Changes in core temperature compartment size on induction of general anaesthesia

A two-compartment model of temperature distribution estimates the core compartment to occupy 66% of body mass at rest, while the peripheral compartment comprises the remainder. General anaesthesia impairs thermoregulation by central and peripheral actions. Peripheral vasodilatation accelerates heat transfer from the core to peripheral compartment causing the core compartment to cool and expand in size. Core hypothermia may be a significant cause of postoperative morbidity. This undocumented change in the size of the core compartment on induction of anaesthesia can be calculated. Core size increased from the established value of 66% before induction of general anaesthesia to 71.2 (SD 6)% of body mass, 20 min after induction of anaesthesia (P = 0.0001). On induction of general anaesthesia, the core compartment cools and expands while the peripheral compartment warms and contracts by a corresponding amount. Measurement of the magnitude of changes in core:periphery heat distribution on induction of anaesthesia contribute to a clearer understanding of the pathophysiology of perioperative hypothermia.