Canonical Question
Therm – Responses
Master answer
Impact of sedative agents on Thermoregulation
3 phases:
- Phase I – 1st hour – Redistribution of heat from core → periphery
- significant decrease in core temperature from 0.5 to 1.5 C
- thermoregulatory center is depressed by anaesthesia
- Contributors:
- undressed
- skin prep
- evaporative loss
- induction → cutaneous vasodilation
- cool IVF
- cool dry ventilation of lungs
- temp drop in spinal > epidural
- Phase II – Hour 2 to 3 – Slow linear decrease in core temperature
- Increased heat loss
- anaethetic induced peripheral vasodilation
- → increased radiant & evaporative loss.
- → redistribution of heat from core to periphery
- Decreased heat production
- NMBD mean cannot shiver
- anaesthesia means cannot get clothes or eat or curl up.
- Increased heat loss
- Phase III – 3+ hours – Plateau in core temperature
- core temperature plateaus
- peripheral temperature continues to decrease.
Mechanisms of ↓ in core body temperature:
- Resetting of interthreshold range (phase II)
- ↑ width of range to 4 °C → ↓ threshold to cold by 3 °C and ↑ threshold to heat by 1° C → ↓ thermoregulatory responses to Δ in body temperature
- Caused by a central effect → GA agent interferes with normal hypothalamic function to maintain a narrow threshold range
- GA agent-induced vasodilation → redistribution of heat from central to peripheral compartments (phase I)
- Muscle paralysis → loss of shivering response and muscle activity (phase II)
- LOC and paralysis → loss of behavioural responses (phase II)
- GA-induced ↓ BMR/heat production (phase II)
- Cold gases and IVF (phase II) → temp. generally ↓ 0.25-0.5 °C/L of IVF
Physiological effects of low body temperature
| CVS | – Tachycardia initially, then progressive bradycardia with ↑ cold – ↑ cardiac arrhythmias and myocardial ischaemia due to catecholamines released from stress response – ↑ SVR and MAP due to peripheral vasoconstriction – ↓ C.O. due to direct –ve inotropic effect of cold and ↑ afterload/SVR |
| Respiratory | – Left-shift in Hb O2 dissociation curve – V/Q mismatching 2° to inhibition of hypoxic pulmonary vasoconstriction – Bronchospasms – ↓ MV (and in severe cases apnoea) – ↑ solubility of gases (incl volatiles) |
| CNS | – Altered mental state (esp ↑ drowsiness, unconsciousness and delayed awakening from GA) – ↓ CBF |
| Haematological and immunological | – Coagulopathy (due to platelet dysfunction and loss of CF enzyme function) → ↑ transfusion requirements due to bleeding – ↓ WBC activity → ↑ incidence of infections |
| Hepatic / renal | – Impaired renal function (oliguria) – Impaired hepatic metabolic function (esp ↓ drug metabolism, such as muscle relaxants) |
| Metabolic and endocrine | – Impaired wound healing (due to catabolic state and wound vasoconstriction) – ↑ protein catabolic state – ↑ stress response (steroids and catecholamine released) – Shivering → 5x ↑ general MRO2 → causes hypoxaemia (risk of myocardial and cerebral ischaemia) – In absence of shivering → general ↓ MRO2 (by up to 50%) and ↓ BMR |
| Others | – ↑ morbidity and mortality rate (due to above reasons) – Hypothermia may be beneficial during cerebral or cardiac ischaemia as it ↓ metabolic O2 requirements (provided shivering response is blunted) – ↓ anaesthetic requirements (MAC-sparing) – ↓ triggering and severity of MH |
JF / Bianca 2016
Exam appearances
| Exam | Exact wording | Relationship | Success |
|---|---|---|---|
| 2022A Q16 | Outline the impact of sedative agents on thermoregulation (40% marks) and describe the physiological effects of a low body temperature (60% marks). | historical_member | 33.00% |
| 2023B Q10 | Outline the impact of sedative agents on thermoregulation (40% marks). Describe the physiological effects of a low body temperature (60% marks). | historical_member | 66.00% |