Maintaining normothermia in surgical patients is critical for reducing complications associated with both hypothermia and hyperthermia. Anesthesia providers have several strategies at their disposal, ranging from passive warming techniques to active warming devices, which are key in preventing significant temperature fluctuations during surgery.
Pre-warming Techniques
Pre-warming patients before induction of anesthesia has been shown to reduce the incidence of perioperative hypothermia. Warming patients with forced-air warming devices for as little as 10 to 30 minutes before induction (longer periods add little, although 30 to 60 minutes is reasonable when the schedule allows) helps increase the body’s peripheral heat content, thereby reducing the core-to-periphery temperature gradient that typically leads to heat redistribution and hypothermia following anesthetic induction.
Intraoperative Warming Methods
Active warming methods during surgery are essential to maintaining normothermia, particularly during long procedures or in patients who are at higher risk for hypothermia. Common techniques include:
- Forced-Air Warming Systems: These devices blow warm air over the patient’s body, maintaining a stable core temperature even during procedures involving large incisions or exposure to a cool environment. Forced-air warming is the best-studied active method and reduces hypothermia and its complications compared with passive insulation; circulating-water garments and resistive (electric) heating blankets are effective alternatives. A single layer of passive insulation cuts cutaneous heat loss by only about 30% and does not prevent hypothermia on its own.
- Warmed IV Fluids: Administering warmed IV fluids is especially important for surgeries involving large fluid shifts or blood transfusions, as cold fluids can rapidly decrease core body temperature: each liter of room-temperature crystalloid, or each unit of refrigerated blood, lowers mean body temperature by about 0.25 C in an adult.
- Warmed Irrigation Fluids: For surgeries involving body cavities (e.g., abdominal or thoracic surgeries), using warmed irrigation fluids can prevent core temperature drops.
- Humidified and Warmed Gases: Anesthesia providers can also maintain normothermia by using humidified and warmed anesthetic gases, which help minimize evaporative heat loss from the airway, particularly in patients undergoing prolonged mechanical ventilation. Because less than 10% of metabolic heat is lost through the respiratory tract, airway heating and humidification add little in adults and matter mainly in infants and small children; a heat and moisture exchanger is usually sufficient.
Postoperative Temperature Management
Following surgery, ongoing temperature monitoring and warming are critical. Patients emerging from anesthesia may still be at risk for hypothermia, especially if their core temperature dropped significantly during the procedure. Forced-air warming devices and warmed blankets should continue to be used in the post-anesthesia care unit (PACU) until patients return to normothermia.
| Phase | Method | What it achieves |
|---|---|---|
| Before induction | Forced-air warming for 10 to 30 minutes before induction (up to 60 minutes when the schedule allows) | Raises peripheral heat content and narrows the core-to-periphery gradient that drives redistribution hypothermia after induction. |
| Intraoperative | Forced-air warming systems | Warm air blown over the body holds core temperature stable through large incisions and exposure to a cool environment. |
| Intraoperative | Warmed intravenous fluids | Prevents the rapid fall in core temperature that cold fluid causes during large fluid shifts or blood transfusion. |
| Intraoperative | Warmed irrigation fluids | Prevents core temperature drops when body cavities are opened, as in abdominal or thoracic surgery. |
| Intraoperative | Humidified and warmed anesthetic gases | Minimizes evaporative heat loss from the airway; a minor contributor in adults (under 10% of heat loss), more useful in infants and small children. |
| Postoperative | Forced-air warming devices and warmed blankets in the PACU | Continued until the patient returns to normothermia, since core temperature may still be low on emergence. |
Malignant Hyperthermia Management
While rare, malignant hyperthermia is a critical temperature regulation emergency that anesthesia providers must be prepared to manage. It requires prompt recognition, discontinuation of the triggering agent, hyperventilation with 100% oxygen, and intervention with dantrolene 2.5 mg/kg IV, repeated until the hypermetabolic state resolves; dantrolene works by blocking calcium release through the skeletal muscle ryanodine receptor. Cooling measures, including cold IV fluids and external cooling devices, are started when core temperature exceeds 39 C and stopped at 38 C, while hyperkalemia, acidosis, and arrhythmias are treated as they arise. After the crisis, dantrolene is continued (about 1 mg/kg every 4 to 6 hours, or 0.25 mg/kg per hour, for at least 24 hours) because recrudescence occurs in up to a quarter of cases.
