Hypothermia eliminates isoflurane requirements at 20 degrees C

Hypothermia decreases anesthetic requirements, but the temperature that completely eliminates anesthetic needs has not been previously determined.

Eight female goats were anesthetized with isoflurane and catheters were placed in the femoral artery and cranial vena cava, after which the right carotid artery and external jugular vein were dissected free. Peripheral temperature was monitored in the rectum and core temperature in the vena cava. A thermistor was placed in the epidural space via a small burr hole to monitor brain temperature. Minimum alveolar concentration (MAC) for isoflurane was determined by eliciting gross, purposeful movement with a tail clamp. Cardiopulmonary bypass (CPB) was established using bubble oxygenators with venous blood drained from a jugular vein and arterial blood infused with a roller pump into the carotid artery. The animals were cooled to approximately 29 degrees C, and MAC redetermined, after which further cooling to 20 degrees C was accomplished. Isoflurane was eliminated, core and brain temperature adjusted in 2-3 degrees C increments, and the tail clamp applied until two temperatures were found that just permitted and just prevented movement. The animals were rewarmed, isoflurane added, and post-CPB MAC determined.

At 38.5 degrees C, pre-CPB MAC was 1.3 +/- 0.1% (mean +/- SEM). At 29.0 degrees C, MAC was 0.7 +/- 0.1%, and the anesthetizing temperature was 20.1 +/- 0.6 degrees C. At 37.3 degrees C, post-CPB MAC was 1.0 +/- 0.1% (P < 0.05 vs. pre-CPB).

These results confirm the rectilinear decrease in MAC seen in previous studies and establishes the anesthetizing temperature at 20 degrees C.