Hyperthermia (Perioperative)

Definition

Hyperthermia is defined as a temperature above the normal core body temperature (generally 36.7°C–37.0°C ± 0.2°C–0.4°C). Elevated temperatures increase metabolic requirements, oxygen consumption, and insensible losses and can cause hypernatremia, seizures, and coagulopathy ( Box 145.1 ). Although hypothermia is more commonly seen in the perioperative period, intraoperative hyperthermia is also a significant concern that has its own differential diagnosis, broader than simply fever due to infection and malignant hyperthermia.

Thermoregulation occurs via a feedback loop controlled by the hypothalamus, coordinating heat production and loss to maintain normothermia near the regulatory set point. This set point, the interthreshold range, and the hypothalamic regulatory reflexes are all affected by both general and regional anesthetics. Hyperthermia is distinct from fever, with fever being an elevation in core body temperature that occurs due to an increased hypothalamic set point. In fever, the elevated set point is due to circulating pyrogens (such as endotoxin from gram-negative bacteria) and cytokines (interleukin [IL]–1, IL-6, tumor necrosis factor, interferon-α); as such, fever responds to antipyretics (such as acetaminophen), whereas hyperthermia is not responsive to these pharmacologic agents.

Recognition

Recognizing a hyperthermic state requires accurate temperature monitoring. Intraoperative temperature measurements can be obtained via a variety of techniques with differing specificity. Skin surface temperatures, which vary with environmental influences much more than core temperatures, are the least invasive to obtain, and generally 2°C cooler than core temperatures in operative environments. Although forehead skin temperature is usually reflective of core temperatures after adjusting for the 2°C difference, transitional zone (rectal, bladder) and core temperature monitoring is preferred when feasible. Bladder temperatures are easily obtained using Foley catheterization systems with built-in thermistors; however, these temperatures are falsely low during low urine flow states. Other situations may require more reliable core temperature measurements using temperature probes within the pulmonary artery (as with a Swan-Ganz catheter) or distal esophagus.

Risk Assessment

Hyperthermia itself has a variety of physiologic implications. Although it may be iatrogenic and secondary to hypothermia prevention techniques, hyperthermia may also be an indicator of an underlying pathologic process ( Box 145.2 ). It is often necessary to reevaluate all vital signs and monitors, and possibly obtain an arterial blood gas for analysis.