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Mechanism of Intraoperative Hypothermia

Notes for MD Anaesthesiology Examination

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Definition

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Normal Thermoregulation (Baseline)

• The hypothalamus maintains core body temperature between 36.5°C and 37.5°C under normal conditions
• Heat production = heat loss under steady state
• Mean body temperature: Tmean = 0.85 × Tcore + 0.15 × Tskin
• Thermoregulatory effectors: vasoconstriction, vasodilation, shivering, sweating, non-shivering thermogenesis

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Mechanisms of Intraoperative Hypothermia

1. Anesthetic-induced impairment of thermoregulation
2. Redistribution of body heat (core → periphery)
3. Excess heat loss via four physical mechanisms
4. Additional surgical and environmental factors

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1. Anesthetic Impairment of Thermoregulation

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2. Core-to-Periphery Heat Redistribution (Phase 1 — Most Important)

1. Anesthesia (particularly volatile agents and propofol) causes peripheral vasodilation
2. Blood flow increases from the core to the previously vasoconstricted peripheral compartments (limbs, skin)
3. Warm core blood perfuses the cooler peripheral tissues
4. Heat redistributes from the core to the periphery
5. Core temperature drops despite total body heat content remaining relatively unchanged

Anesthetic-induced vasodilation redistributes blood from the core to the periphery, where heat is rapidly lost via radiation to the surrounding environment. — Sabiston Textbook of Surgery, p. 351

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3. Heat Loss via Physical Mechanisms (Phase 2 — Sustained Loss)

A. Radiation (50–70% of total heat loss — largest component)

• Transfer of heat as infrared electromagnetic waves from the skin surface to cooler surrounding objects
• Does not require a medium or contact
• Proportional to the temperature gradient between skin and environment
• Operating theatre temperature (typically 18–21°C) greatly increases radiative loss
• Skin exposure (draping, open body cavities) amplifies this

B. Convection (25–30%)

• Heat loss due to ambient air currents passing over the exposed body surface
• Air-conditioned OR environments with laminar airflow dramatically increase convective loss
• Cool, moving air sweeps away the warm boundary layer of air around the patient

C. Evaporation (10–15%)

• Heat loss when water vaporizes from exposed skin, mucous membranes, and open body cavities
• For every gram of water evaporated, 0.58 kcal of heat is lost
• Sources: respiratory tract (unhumidified dry anesthetic gases), open abdominal/thoracic wounds, wet skin prep (antiseptic solutions)
• Greater in prolonged or open abdominal/thoracic surgeries

D. Conduction (5%)

• Transfer of heat by direct contact with cooler objects
• Cold operating table surface, cold IV fluids, cold irrigation fluids, unwarmed blood products
• Administration of large volumes of room-temperature (21°C) or refrigerated (4°C) IV fluids significantly adds to conductive heat loss

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4. Additional Surgical & Environmental Factors

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Phases of Intraoperative Temperature Change

Temperature
     │
36.5°│──────────╮
     │           ╲   Phase 1: Rapid redistribution
36.0°│            ╲  (first 45–60 min; ~1–1.5°C drop)
     │             ╲___________
35.5°│                         ╲  Phase 2: Slow linear decline
     │                          ╲ (heat loss > heat production)
35.0°│                           ╲__________
     │                                      ╲  Phase 3: Plateau
34.5°│                                       ──── (new equilibrium;
     │                                            vasoconstriction-limited)
     └──────────────────────────────────────────────► Time
       0      1 hr      2 hr      3 hr       4 hr

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Consequences of Intraoperative Hypothermia

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Temperature Monitoring Sites

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Prevention

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Key Exam Points (Summary)

1. Definition: Core temperature < 36°C during surgery
2. Most rapid drop: Phase 1, due to core-to-peripheral redistribution from anesthetic vasodilation
3. Largest absolute heat loss mechanism: Radiation (50–70%)
4. Heat loss for each gram of water evaporated: 0.58 kcal
5. Both GA and neuraxial anesthesia impair thermoregulation by different but complementary mechanisms
6. Anesthesia effect: Expands interthreshold range from ~0.2°C to ~4°C
7. Coagulation impaired at 33–35°C
8. Most effective active warming: Forced-air warming blanket
9. Best core temperature monitoring site in anesthetized patient: Distal esophagus or nasopharynx

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