The effects of preinduction warming on temperature and blood pressure during propofol/nitrous oxide anesthesia

Core temperature decreases rapidly after induction of anesthesia, largely because heat is redistributed to peripheral tissues. The hypothesis that warming peripheral tissues before induction of general anesthesia (prewarming) minimizes hypothermia was tested. Because circulating blood volume may be greater during exposure to heat compared to cold, the hypothesis that prewarming decreases the amount of hypotension associated with induction of anesthesia was tested also. Finally, the hypothesis that the difference between direct radial arterial blood pressure and blood pressure measured oscillometrically at the brachial artery depends on thermoregulatory and anesthetic conditions was tested.

Each of six volunteers underwent general anesthesia (propofol and nitrous oxide) twice on the same day. Each anesthetic lasted 1 h and was preceded by either 2 h of active warming with forced air or 2 h of passive cooling by exposure to a typical operating room environment. After induction of each anesthetic, volunteers were fully exposed to the ambient environment. Volunteers recovered for 2 h before starting the second preinduction treatment.

Initial tympanic membrane temperatures were similar before each preinduction treatment: 36.7 +/- 0.4 degrees C when volunteers were not warmed and 36.7 +/- 0.6 degrees C when volunteers were warmed. Tympanic membrane temperature did not change during the preinduction period without warming but increased slightly (delta T = 0.4 +/- 0.2 degree C) during warming. After induction of anesthesia, core temperatures decreased to 36.1 +/- 0.4 degree C over 1 h when volunteers were prewarmed but decreased to 34.9 +/- 0.4 degrees C when they were not. Radial arterial systolic, diastolic, and mean blood pressures were lower before induction of anesthesia when volunteers were warmed compared to when no warming was given. Oscillometric diastolic and mean pressures also were lower during prewarming; however, oscillometric systolic pressure did not differ significantly. Prewarming did not result in less hypotension after induction. Without warming, the difference (radial arterial minus oscillometric) in systolic blood pressure measurements was approximately 17 mmHg. Warming was associated with a reversal of the systolic pressure difference to approximately -6 mmHg. After induction of anesthesia, the differences in systolic and mean pressure measurements became more negative with respect to the preinduction values regardless of preinduction warming treatment.

These data confirm our hypothesis that redistribution hypothermia can be minimized by preinduction warming of peripheral tissues. Prewarming decreases blood pressure but does not prevent subsequent hypotension after induction. The difference between radical arterial blood pressure and oscillometric blood pressure depends on thermoregulatory vasomotor changes but also may be influenced by vasodilation associated with administration of propofol and nitrous oxide.