Thermoregulation, the three phases of perioperative heat loss, and active warming

A First-examination applied-physiology SAQ testing the full thermoregulation framework: the hypothalamus as thermostat integrating central (preoptic) and peripheral (skin) thermoreceptor input through a threshold system; heat production (basal metabolic rate from liver, brain, heart; shivering 4 to 5 times BMR; brown-fat non-shivering thermogenesis via UCP1 uncoupling, norepinephrine-driven, the neonate's main mechanism; thyroid and catecholamine endocrine contributions); heat loss at rest (radiation 40 to 60 per cent, convection 25 to 30 per cent, evaporation 10 to 15 per cent, conduction 3 to 5 per cent); the threshold shift of 2 to 4 degrees C under anaesthesia rendering the patient poikilothermic; the three phases (redistribution 1 to 1.5 degrees C in the first hour and not true heat loss, linear 0.5 to 1 degree C per hour, plateau when vasoconstriction returns); the consequences with the four landmark trials (Frank cardiac relative risk 2.2, Kurz wound infection 19 versus 6 per cent, Schmied blood loss, Heier vecuronium doubled) plus shivering oxygen demand 200 to 500 per cent; and warming (prewarming prevents phase 1, forced-air warming is the mainstay, fluid warming for large volumes, insulation, ambient at least 21 degrees C) and measurement sites. All values are standard reference values (Sessler, Guyton).

Prompt

Describe the normal control of body temperature, including heat production and heat loss and the role of the hypothalamus. Explain the effect of general anaesthesia on thermoregulation and describe the three phases of perioperative heat loss. Outline the consequences of mild perioperative hypothermia and the measures used to prevent it. (20 marks)

Model answer plan (20 marks, 20 minutes)

A high-scoring answer is structured in four blocks that mirror the stem: (1) normal control of temperature; (2) the effect of anaesthesia and the three phases; (3) the consequences; (4) prevention and monitoring. Allocate roughly 5 marks to each block.^[6]

1. Normal control (5 marks)

Core temperature is regulated near 37 degrees C with a narrow thermoneutral zone.^[1]
The hypothalamus is the thermostat, mediating heat loss (sweating, vasodilation) and heat production and conservation (vasoconstriction, shivering, non-shivering thermogenesis).^[1]
Afferents: central thermoreceptors sense core/blood temperature (the dominant input); peripheral skin thermoreceptors sense surface temperature and rate of change.^[1]
Threshold system: responses are triggered at defined thresholds with a narrow inter-threshold range in the awake person.^[1]
Heat production: basal metabolic rate plus shivering (involuntary skeletal-muscle contraction markedly raising heat production), and non-shivering thermogenesis from brown fat where uncoupling releases heat directly — the neonate's main mechanism.^[4]
Heat loss at rest: radiation (the largest share), convection, evaporation from skin and respiratory tract, and conduction.^[1]

2. Effect of anaesthesia and the three phases (5 marks)

General anaesthetics greatly impair thermoregulation, reducing the thresholds for vasoconstriction and shivering; unwarmed anaesthetised patients become hypothermic.^[1]
Neuraxial anaesthesia also blocks the vasoconstrictor and shivering efferents to the legs and the hypothalamus misperceives the vasodilated legs as whole-body warmth, so hypothermia can be equally severe but less noticed.
Phase 1 (redistribution): the first hour, the steepest fall — core-to-peripheral redistribution when vasoconstriction is abolished. Cannot be treated, only prevented.^[1]
Phase 2 (linear decline): actual heat loss exceeding production over the next few hours. Reduced by active warming.^[1]
Phase 3 (plateau): core stabilises when vasoconstriction returns and loss equals production.^[1]

References6Show

[1]Sessler DI Perioperative thermoregulation and heat balance. Lancet, 2016.PMID 26775126
[2]Sessler DI Temperature monitoring and perioperative thermoregulation. Anesthesiology, 2008.PMID 18648241
[3]Frank SM, Fleisher LA, Breslow MJ, et al Perioperative maintenance of normothermia reduces the incidence of morbid cardiac events. JAMA, 1997.PMID 9087467
[4]Kurz A, Sessler DI, Lenhardt R Perioperative normothermia to reduce the incidence of surgical-wound infection and shorten hospitalization. New England Journal of Medicine, 1996.PMID 8606715
[5]Heier T, Caldwell JE, Sessler DI, et al Mild intraoperative hypothermia increases duration of action and spontaneous recovery of vecuronium blockade. Anesthesiology, 1991.PMID 1673591
[6]Bersten AD, Soni N (eds) Oh's Intensive Care Manual. 8th edition, Elsevier, 2018.

Thermoregulation, the three phases of perioperative heat loss, and active warming

Model answer plan (20 marks, 20 minutes)

1. Normal control (5 marks)

Core temperature is regulated near 37 degrees C with a narrow thermoneutral zone.^[1]

The hypothalamus is the thermostat, mediating heat loss (sweating, vasodilation) and heat production and conservation (vasoconstriction, shivering, non-shivering thermogenesis).^[1]

Afferents: central thermoreceptors sense core/blood temperature (the dominant input); peripheral skin thermoreceptors sense surface temperature and rate of change.^[1]

Threshold system: responses are triggered at defined thresholds with a narrow inter-threshold range in the awake person.^[1]

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

Heat loss at rest: radiation (the largest share), convection, evaporation from skin and respiratory tract, and conduction.^[1]

2. Effect of anaesthesia and the three phases (5 marks)

General anaesthetics greatly impair thermoregulation, reducing the thresholds for vasoconstriction and shivering; unwarmed anaesthetised patients become hypothermic.^[1]

Neuraxial anaesthesia also blocks the vasoconstrictor and shivering efferents to the legs and the hypothalamus misperceives the vasodilated legs as whole-body warmth, so hypothermia can be equally severe but less noticed.

Phase 1 (redistribution): the first hour, the steepest fall — core-to-peripheral redistribution when vasoconstriction is abolished. Cannot be treated, only prevented.^[1]

Phase 2 (linear decline): actual heat loss exceeding production over the next few hours. Reduced by active warming.^[1]

Phase 3 (plateau): core stabilises when vasoconstriction returns and loss equals production.^[1]

References6Show

[1]Sessler DI Perioperative thermoregulation and heat balance. Lancet, 2016.PMID 26775126
[2]Sessler DI Temperature monitoring and perioperative thermoregulation. Anesthesiology, 2008.PMID 18648241
[3]Frank SM, Fleisher LA, Breslow MJ, et al Perioperative maintenance of normothermia reduces the incidence of morbid cardiac events. JAMA, 1997.PMID 9087467
[4]Kurz A, Sessler DI, Lenhardt R Perioperative normothermia to reduce the incidence of surgical-wound infection and shorten hospitalization. New England Journal of Medicine, 1996.PMID 8606715
[5]Heier T, Caldwell JE, Sessler DI, et al Mild intraoperative hypothermia increases duration of action and spontaneous recovery of vecuronium blockade. Anesthesiology, 1991.PMID 1673591
[6]Bersten AD, Soni N (eds) Oh's Intensive Care Manual. 8th edition, Elsevier, 2018.