Heat Loss Flashcards
What is the primary mechanism by which general anesthesia induces hypothermia in the first hour?
Redistribution of heat from the core to peripheral compartments due to anesthetic-induced vasodilation.
How does the critical temperature differ between a preterm neonate and an adult?
Preterm neonate: 28°C; Adult: 1°C.
Explain why radiation accounts for ~60% of heat loss in the operating room.
Radiation transfers heat via infrared waves to cooler surrounding objects, proportional to the fourth power of the temperature difference (e.g., a 2°C drop increases loss by 16x).
Compare non-shivering thermogenesis in neonates vs. adults.
Neonates rely on brown adipose tissue for non-shivering thermogenesis due to limited muscle mass. Adults primarily use shivering.
Design a protocol to prevent hypothermia in a preterm neonate during surgery.
- Warm OR to ≥24°C. 2. Use forced-air warming. 3. Administer warmed IV fluids. 4. Cover with impervious drapes. 5. Use overhead radiant heaters during induction.
Why is pethidine (0.3 mg/kg IV) effective for postanesthetic shivering?
Pethidine inhibits shivering centrally without suppressing respiratory drive, but active warming is still required to address hypothermia.
Evaluate the statement: ‘Humidifying inspired gases significantly warms hypothermic patients.’
False: Respiratory heat loss accounts for <10% of total loss. Humidification minimizes airway heat loss but does not actively rewarm.
What physiological change explains the 1°C core temperature drop within 30-60 minutes of anesthesia induction?
Vasodilation redistributes heat from the core (thoracic/abdominal organs) to cooler peripheral tissues (limbs/skin).
Analyze the clinical impact of a core temperature of 35°C on drug metabolism.
Hypothermia slows metabolism: vecuronium duration doubles, propofol plasma concentration increases 30%, and MAC for inhaled anesthetics decreases 5%/°C.
Which temperature measurement site is least reliable for core temperature monitoring?
Rectal or bladder temperatures, due to poor perfusion and lag in reflecting rapid core changes.
Propose a treatment plan for intraoperative malignant hyperthermia.
- Stop triggering agents. 2. Administer dantrolene. 3. Cool with IV fluids, surface cooling, and cold irrigation. 4. Monitor electrolytes/acidosis.
Explain why forced-air warming is more effective than passive insulation.
Forced-air actively transfers heat to the periphery, counteracting redistribution. Passive insulation only reduces conductive/convective loss by ~30%.
How does anesthesia alter the inter-threshold range for thermoregulation?
General anesthesia increases the inter-threshold range from 0.2–0.5°C to 5°C, delaying vasoconstriction/sweating responses.
Justify the use of warmed IV fluids during major surgery.
1 L of room-temperature IV fluid lowers core temperature by ~0.25°C. Warming prevents iatrogenic hypothermia and stabilizes metabolism.
Critique the statement: ‘Shivering increases oxygen consumption but does not cause hypoxemia.’
True: Shivering increases O₂ demand, but hypoxemia itself inhibits shivering, preventing a vicious cycle.
Identify three factors that increase evaporative heat loss in neonates.
- Thin skin. 2. High surface-area-to-volume ratio. 3. Use of skin-preparation solutions in the OR.
Create a differential diagnosis for intraoperative hyperthermia (Temp >38°C).
- Sepsis. 2. Malignant hyperthermia. 3. Thyroid storm. 4. Transfusion reaction. 5. Drug-induced hyperthermia.
Why is the thermoregulatory range narrower in neonates than adults?
Neonates have immature hypothalamic control, limited shivering capacity, and rely on non-shivering thermogenesis, which is easily overwhelmed.
Assess the risk of hypothermia in a patient under spinal anesthesia.
Spinal anesthesia blocks vasoconstriction, preventing the plateau phase of hypothermia. Risk is higher due to continuous heat loss without compensation.
What intervention addresses both conductive and convective heat loss?
Using forced-air warming blankets, which reduce convective loss by heating ambient air and conductive loss by warming skin contact surfaces.
Front
Back
Define normothermia and its clinical significance.
Normothermia is a core body temperature of 36.5–37.1°C, maintained within 0.2°C to ensure optimal enzyme function and physiological stability.
How does the peripheral compartment contribute to thermoregulation?
It acts as a heat sink, absorbing or releasing thermal energy to maintain the core temperature, with its temperature varying between 36°C and 28°C.
What are the physiological mechanisms that regulate body temperature?
Behavioral, hormonal, and neuronal responses integrate through hypothalamic control based on afferent thermal inputs from skin, viscera, and spinal cord.
