Elsevier

The Lancet

Volume 387, Issue 10038, 25 June–1 July 2016, Pages 2655-2664
The Lancet

Review
Perioperative thermoregulation and heat balance

https://doi.org/10.1016/S0140-6736(15)00981-2Get rights and content

Summary

Core body temperature is normally tightly regulated to within a few tenths of a degree. The major thermoregulatory defences in humans are sweating, arteriovenous shunt vasoconstriction, and shivering. The core temperature triggering each response defines its activation threshold. General anaesthetics greatly impair thermoregulation, synchronously reducing the thresholds for vasoconstriction and shivering. Neuraxial anaesthesia also impairs central thermoregulatory control, and prevents vasoconstriction and shivering in blocked areas. Consequently, unwarmed anaesthetised patients become hypothermic, typically by 1–2°C. Hypothermia results initially from an internal redistribution of body heat from the core to the periphery, followed by heat loss exceeding metabolic heat production. Complications of perioperative hypothermia include coagulopathy and increased transfusion requirement, surgical site infection, delayed drug metabolism, prolonged recovery, shivering, and thermal discomfort. Body temperature can be reliably measured in the oesophagus, nasopharynx, mouth, and bladder. The standard-of-care is to monitor core temperature and to maintain normothermia during general and neuraxial anaesthesia.

Section snippets

Normal thermoregulation

Normal core body temperature varies by at least 1°C based on circadian and menstrual cycles.1 But at any given time, core temperature is tightly regulated, to within a few tenths of a degree during the day2 with slightly more variability at night.3 There are three major components to the control of body temperature: (1) afferent sensing, (2) central regulation, and (3) autonomic and behavioural defences (figure 1).

Temperatures are sensed peripherally and throughout the body by various receptors

General anaesthesia and thermoregulation

Volatile anaesthetics such as isoflurane and sevoflurane,27 the inhaled anaesthetic nitrous oxide,28 intravenous anaesthetics such as propofol,29 and opioids30 all substantially impair thermoregulatory control. None of these drugs has much effect on sweating thresholds, but each greatly and synchronously reduces the vasoconstriction and shivering thresholds. Threshold reductions are concentration dependent over the entire clinical range, and the slopes vary between drugs.

Neuraxial anaesthesia and thermoregulation

Epidural anaesthesia results from injection of moderate amounts of local anaesthesia into the epidural space; spinal anaesthesia similarly results from injection of small amounts of local anaesthetic into the spinal canal. Both types of anaesthesia, termed neuraxial, prevent most efferent and afferent neural activity to the lower body. Although local anaesthetics used in neuraxial anaesthesia do not normally reach the brain, each type of block nonetheless impairs thermoregulatory control via

Hyperthermia and fever

Hyperthermia is any elevation in core temperature. It can result from excessive heating, excessive heat production, inadequate heat loss, or setpoint elevation. Routine intraoperative warming systems can produce hyperthermia, especially during long operations. Hyperthermia is rare in adults using modern warming systems, but occasionally occurs in infants and children. Intraoperative hyperthermia can also result, either intentionally or not, from peritoneal lavage with heated chemotherapy

Heat balance

Human tissues can be divided into core and peripheral thermal compartments. The core compartment is generally defined as the tissues that have a high and nearly constant temperature over a wide range of environments and thermoregulatory responses. The core, mostly the trunk and head, constitutes about half the body mass. The remaining mass, mostly in the arms and legs, represents the peripheral thermal compartment. Although both core and peripheral temperatures influence central

Consequences of mild perioperative hypothermia

Most cellular functions are temperature dependent. Furthermore, hypothermia provokes systemic responses, some of which are potentially harmful. Thus, even mild hypothermia causes various complications. Although few patients are susceptible to all potential complications, most are susceptible to at least some. Throughout this section, hypothermia will refer to a 1–2°C reduction in core temperature unless otherwise specified. Most trials were done in the 1990s, when patient warming was rare;

Maintaining normothermia

Mean body temperature remains unchanged as long as metabolic heat production equals heat loss to the environment. Human beings normally have no difficulty balancing heat loss and production in hospital environments, but environmental heat loss can be substantial during surgery, and general anaesthesia reduces metabolic heat production by about 30%. Furthermore, core-to-peripheral redistribution of heat during the first hour of anaesthesia reduces core temperature even though body heat content

Temperature monitoring

Precision and accuracy of temperature monitoring depends on both the measurement system and the measurement site. There is no single body temperature; tissue temperatures vary greatly from site to site. The core thermal compartment (ie, the trunk and head) is highly perfused and relatively homogeneous. By contrast, peripheral (ie, arm and leg) tissue temperatures are typically well below core temperature, and skin temperatures are usually still lower. Skin temperatures also vary considerably

Recommendations

Both general and neuraxial anaesthesia greatly impair thermoregulatory control, with the consequence that unwarmed surgical patients become hypothermic. Randomised trials show that even mild hypothermia causes numerous severe complications. Core temperature should be monitored during general anaesthetics lasting more than 30 min and during neuraxial anaesthesia for substantial procedures. Surgical patients should be kept normothermic.

Search strategy and selection criteria

I supplemented my collection of published articles by searching MEDLINE with (“temperature” or “thermoregulation”) and (“anaesthesia” or “surgery”) for articles published between 2000 and 2014, with no language restrictions. I also considered relevant references from within citation lists. All articles were considered; those with the most robust methodology and largest sample size were given most weight. In general, the most recent reliable evidence is cited. Major reviews are cited to provide

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