Brain temperature during reversible upper respiratory bypass.

Seven male New Zealand white rabbits were trained to run in an exercise wheel. At least 5 days prior to experimentation, each rabbit had a reversible tracheal bypass canula and a hypothalamic thermocouple guide tube chronically implanted. The reversible tracheal bypass enabled the rabbit to breathe normally (through its upper respiratory pathways) or to be placed on bypass (breathe through its neck). Prior to and following exercise, hypothalamic temperature in rabbits breathing normally was lower than rectal temperature. This difference was greatest following a mild heat stress induced by exercise. When the rabbits breathed through the bypass, there were no differences between hypothalamic and rectal temperature. These data suggest that even in a species such as the rabbit, which lacks a carotid rate, hypothalamic temperature is influenced by upper respiratory cooking of venous blood and that the ensuing transfer of heat from the warmer internal carotid artery to the cooler venous sinuses can effectively cool the brain.