Temperature Responses — Post-op Hypothermia

• Temperature
  • average kinetic energy of the atoms and molecules that make up a substance
• Mild hypothermia
  • Temp <35 degrees
• Interthreshold range
  • the range of core temperature over which no autonomic thermoregulatory responses occur.
  • Under normal conditions: 36.8 ~37.2 degrees
• Under anaesthesia
  • Lower threshold decreases 2.5 degrees and upper threshold increases 1.3 degrees
  • Therefore decreased ability to maintain core normothermia

Physiology

Cardiovascular system

• Decreased cardiac output
  • Peripheral vasoconstriction
    • Increased afterload
  • Decreased diastolic relaxation
    • Decreased preload
  • Decreased contractility
  • Increased blood viscosity
    • Haemoconcentration
      • Cold diuresis: Due to decreased responsiveness to ADH and shunting of peripheral blood to central circulation
      • Extravasation of intravascular H₂O
  • Increased risk of myocardial ischaemia
• Coagulopathy
  • PT and aPTT increase 9% at 34c compared to 37c
• Increase incidence of atrial arrythmias → ventricular fibrillation at severe hypothermia
• Every degree decrease in temp increases pH 0.015 and decreases pCO₂ 4.4%

Respiratory

• Initially tachypnoea due to sympathetic stimulation
• Decreased responsiveness to CO₂
• Oxyhaemoglobin dissociation curve shifts to left → decreased tissue O₂ delivery

CNS

• CMRO₂ decreases 7% per 1 degree decrease in core body temperature

GI

• Decreased GI motility

Other

• Post-operative shivering increases total body O₂ consumption
• Decreased enzyme activity
  • Decreased temperature decreases rate constant in Arrhenius equation
• Decreasesd neutrophil degranulation
• Impaired macrophage phagocytosis

Pharmacology

Pharmacodynamics

• Arrhenius equation for rate constant K
  • Where T = temperature
    • Therefore as T decreases, the rate constant decreases
    • Drug-Receptor interactions are slowed as temperature decreases

Pharmacokinetics

• Decreased absorption of drugs due to delayed gut transit times
• Metabolism of drugs reduced → slow offset of sedative agents given in anaesthesia
• Decreased cardiac output → decreased renal filtration à decreased drug elimination of unchanged drugs → slow offset
• Specific drugs
  • Prolonged muscle relaxation from non-depolarizing
  • Opioids blunt shivering thermogenesis mechanism → decreased ability to thermoregulate

Sakurai 2016

Answer by Sakurai