Thermoregulation and perioperative hypothermia — physiology viva

Structure of the viva and expected answers

Opening (2 minutes) — normal control

The examiner opens with: "How does the body normally maintain its temperature?" A model opening:

"The hypothalamus is the body's thermostat. Core temperature is held near 37 degrees C. The hypothalamus senses core temperature and mediates heat loss (sweating, vasodilation) and heat production and conservation (vasoconstriction, shivering, non-shivering thermogenesis). Responses are triggered at defined thresholds with a narrow inter-threshold range in the awake person.^[1]"

Heat production and heat loss (2 minutes)

"Heat production is basal metabolic rate plus shivering (involuntary skeletal-muscle contraction markedly raising heat production), and non-shivering thermogenesis from brown adipose tissue, where uncoupling releases heat directly — the neonate's main mechanism.^[4]"

"Heat is lost by radiation (the largest share at rest), convection, evaporation from skin and respiratory tract, and conduction.^[1]"

The examiner may probe: "Which is largest at rest, and which rises most in open-cavity surgery?" Answer: radiation is largest at rest; evaporation rises most with an open bowel or burns.^[1]

The effect of anaesthesia and the three phases (4 minutes)

"General anaesthetics greatly impair thermoregulation, synchronously reducing the thresholds for vasoconstriction and shivering; unwarmed anaesthetised patients become hypothermic, typically by 1-2 degrees C.^[1]"

"Hypothermia results initially from an internal redistribution of body heat from the core to the periphery, followed by heat loss exceeding metabolic heat production.^[1] Prewarming prevents the redistribution fall by abolishing the core-to-peripheral gradient; forced-air warming is the mainstay intraoperatively, and standard of care is to monitor core temperature and maintain normothermia.^[1]"