#Temperature regulation

#Definitions

Core body temperature

• the temperature of the body's deep tissues and organs
• maintained by hypothalamus at 37 +/- 0.5C

Interthreshold range

• the range of core body temperatures when thermoregulatory responses do not occur
• 37 +/- 0.2°C in normal adult (i.e. 0.4°C)
• 37 +/- 2°C in anaesthetised adult (i.e. 4°C)

Thermoneutral zone (TNZ)

• the range of environmental temperatures where core body temperature is maintained by vasomotor activity without a significant change in metabolic rate
• i.e. there is no active heat production (shivering, non-shivering thermogenesis) or active heat dissipation (sweating, behavioual response)
• 27-31°C in a naked adult, lower if clothed

From Solar radiation by Amy L. Norris and Thomas H. Kunz, licensed under CC-BY 3.0

#Outline the mechanisms of heat production

The body generates heat by increasing metabolic activity through various mechanisms.

• Voluntary muscle activity

  • e.g. moving on the spot when you are cold

• Involuntary mechanisms work by increasing basal metabolic rate (BMR) and kick in below the thermoneutral zone

  • shivering
  • non-shivering thermogenesis
  • endocrine response
  • hunger

#Shivering

Rapid, oscillatory muscle activity that greatly increases heat production.

Comprised of

• cyclical contractions up to 250Hz - likely peripherally mediated
• superimposed slow synchronised waves - likely centrally mediated
• rigors appear when shivering is more intense (10-20Hz)

Shivering can increase BMR by 100% in adults.

#Non-shivering thermogenesis

Mediated by beta3 receptors in brown adipose tissue and skeletal muscle.

Mechanism

• brown fat has rich vascular supply and adrenergic innervation with abundant mitochondria
• lipolysis liberates substrates for the electron transport chain (ETC)
• oxidative phosphorylation is uncoupled
  • energy from ETC is not used to produce ATP and is instead dissipated as heat

In newborns, brown fat is present in interscapular, perinephric areas, and around the great vessels. Non-shivering thermogenesis can triple BMR in newborns.

#Endocrine response

Thyrotropin releasing hormone (TRH) is released.

• ↑TRH → ↑TSH → ↑T3/T4
• thyroid hormones increase metabolic activity in virtually all tissues, generating heat

#Hunger

The hunger response is mediated by the hypothalamus in mild/early cold exposure.

The goal is to restore energy stores to maintain heat production.

Hunger may disappear in severe hypothermia due to impaired consciousness and slowing of metabolic processes.

#Describe how thermoregulation is altered in a critically ill patient

Sedated, mechanically ventilated patients can lose 3-4°C if not actively warmed. Heat loss occurs through multiple pathways.

#Airway

• Endotracheal tube exposes the airway to the environment → significant evaporative and convective heat loss
• Inspired gases must be actively heated and humidified to reduce further loss

#Breathing

• Mechanical ventilation decreases work of breathing → less heat production
• Mandatory ventilation prevents autoregulation of respiratory rate and tidal volume
  • hyperventilation increases evaporative and convective heat loss

#Circulation

Vasodilators

• many drugs commonly used in ICU are vasodilators
  • e.g. sedatives (propofol)
  • e.g. vasoactivers (GTN, SNP)
• sedatives also blunt the normal vasoconstrictive response to cold exposure
• results in increased convective heat loss at the skin

Vasopressors

• decrease convective heat loss through the skin by causing cutaneous vasoconstriction (e.g. noradrenaline, vasopressin)

Inotropes

• augmentation of cardiac output increases cutaneous blood flow, increasing convective heat loss (e.g. milrinone)

#Drugs

Sedation

• eliminates voluntary heat-preserving behaviours (seeking warmth, covering up)
• inhibits mechanisms that prevent heat loss
  • circulatory effects as above
  • inhibition of piloerection (goosebumps), which would otherwise trap warm air near the skin
• also inhibits mechanisms that increase heat loss (e.g. sweating), but this is not commonly an issue in ICU
• interthreshold range widens to 4°C (35-39°C)
  • active heat production is inhibited within this range, preventing:
    • shivering
    • non-shivering thermogenesis
    • endocrine response
    • maximal vasoconstriction may not overcome heat loss by other mechanisms

Paralysis

• prevents heat production due to voluntary and involuntary muscle activity

Metabolic rate may be altered by administration of some drugs

• e.g. thyroxine → ↑BMR → heat production

#Environment

Convection

• critical care environments (ICU, operating theatre) are typically cool
• patients wear thin gowns and have limited blankets
• many patients have increased exposure to the environment due to:
  • open abdomen or chest
  • extracorporeal circuits (dialysis, ECMO)
  • wounds or burns
  • large bore surgical drains

Evaporation

• diaphoresis in critically ill patients increases evaporative heat loss

Conduction

• contact with cool surfaces (i.e. bed in ICU or operating theatre)