Selective brain hypothermia suppresses noxious-evoked movement in canines

Systemic hypothermia suppresses noxious-evoked movement, but its main site of action is unknown. We examined the effect of hypothermia in the brain on noxious-evoked movement by selectively cooling the brain. Sixteen beagles were randomly divided into two groups and anesthetized with isoflurane/oxygen. After being deeply anesthetized, the dogs' lungs were artificially ventilated, and several major vessels were cannulated for perfusion and monitoring. Cold lactate Ringer's solution was infused into the right vertebral artery to cool the brain while maintaining the trunk temperature relatively warmer. When the brain temperature decreased to 20 degrees C or 25 degrees C, isoflurane administration was discontinued; the trunk temperatures at this stage were approximately 34.7 degrees C and 34.6 degrees C, respectively. After the end-tidal isoflurane concentration reached 0%, the base of the tail was stimulated with an electric current through 2 25-gauge needles. None of the dogs reacted to tail stimulation when the brain temperature was at 20 degrees C, whereas 7 of 8 reacted at 25 degrees C. These results indicate that selective brain hypothermia (20 degrees C) results in suppressing noxious-evoked movement in canines.