Question 10

Outline the impact of sedative agents on thermoregulation (40% marks). Describe the physiological effects of a low body temperature (60% marks)

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College Answer

The first part of the question required candidates to outline the impact of sedatives on the interthreshold range with an explanation of what this is, how heat is lost, how heat generation is impaired and the mechanism by which these occur (ie. radiation/conduction/convection via vasodilation, with absence of vasoconstriction/heat generation strategies). The second part of the question required a systems based approach with an outline of the perturbation as a result of the low body temperature. Temperature thresholds for certain physiological effects ie. loss of consciousness or arrhythmia was also expected for an overall thorough answer to this question.

Discussion

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

Hypothermia:

  • Endocrine and metabolic consequences
    • Decreased metabolism and oxygen consumption
    • Decreased carbohydrate metabolism and hyperglycaemia
    • Essentially unchanged electrolytes
  • Haematological consequences
    • Increased hematocrit and blood viscosity
    • Neutropenia and thrombocytopenia
    • Coagulopathy and platelet dysfunction
  • Respiratory consequences
    • Decreased respiratory rate and medullary sensitivity to CO2
    • Acid-base changes: alkalosis and hypocapnea
    • Rise of pH with falling body temperature
    • Fall of PCO2 with falling body temperature
    • Increased oxygen solubility and O2-haemoglobin affinity
  • Pharmacological consequences
    • Delayed absorption
    • Decreased drug metabolism, especially hepatic metabolism
    • Delayed hepatic and renal clearance
    • Poorer affinity of receptors (eg. for catecholamines)
  • Cardiovascular consequences
    • Decreased cardiac output and bradycardia
    • QT prolongation and the J wave
    • Arrhythmias - classically AF and VF
    • Resistance to defibrillation
    • Vasoconstriction
  • Renal consequences
    • "Cold diuresis" due to decreased vasopressin synthesis
  • Central nervous system effects
    • Confusion and decreased level of consciousness
    • Shivering
    • Increased seizure threshold
  • Immunological consequences
    • Decreased granulocyte and monocyte activity

References

Wong, K. C. "Physiology and pharmacology of hypothermia." Western Journal of Medicine 138.2 (1983): 227.

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.

Conway, Aaron. "A review of the effects of sedation on thermoregulation: insights for the cardiac catheterization laboratory." Journal of PeriAnesthesia Nursing 31.3 (2016): 226-236.

Clark, Robert E., Louis R. Orkin, and E. A. Rovenstine. "Body temperature studies in anesthetized man: effect of environmental temperature, humidity, and anesthesia system.Journal of the American Medical Association 154.4 (1954): 311-319.