The effect of prior hypothermia on the physiological response to norepinephrine

Objective: this study determines the effect of prior hypothermia on the car diovascular responses to norepinephrine (NE) after rewarming. Methods: the experiment was a 2 x 2 controlled design with four groups of feline animals . The two variables were the presence or absence of previous cooling, and t he use or non-use of NE after rewarming. During the 'cooling' phase, animal s were either cooled using an external arterial-venous femoral shunt to 30 degrees C or maintained at 37 degrees C. After 'rewarming' animals were str atified to receive either NE at rates to deliver 0.2, 1.0 or 5 mu g/kg per h or normal saline infusions. Animals were instrumented to measure mean art erial pressure (MAP) and cardiac output (CO) and systemic vascular resistan ce (SVR) was calculated. Results: there were no differences between groups at baseline and low dose NE (0.2 mu g/kg per min). At 1.0 mu g/kg per min, NE caused a significant increase in CO (P < 0.01) and no effect of MAP or S VR in the rewarmed animals when compared with normothermic controls. In rew armed animals 5.0 mu g/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 ca rdiovascular responses to moderate and high doses of NE is an improved CO w ith no affect on SVR and MAP. This could alter the clinical utility of NE i n this situation. (C) 2000 Elsevier Science Ireland Ltd. All rights reserve d.