Question 16

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

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College Answer

Sedation reduces body temperature by interfering with heat production and increasing heat loss, along with widening of hypothalamic inter-threshold range. This portion of the question was generally well answered. The question asked to "outline" the answer. Many candidates actually "described" the thermoregulation process in general but were unable to relate those with the impact of sedation. The second part of the question (physiological effect of low body temperature) was answered by most of the candidates with the structure of organ-system wise description. A few candidates scored extra marks by relating these effects with degree of hypothermia and by describing how thermogenesis responses (including shivering) can influence those effects. Some candidates restricted their answers to the effect of thermogenesis in response hypothermia and did not include the overall physiological consequences of low body temperature. Better answers displayed an understanding of core temperature regulation, inter-threshold range and the effects of sedatives on thresholds for thermogenic responses, although only a few mentioned gain and maximal response. Better answers included specific detail (mentioned bradyarrhythmia, slow AF, VF, prolonged PR/QRS / J waves rather than just stating arrhythmia) across several organ systems. Marks were not awarded for generic statements such as 'decreased liver function' without some additional detail. Inadequate depth of knowledge was main reason behind overall poor scores.

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.