Regulation of Body Temperature

Outline the mechanisms for heat transfer between the body and its environment.

Define the thermoneutral zone, and describe the mechanisms by which normal body temperature is maintained.

• Regulation of body temperature is done by balancing heat loss and heat production, predominantly through behavioural mechanisms and skin
• The body is able to maintain a relatively constant core temperature under a wide range of environmental conditions
  • The thermoneutral zone is the range across which the basal rate heat production (and oxygen consumption) is balanced by the rate of heat loss
    • For an adult it is typically 27-31°C
    • In neonates it is higher, typically 32-34°C.

Principles

Net flux of heat is determined by the balance of metabolic heat production and the contribution of four mechanisms of heat loss:

• Radiation
• Conduction
• Convection
• Evaporation

Radiative

Radiative heat exchange:

• Describes the loss of heat through EMR by all objects above 0°K
  • Radiative heat loss is proportional to temperature
  • Radiative heat loss does not require a transfer medium
• Makes up ~45% of heat loss under thermoneutral conditions.
• Depends on the temperature differential between an individual and their environment
  • A cold environment (e.g. operating theatre) causes a large radiant heat loss
    The heat loss from the patient is greater than the heat gain from the surrounding environment.

Conduction

Conduction is the transfer of heat (as kinetic energy) by direct contact from a higher temperature object to the lower temperature one. Conduction:

• Requires physical contact between bodies to conduct heat
  • Solids conduct heat better than gases
  • There is no conduction in a vacuum
• Heat loss via conduction is minimal in air but is a major cause of heat loss in immersion
  • As arteries and veins typically run next to each other, arterial heat tends to be transferred to the (cooler) veins, limiting further heat loss
    This is similar to counter-current exchange in the kidney.
• As fat is a poorer conductor of heat than muscle, increased body fat will slow heat loss by conduction

Convection

Convection is loss of heat by conduction by a moving object. Convection is:

• The predominant mechanism of heat loss in the naked human
  Effects are greater effects at higher wind speeds.

Evaporation

Evaporative losses describe the loss of heat energy due to the latent heat of vapourisation of water. Evaporation of 100ml of water will reduce body temperature by ~1°C.

Temperature Sensation and Regulation

Temperature sensors are central and peripheral, whilst regulation occurs centrally.

Central Sensation

Central temperature sensors exist in the:

• Abdominal viscera
• Spinal cord
• Hypothalamus
  Anterior hypothalamus is the most important central thermoreceptor, and responds to both increased and decreased temperatures by altering their rate of depolarisation, eliciting an array of neuronal and hormonal responses.
• Brainstem

The inter-threshold range is the range of core temperatures not triggering a response.

• Normal is 0.2 to 0.4°C.
• Widens under anaesthesia to ~4°C

Peripheral Sensation

Peripheral temperature sensors are:

• Free nerve endings
• Extremely sensitive
  Alter their rates of firing by orders of magnitude in response to temperature change.
• Divided into:
  • Cold receptors
    Lie beneath the epidermis, and are excited by cooling (inhibited by warming), active from 10-40°C, with a static maxima at 25°C.
  • Warm receptors
    Lie deep to the dermis, are excited by warming (and inhibited by cooling), active from 30-50°C, with a static maxima at 44°C.

Regulation

Temperature sensation runs from cutaneous receptors via the spinothalamic tracts and medulla to the hypothalamus. Cortical input is received via the thalamocortical relay, whilst primitive responses are effected via the midbrain.

Effector Responses

• Vascular changes are the least metabolically costly and can result in dramatic increases (up to 60% of cardiac output) in skin blood flow
• When environmental temperature exceeds body temperature, conduction and convection result in heat gain - evaporative cooling via sweating is the only way to reduce body temperature

• Efficacy of sweating is related to relative humidity

• Piloerection (hair standing on end) traps a layer of warm air close to the body to act as an insulator
  This is of more importance in other primates than in man, as they have enough body hair to make it effective.

• Increasing basal metabolic rate and 'waste' heat production is essential to maintain temperature in cold environments. This can be through:
  • Shivering
    The simultaneous contraction of agonistic and antagonistic muscles.
  • Non-shivering thermogenesis:
    • Hormonal
      Levels of thyroid hormone and adrenaline increase, raising metabolic rate in all cells
    • Brown fat
      Brown fat produces heat through uncoupled oxidative phosphorylation, which uses the electron transport chain to produce heat rather than ATP. Brown fat is:
      • A vital mechanism for heat production in the neonate (they have an immature shivering response), and forms ~5% of neonatal mass
      • Located in:
        • Neck
        • Supraclavicular
        • Interscapular
        • Suprarenal
      • Sympathetically innervated
        Contains large numbers of β₃ receptors

Effect of Anaesthesia

General anaesthesia causes a 1-3°C drop in core body temperature, which occurs in three phases:

• Rapid reduction
  Core temperature falls by 1-1.5°C in the first 30 minutes.
  • Predominantly due to vasodilation, which is due to:
    • Reduction in SVR, with generalised vasodilation and increased skin blood flow
      Heat redistribution is the major initial factor (rather than heat loss), as vasodilation leads to increased heat content of peripheries.
    • Impairs thermoregulatory vasoconstrictive responses
      Inter-threshold range is widened to 4°C (up from 0.4°C)
• Gradual reduction
  Further drop in core temperature of 1°C over following 2-3 hours.
  • Due to heat loss exceeding heat production
    Non-shivering thermogenesis is the only response available to paralysed, anaesthetised patient.
• Plateau
  Once core body temperature falls far enough, thermoregulatory responses are activated and further heat loss is attenuated by increased metabolic heat production.

Neuraxial anaesthesia:

• Hypothermia is less extreme as thermoregulation is only affected in areas covered by the blockade
• Plateau does not occur as vasoconstrictive responses are inhibited by the blockade

References

1. Auerbach. Wilderness Medicine. Sixth Edition. Chapter 4: Thermoregulation.
2. Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.
3. Diaz A. Define "thermoneutral zone". Briefly explain how the body regulates temperature when the ambient temperature exceeds the thermoneutral zone. Primary SAQs.
4. Buggy DJ, Crossley AW. Thermoregulation, mild perioperative hypothermia and postanaesthetic shivering. Br J Anaesth. 2000 May;84(5):615-28.