Perioperative thermoregulation and temperature monitoring.

Traditionally, hypothermia has been thought of and used perioperatively as a presumptive strategy to reduce cerebral and myocardial tissue sensitivity to ischemia. Evidence, however, is mounting that maintenance of perioperative normothermia is associated with improved outcomes in patients undergoing all types of surgery, even cardiac surgery. Ambient environmental temperature is sensed by free nerve endings in the dermal and epidermal layers of the skin, which are the axonal extensions of thermosensitive neurons found in the dorsal root ganglia. Free nerve endings in the skin, by means of transient receptor ion channels that are specifically thermosensitive, also may directly sense environmental temperature. This information is transmitted to the preoptic/anterior hypothalamic region of the brainstem, which coordinates efferent responses to abnormal temperature deviation. People have evolved a highly integrated thermoregulatory system that maintains core body temperature in a relatively narrow temperature range. This system, though, is impaired by the stress of regional and general anesthesia, and the added exposure that occurs during the surgical procedure. When combined, these factors can lead to unwanted thermal disturbances. In a cold operating room environment, hypothermia is the usual perioperative consequence; however, hyperthermia is more dangerous and demands immediate diagnosis. Intraoperative hypothermia usually develops in three phases. The first is a rapid decrease in core temperature following anesthetic induction, which mostly results from redistribution of heat from the core thermal compartment to the outer shell of the body. This is followed by a slower, linear reduction in the core temperature that may last several hours. Finally, a core temperature plateau is reached, after which core temperature remains virtually unchanged for the remainder of the procedure. The plateau can be passive or result from re-emergence of thermoregulatory control in patients becoming sufficiently hypothermic. Mild hypothermia in the perioperative period has been associated with adverse outcomes, including impaired drug metabolism, prolonged recovery from anesthesia, cardiac morbidity, coagulopathy, wound infections, and postoperative shivering. Perioperative temperature monitoring devices vary by transducer type and site monitored. More important than the specific device is the site of temperature monitoring. Sites that are accessible during surgery and give an accurate reflection of core temperature include esophageal, nasopharynx, bladder, and rectal sites. Core temperature also may be estimated reasonably using axillary temperature probes except under extreme thermal conditions. Rather than taking a passive approach to thermal management, anesthesiologists need to be proactive in monitoring patients in cold operating rooms and use available technology to prevent gross disturbances in the core temperature. Various methods are available to achieve this. Prewarming patients reduces redistribution hypothermia and is an effective strategy for maintaining intraoperative normothermia. Additionally, forced-air warming and circulating water garments also have been shown to be effective. Heating intravenous fluids does not warm patients, but does prevent fluid-induced hypothermia in patients given large volumes of fluid. This article examined the evolutionary adaptations people possess to combat inadvertent hypothermia and hyperthermia. Because thermal disturbances are associated with severe consequences, the standard of care is to monitor temperature during general anesthesia and to maintain normothermia unless otherwise specifically indicated.