Mild core hypothermia and anesthetic requirement for loss of responsiveness during propofol anesthesia for craniotomy

Mild hypothermia may be induced during neurosurgery for brain protection. However, its effect on propofol requirement has not been defined. Accordingly, we tested the hypothesis that 3degreesC of core hypothermia decreases the propofol blood concentration at which patients respond to command (CP50-awake) in neurosurgical patients. Forty patients were anesthetized with alfentanil 50 mug/kg IV, nitrous oxide, propofol target-controlled infusion, and rocuronium. The bispectral index (version 3.12) was monitored continuously. Patients were randomized to a core temperature of 34degreesC or 37degreesC, At the end of surgery, neuromuscular blockade was reversed, nitrous oxide was ceased, and propofol was infused to achieve a blood target determined by the previous patient's response. Responsiveness to command was assessed 15 min later. Results were analyzed with logistic regression models; P < 0.05 was considered statistically significant. The CP50-awake of propofol was 3.05 mug/mL (95% confidence interval, 2.34-3.66). Propofol concentration, but not core temperature, predicted loss of response to command (odds ratio, 11.76; 95% confidence interval, 2.40-57.63; P < 0.01). Core temperature did not alter the relationship between bispectral index and response to command. Propofol infusion regimens may not require adjustment during mild hypothermia.