Overview of thermoregulation

The body maintains its temperature within the narrow range of 36° C and 38° C. Although temperature in the peripheral thermal compartment (consisting of the arms and legs) may vary with environmental and thermoregulatory responses, temperature in the core body compartment is controlled within a 0.2° C range by a balance of heat production and heat loss typically regulated by thermoregulatory mechanisms in the central nervous system. These mechanisms receive input from various thermoreceptors located in the skin, nose, oral cavity, thoracic viscera, and spinal cord. These thermoreceptors send sensory information in hierarchical order: spinal cord, reticular formation, and primary control in the preoptic hypothalamic region of the brain.1^,4–8

The central temperature controls body temperature with two primary responses: physiologic and behavioral. The physiologic thermoregulatory response consists of sweating, shivering, and alterations in the peripheral vasomotor tone. These responses fine-control the regulatory process of body temperature; consequently, heat loss is reduced with vasoconstriction and increased with vasodilation and sweating. They also work by reducing heat production, lowering the metabolic rate, and increasing muscle tone and shivering to enhance heat production. The behavioral thermoregulation results from subjective feelings of discomfort or comfort. For example, in a hot environment, a person seeks air conditioning; in a cold environment, the person seeks heat. This response mechanism is strong, but does not exhibit fine control as in the physiologic thermoregulatory response system.^1,4–8

Body heat is produced by metabolism and has a circadian cycle with the core temperature lower in the morning than in the afternoon.¹¹ Body heat loss occurs by four methods of heat transfer: radiation, conduction, convection, and evaporation, all of which play a significant role in the development of perioperative hypothermia (Fig. 53.1).

A patient lies supine depicting mechanism of heat loss. Three downward arrows from back side of patient are marked conduction. One upward arrow from nose is marked evaporation. A clockwise arrow above the body indicates convection. Three wavy arrows above legs are marked radiation.

Temperature measurement

Patients have rapid core temperature changes during the perioperative period. During such periods of rapid temperature fluctuation, a core temperature measurement provides the most accurate indication of body temperature. Temperature measurement during this period must be accurate and consistent and provide a true reflection of the core temperature measurement. The relationship between temperatures measured at various body sites during this period, however, may differ significantly from a true core reading.1^,2,6–8,12 Consideration of the best method for obtaining a temperature must also take into account accessibility of the measurement site, patient comfort and safety, and the practitioner’s ability to consistently use the temperature measurement device correctly. The same route of temperature measurement should be used throughout the perianesthesia period to allow for accurate temperature comparisons across the surgical continuum, and any extreme temperature (hypothermic or hyperthermic) taken with a noncore measurement instrument should be interpreted with caution.^3,4,13

The most accurate core temperature measurement is obtained via use of a pulmonary artery (PA) catheter because the artery bathes the catheter with blood from the core compartment and its surroundings. Temperature readings at the site can be affected by the rapid infusion of large amounts of warmed or cold IV fluids, by respiratory cycles, and by lower limb pneumatic compression devices. Readings from the distal esophagus and nasopharynx provide accurate alternatives to the PA catheter and are commonly used during surgery; however, like the PA catheter, these methods are invasive in nature and are not appropriate outside of the operative setting once the patient has been extubated.^3,4,12

Oral temperature measurement with electronic digital thermometers is a popular method of temperature measurement that is easily accessible, less prone to operator error, and accurately reflects changes in core body temperature. Oral temperature readings vary based on placement in the oral cavity (Fig. 53.2).^12,14 Oral temperature measurement taken in the right or left posterior sublingual (buccal) pocket provides an accurate reflection of core temperature even in the presence of oxygen therapy, warmed and cooled inspired gases, and varied respiratory rates.^12,14 Zero-heat-flux (ZHF) thermometry, a noninvasive temperature measurement device using a thermal probe on the lateral forehead (Fig. 53.3) to estimate core temperature, has also been found to be accurate to PA and esophageal probe temperature readings across multiple studies.^15–19

Oral cavity shows two oval-shaped sublingual pockets close to molar teeth on both sides. Oral temperature measurements based on their positions are as follows: Behind last molar on left: 98.4, near second left molar: 98.4, near first left molar: 98, near first premolar: 97.4, near left canine: 98, near incisors: 96.8, near right canine: 98.2, near second right premolar: 97.2, near right first molar: 98, near right second molar: 98.2, and near right third molar: 98.2.

A) Close-up of patient’s face and upper chest region with thermal insulator probe attached to forehead. B) Illustration of zero-heat-flux thermometer shows a wide, Y-shaped red region at the center. Probe is attached above on the surface.

Temporal artery thermometry is a noninvasive radiation thermometer with use of a scanner probe to scan the forehead and capture the infrared heat from the arterial blood supply and lock in the highest temperature sensed. Meta-analysis does not support the accuracy of temporal artery measures as a reflection of core temperature.²⁰

Infrared tympanic thermometry, previously considered a preferred method for noninvasive core temperature measurement, can be adversely affected by common sources of instrument error including poor operator technique, patient anatomy (site), and the calibration, accuracy, and inherent instrument error of the thermometer used. Given the issues surrounding temperature measurement with this instrument, it is no longer recommended as an accurate means of temperature measurement during the perianesthesia period.^3,4,12

Perioperative hypothermia

Perioperative hypothermia is defined as a core body temperature lower than 36° C.1^–4 As many as 78% of surgical patients have hypothermia in the course of the surgical experience.^21,22 All patients undergoing general and/or regional anesthesia are at risk for developing unplanned perioperative hypothermia; additional risk factors include:^{2–4,22–24}

• • Extremes of ages
  
• • Female gender
  
• • Systolic blood pressure less than 140 mm Hg
  
• • Level of spinal blockade
  
• • Length and type of surgical procedure
  
• • Normal or lower than normal body mass index
  
• • Body surface/wound area uncovered
  
• • History of diabetes with autonomic dysfunction
  
• • Use of cold irrigants
  
• • Use of general or regional anesthesia

Adverse effects associated with perioperative hypothermia include:^{2–4,8,23,24}

• • Patient discomfort
  
• • Increased adrenergic stimulation
  
• • Untoward cardiac events
  
• • Coagulopathy
  
• • Altered drug metabolism
  
• • Impaired wound healing
  
• • Surgical site infection
  
• • Increased PACU and hospital length of stay
  
• • Increased hospital costs

The typical temperature drop associated with perioperative hypothermia is between 1° C and 3° C and depends on the type and dose of anesthesia, amount of surgical exposure, and ambient room temperature. This temperature drop occurs from a loss of normal physiologic thermoregulatory mechanisms impaired by anesthetic drugs. As a result, the patient becomes poikilothermic and, without intervention, takes on the cooler temperature of the operative environment.^5,6,8,24

Intraoperative temperature loss typically occurs in a characteristic pattern as a result of core-to-peripheral redistribution (Fig. 53.4). Redistribution occurs as a result of a reduction in the vasoconstriction threshold related to the inhibitory effect of general anesthesia, resulting in a drop in core temperature and peripheral vasodilation triggered by both general and regional anesthesia, which causes an increase in the blood flow to the skin and a resulting loss in core body heat. An initial heat loss of 1° C to 1.5° C occurs during the first hour of surgery followed by a slower, more linear drop over the next 2 to 3 hours. Core temperature loss generally does not stabilize until 2 to 4 hours into the surgical procedure (Fig. 53.5). Postoperative return to normothermia occurs once the brain anesthetic concentration decreases enough to allow a normal thermoregulatory response. This response may take as long as 2 to 5 hours to kick in and may be inhibited by residual anesthetics and postoperative opioids.^5,6,8,24

Anterior view of human labeled vasoconstricted shows core temperature 37 degrees Celsius, skin temperature 28 through 32 degrees Celsius, and periphery temperature 31 through 35 degrees Celsius. After anesthesia, body is vasodilated. Body now shows core temperature 36 degrees Celsius, skin temperature 32 through 34 degrees Celsius, and periphery temperature 33 through 35 degrees Celsius.

Graph plots core temperature in degrees Celsius ranging from negative 4 through 0, in increments of 0.5, on the vertical axis against elapsed time in hours ranging from negative 0.5 through 6, in increments of 0.5, on the horizontal axis. A curve starts at (negative 0.4, 0.1), passes through (0, 0.1), (0.2, negative 0.8), (0.9, negative 1.7), (1.9, negative 2.9), (2.9, negative 3.3), (4, negative 3.5), (5, negative 3.5), and ends at (5.7, negative 3.5). All data are approximate.

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