Question 20

Describe the mechanisms by which heat is lost from the body (40% of marks). Discuss the importance of each of these in a sedated and intubated adult patient  (60% of marks)

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College Answer

A satisfactory answer required use of terms radiation, convection, conduction etc. in the 
manner defined in the texts, rather than the layman’s use of the terms. Better answers 
displayed understanding of the meaning, relative importance and mechanism of methods of 
heat loss. 
The second part of the question required application of these concepts in patients with artificial 
airway and sedatives, particularly change in control of vascular tone and voluntary behavioural 
control.

Discussion

  • The total heat transfer of a human is the same as the metabolic heat production, and is approximately 100 watts, or 400 kJ/hr, or 60-70 kcal/hr at rest.
  • It occurs by:
    • Radiation (50%)
    • Convection (30%)
    • Evaporation (20%)
    • Conduction (usually 0%)
  • Radiation is heat transfer by emission of IR-spectrum electromagnetic radiation, 
  • Conduction is the transfer of heat energy to a lower-temperature object by direct surface contact
  • Convection is heat transfer by conduction to a moving gas or liquid, 
  • Evaporation is loss of heat energy to the latent heat of vapourisation of water. 

Impact of sedative agents on thermoregulation:

  • Effect of anaesthesia on interthreshold range:

    • Normal interthreshold range: 36.7–37.1°C (i.e. 0.4 °C)
    • With anaesthesia, widens to 4.0 °C, i.e 34.9 - 38.9 °C
  • Reduced total body thermogenesis by about 30%; due to:
    • Reduced skeletal muscle tone and voluntary contraction
    • Reduced respiratory work (mechanical ventilation)
    • Reduced thermodgenic activity of brown adipose tissue (by volatile anaesthics)
  • Increased conductive heat loss
    • Mostly from the core to the peripheries
    • Due to cutaneous vasodilation and blood flow redistribution
    • Vasodilated skin then acts as an excellent heat exchanger, facilitating heat loss in context of environmental influences:
  • Impaired thermoregulation:
    • Decreased cutaneous vasoconstriction by direct vasodilator effects of the drugs themselves as well as by decreased sympathetic activation
    • Shivering is inhibited, and completely abolished by muscle relaxants
    • Brown adipocyte thermogenesis is inhibited
    • Sweating is preserved, but occurs at a higher threshold temperature
    • Voluntary behavioural thermoregulation is made impossible

References

Gagge, A. Pharo, and Richard R. Gonzalez. "Mechanisms of heat exchange: biophysics and physiology." Comprehensive physiology (2010): 45-84.

Ravanelli, Nicholas, Coen CWG Bongers, and Ollie Jay. "The biophysics of human heat exchange." Heat Stress in Sport and Exercise: Thermophysiology of Health and Performance (2019): 29-43.

Sessler, Daniel I., and Michael M. Todd. "Perioperative heat balance." The Journal of the American Society of Anesthesiologists 92.2 (2000): 578-578.

Sessler, Daniel I. "Perioperative thermoregulation and heat balance." The Lancet 387.10038 (2016): 2655-2664.

Sessler, Daniel I., David S. Warner, and Mark A. Warner. "Temperature monitoring and perioperative thermoregulation." The Journal of the American Society of Anesthesiologists 109.2 (2008): 318-338.