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Temperature Management and Warming
TEXTIntraop II · 9 min read
Every 1°C below 36 doubles wound infection risk + impairs coagulation. Forced-air warming is the gold standard.
After this lesson you can
3 min read8 sections- Recall normothermia targets + active warming techniques.
- Identify temperature drop mechanisms intraop.
- Manage hypothermia and hyperthermia.
- Distinguish therapeutic vs incidental hypothermia.
Heat loss mechanisms
Patient loses heat by four physical mechanisms during anesthesia: RADIATION 60% (largest, infrared heat to cooler surroundings), CONVECTION 15% (air currents over skin), EVAPORATION 10% (insensible losses + open body cavity + skin prep), CONDUCTION 5% (contact with cold OR table/IV fluids), and RESPIRATION 10% (humidifying cold dry gases).
Anesthetic vasodilation triggers Phase I redistribution: warm core blood shunts into the cooler peripheral compartment — drops core temp 1-1.5°C in the first hour.
Phase II: slower linear loss from radiation + convection ~0.5°C/hr.
Phase III: steady state once core 33-34°C and skin vasoconstriction returns.
Consequences of perioperative hypothermia
1°C below 36 (Kurz NEJM 1996, foundational evidence).35°C; clotting factor enzymatic activity drops ~10% per °C.Prolonged drug effect — volatile MAC decreases ~5% per °C, NMB metabolism slows.
POST-OP SHIVERING at emergence: 5× O₂ consumption + cardiac demand — dangerous in CAD, fragile elderly.
Cardiac events: 3-fold increased MI rate in hypothermic CAD patients.
Increased blood loss + transfusion.
Prolonged PACU stay.
Patient discomfort + dissatisfaction as a quality metric.
Forced-air warming (Bair Hugger + equivalents)
Lower-body, upper-body, or full-body covers.
Effective warming rate ~0.5-1.0°C/hr.
Lower-body warmer is most common because surgical site is usually upper torso/abdomen.
Concerns about laminar-flow OR contamination from forced-air warmers have been largely refuted by SSI data — they remain standard.
Under-body warming mattresses (HotDog, Augustine) are an alternative, safer in burn or large-skin-loss patients where blowing air across raw skin is harmful.
Warmed IV fluids + blood products
21°C drops adult body temperature ~0.25°C.Warmed IVF (37-39°C) prevents this cooling.
- Hotline (counterflow heat exchanger)
- Ranger (foil heat plates)
- Belmont and Level 1 (high-flow rapid infusers for massive transfusion)
CRITICAL in massive transfusion — cold refrigerated blood products at 4°C dropped through a 16-gauge IV will rapidly cause severe hypothermia, coagulopathy, ionized hypocalcemia from citrate, and hyperkalemia.
Cold-blood platelet dysfunction is reversible with warming.
Pre-warming — the single biggest intervention
Mechanism: warms the peripheral compartment so anesthetic-induced vasodilation does NOT cause Phase I redistribution drop (there's no thermal gradient to redistribute).
Drops the average core temperature loss by 50% or more.
Operationally: bring patient back early, drape with forced-air warmer in pre-op holding before induction.
- warm blankets
- OR temperature ≥
21°C(warmer for pediatric, burn, prolonged cases) - humidification of inspired gases for cases >2 hr
Monitoring sites + accuracy
- ESOPHAGEAL (lower 1/3, gold standard for cardiac surgery, fast response)
- NASOPHARYNGEAL (brain surrogate, useful but can be inaccurate with leak around uncuffed ETT)
- BLADDER (rapid response in urine-output cases)
- PA CATHETER (cardiac output stream temperature, gold standard but invasive)
- TYMPANIC (infrared, can be inaccurate)
SKIN + ORAL are not core temperature — they reflect peripheral compartment and lag behind core.
Target 36-37.5°C through the entire case.
Below 36 = active hypothermia, intervene.
Above 38 = check for MH (ETCO2!), infection, drug effect, transfusion reaction.
Special populations
Warm OR to 24-26°C, plastic wrap exposed skin, warmed prep solution, force-air, monitor closely.
28-32°C for large burnsLARGE-CAVITY (open abdomen, thoracotomy): radiation + evaporation losses high; covers all exposed surfaces.
Therapeutic hypothermia + intentional cooling
32-36°C × 24 hr (TTM trial) for neuroprotection — though most modern protocols target 36°C (less variation than 32-34°C with similar neuro outcomes).CARDIAC SURGERY with cardioplegic arrest: 28-34°C systemic cooling on bypass for myocardial protection.
NEUROSURGERY for cerebral aneurysm / circulatory arrest: 18-22°C deep hypothermia for cerebral protection.
Avoid INADVERTENT hypothermia in routine cases.
Document target + actual temperature in cases where cooling is therapeutic.
⚠ Common pitfalls
- Treating shivering with meperidine without ruling out other causes — anaphylaxis, transfusion reaction.
- Forgetting that the first hour of GA drops core temp 1-2 °C from redistribution.
- Active warming on a stable normothermic — risks overshoot to hyperthermia.
- Ignoring temperature monitoring as 'optional' — ASA standard when changes anticipated.
💎 Clinical pearls
- Forced-air warming (Bair Hugger) is the most effective intra-op warming modality.
- Pre-warming 30-60 min pre-induction reduces post-induction temperature drop.
- Hyperthermia ddx: infection, MH, transfusion reaction, anticholinergic toxicity, warming overshoot.
- Therapeutic hypothermia (33-36 °C) for cardiac arrest survivors; otherwise aim normothermia.
Recap
- Forced-air warming (Bair Hugger) is the most effective intra-op warming modality.
- Pre-warming 30-60 min pre-induction reduces post-induction temperature drop.
- Hyperthermia ddx: infection, MH, transfusion reaction, anticholinergic toxicity, warming overshoot.
- Therapeutic hypothermia (33-36 °C) for cardiac arrest survivors; otherwise aim normothermia.
Mark each section done to complete the module.