Therm – Heat Transfer

Master answer

Definitions

• Thermoneutral zone:
  • The range of environmental temperatures in which the metabolic heat production (and oxygen consumption) is minimal and steady and where core temperature is maintained by vasomotor activity alone.
  • 25-30 C for a naked, upright man in still air
• Inter-threshold range:
  • The range of core temprature over which no autonomic thermoregulatory responses occur
  • Normally 0.2 -0.4 °C in a non-anaesthetized state

Mechanisms of Heat production

Total body heat production = Metabolic rate – Total external work

1. Metabolism of food to form ATP
   • <100% efficient
2. Transfer of ATP to functional systems of cells
   • <100% efficient
3. Basal metabolism of cells
4. Muscle contraction
   • Overcoming resistance in muscles and other tissues -> friction
     • Useful in shivering
5. Pumping blood
   1. Overcoming shear forces -> friction
6. Breaking chemical bonds
7. Flow of ions across membranes

Mechanisms of Heat loss

• Heat is lost by conduction, convection and radiation and evaporation
• The movement of warm blood from the body to skin allows heat loss to the environment

1. Conduction:
   • Heat transfer occurs due to direct contact to another solid body
   • Comprises 3% of total heat loss
2. Convection:
   • Heat is lost to air via conduction
   • This air then moves away, restoring the temperature gradient
   • Comprises 15% of total heat loss
3. Radiation:
   • Heat is emitted from the skin as infrared radiation
   • Comprises 60% of total heat loss
4. Evaporation:
   •  There is a distribution of kinetic energies in liquids
   • Warmer particles transition to the gas phase, lowering the average temperature of the skin
   • Sweat, comprises 22% of total heat loss

Importance in sedated and intubated patient

Radiation:

• Heat loss increased
  • if the patient is uncovered and surrounded by cold objects.
  • by vasodilatation due to sedative drugs or cholinergics.
  • by cool ambient temperature
  • in Extracorporeal circuits
• Heat loss Decreased
  • by vascoconstriction due to adrenergics

Conduction:

• usually a less important route
• Heat loss increased by use of cold irrigating solutions, and in extracorporeal circuits

Convection:

• Heat loss increased
  • if the patient is uncovered
  • by cool ambient temperature
  • in Extracorporeal circuits
• Heat loss decreased
  • Use of Neuromuscular blocking drugs (NMBDs can inhibit shivering, hence decreasing possible convective heat loss via bodily movement)

Evaporation:

• increased if a body cavity is opened, especially if ambient humidity is low.
• Heat loss via evaporation in the trachea and airways may be considerable if inspired gases are not humidified.
• Drugs affecting vascular tone can also alter heat loss by evaporation

Other:

• impaired temperature regulation
  • peripheral (e.g. vasodilatation, shivering and impaired piloerection)
  • central (central effects of drugs)
• Impairment of neurological or cardiovascular function, or deficiencies of substrate for thermogenesis → alter body’s ability to maintain thermal homeostasis

Mooney / JC 2019

Answer by JC, Mooney