Residency · Residency · Anesthesiology
Temperature Monitoring and Perioperative Thermoregulation
Introduction
Perioperative hypothermia is one of the most common yet under-recognized complications of anesthesia, affecting up to 70% of surgical patients who are not actively warmed. Understanding the physiology of thermoregulation and its alteration under anesthesia is essential for safe practice. Even mild hypothermia (core temperature 34 to 36 degrees Celsius) increases wound infection rates, prolongs coagulation, and extends recovery.
Physiology of Normal Thermoregulation
The Thermoregulatory System
The hypothalamus serves as the central thermoregulatory controller, maintaining core temperature within a narrow interthreshold range of approximately 0.2 degrees Celsius. Afferent input arises from thermoreceptors in the skin, deep tissues, spinal cord, and hypothalamus itself. Efferent responses to cold include arteriovenous shunt vasoconstriction, nonshivering thermogenesis (in neonates), and shivering. Efferent responses to heat include active vasodilation and sweating. Core temperature is normally maintained at approximately 37.0 degrees Celsius with circadian variation of 0.5 to 1.0 degrees Celsius.
Effect of General Anesthesia on Thermoregulation
General anesthetics widen the interthreshold range to approximately 4 degrees Celsius, creating a zone in which no thermoregulatory responses are triggered. The vasoconstriction threshold decreases from approximately 36.5 degrees Celsius to approximately 34.5 degrees Celsius. The sweating threshold increases modestly. Volatile anesthetics, propofol, and opioids all contribute to thermoregulatory impairment in a dose-dependent fashion.
Three Phases of Intraoperative Hypothermia
The first phase (redistribution, 0 to 1 hour) involves core-to-peripheral heat redistribution that causes a rapid 1 to 1.5 degree drop and is the largest contributor to early hypothermia. The second phase (linear decline, 1 to 3 hours) occurs when heat loss exceeds metabolic heat production. The third phase (plateau, beyond 3 hours) occurs when thermoregulatory vasoconstriction, if the threshold is reached, constrains further heat loss.
Mechanisms of Perioperative Heat Loss
Radiation accounts for approximately 40% of heat loss and involves electromagnetic heat transfer to cooler surroundings. Convection accounts for approximately 30% and results from airflow over exposed skin surfaces. Evaporation accounts for approximately 20% and occurs from surgical wounds, skin preparation, and the respiratory tract. Conduction accounts for approximately 10% and occurs from contact with cold surfaces such as the operating table and IV fluids.
Temperature Monitoring Sites and Accuracy
Core Temperature Sites
| Site | Accuracy | Clinical Notes |
|---|---|---|
| Pulmonary artery catheter | Gold standard | Invasive; rarely placed solely for temperature |
| Distal esophageal | Excellent | Place in lower one-third of esophagus; avoid upper esophagus (nasopharyngeal artifact) |
| Nasopharyngeal | Excellent | Reflects brain temperature; risk of epistaxis |
| Tympanic membrane | Good | Contact-type most accurate; infrared less reliable intraoperatively |
| Bladder | Good at normal urine output | Lags during rapid temperature changes or low urine output |
Peripheral and Non-Core Sites
Axillary temperature is a reasonable surrogate if the probe is positioned over the axillary artery. Skin (forehead) temperature underestimates core temperature by 1 to 2 degrees Celsius but is useful for trending. Oral and rectal temperature measurements are impractical intraoperatively.
Consequences of Perioperative Hypothermia
Surgical site infection increases 3-fold with just 1.9 degrees Celsius of core hypothermia, as demonstrated in the landmark study by Kurz et al. (NEJM 1996). Coagulopathy results from impaired platelet function and enzymatic clotting cascade dysfunction, increasing blood loss by 16 to 25%. Cardiac morbidity increases from elevated catecholamine release, increased myocardial oxygen demand, and arrhythmias. Drug metabolism is delayed, prolonging the action of neuromuscular blockers, volatile agents, and propofol. Recovery is prolonged with extended PACU stays and delayed discharge. Patient discomfort from shivering increases oxygen consumption by 200 to 400%.
Prevention and Treatment Strategies
Active Warming
Forced-air warming is the most effective and widely studied method, maintaining normothermia in most cases. Prewarming for 15 to 30 minutes before induction prevents redistribution hypothermia by raising peripheral tissue temperature. Resistive polymer (conductive fabric) blankets are an alternative to forced-air warming and have no airflow disruption concerns. Fluid warmers should be used for volumes greater than 500 mL, as each liter of room-temperature crystalloid decreases core temperature by approximately 0.25 degrees Celsius.
Passive Insulation
Reflective blankets, cotton blankets, and surgical drapes reduce radiant and convective losses by approximately 30%. Covering the head reduces heat loss, though its contribution is proportional to surface area (approximately 7 to 10% in adults).
Environmental Control
Increasing OR ambient temperature to 21 to 24 degrees Celsius reduces convective and radiant losses. This is particularly important for neonates and pediatric patients with high surface-area-to-mass ratios.
Special Populations
Neonates and infants have immature thermoregulation, a high body surface area-to-weight ratio, and limited shivering capacity, making radiant warmers and heated mattresses critical. Geriatric patients have a reduced metabolic rate, impaired vasoconstriction, and thinner body habitus. Burn patients lose their skin barrier, dramatically increasing evaporative heat loss. In trauma patients, hypothermia is part of the lethal triad alongside acidosis and coagulopathy.
Therapeutic Hypothermia and Deliberate Cooling
Targeted temperature management (TTM) at 32 to 36 degrees Celsius is used post-cardiac arrest for neuroprotection. Deep hypothermic circulatory arrest (DHCA) at 18 to 20 degrees Celsius is used for complex aortic and neurosurgical procedures. Rewarming must be controlled at 0.25 to 0.5 degrees Celsius per hour to avoid rebound hyperthermia and cerebral edema.
Clinical Pearls
Prewarming is the single most effective intervention to prevent redistribution hypothermia and should begin 15 to 30 minutes before induction. Core temperature should be monitored in all general anesthetics lasting longer than 30 minutes per ASA Standards. Mild hypothermia significantly worsens coagulopathy, and normothermia should be maintained in patients at risk of bleeding. Shivering in the PACU can be treated with low-dose meperidine (12.5 to 25 mg IV), which is the most effective pharmacologic anti-shivering agent. Neuraxial anesthesia impairs thermoregulation below the block level and prevents shivering in affected dermatomes.
References
- Sessler DI. Perioperative thermoregulation and heat balance. Lancet. 2016;387(10038):2655-2664.
- Kurz A, Sessler DI, Lenhardt R. Perioperative normothermia to reduce the incidence of surgical-wound infection and shorten hospitalization. N Engl J Med. 1996;334(19):1209-1215.
- Madrid E, Urrutia G, Roqué i Figuls M, et al. Active body surface warming systems for preventing complications caused by inadvertent perioperative hypothermia in adults. Cochrane Database Syst Rev. 2016;4:CD009016.
- Torossian A, Bräuer A, Höcker J, et al. Clinical practice guideline: Preventing inadvertent perioperative hypothermia. Dtsch Arztebl Int. 2015;112(10):166-172.