• The hypothalamus is the primary thermoregulatory center for the body; it maintains normothermia through internal regulation of heat production or heat loss. Sensory thermoreceptors are located in the skin and subcutaneous tissue. Superficial or shell-zone temperature information is transmitted by thermoreceptors to the posterior hypothalamus through the spinal cord. Thermoreceptors in the brain, heart, and other deep organs transmit the core-zone temperature. Effective temperature regulation depends on the ability of the posterior hypothalamus to receive and integrate the signals received from the core and shell zones.^9,12,22

• Knowledge of terms associated with temperature is needed (Table 94-1).

• The hypothalamus regulates temperature in the range of approximately 36.4° to 37.3°C (97.5° to 99.4°F). By initiating physiologic responses to changes above or below this range, the hypothalamus coordinates heat loss or gain. Vasoconstriction and vasodilation control the distribution and flow of blood to the organs, viscera, and skin surface; thus, the amount of heat loss to the environment is influenced by vasomotor activity. In response to heat loss, shivering and vasoconstriction occur, muscles tense and the extremities are drawn closer to the body, and the person conserves heat. In response to heat gain, sweating and vasodilation occur, muscles relax, and heat is lost through evaporative cooling and to the environment.¹²

• Heat flows from a higher temperature to a lower temperature until the gradient between the two temperatures diminishes. Mechanisms of heat loss include conduction, convection, radiation, and evaporation.^5,9,12,22

• Alteration in thermoregulation can result from a primary central nervous system injury or disease (e.g., subarachnoid hemorrhage, traumatic brain injury, spinal cord injury, or neoplasm) and metabolic conditions (e.g., diabetes mellitus; toxic levels of ethanol alcohol or other drugs, such as barbiturates and phenothiazines).

• Body temperature is the measurement of the presence or absence of heat. Body heat is generated, conserved, redistributed, or dissipated during all physiologic processes. Factors such as age, circadian rhythm, and hormones influence body temperature.

• Body temperature may be measured with a variety of thermometers and at several body sites. Electronic or digital thermometers are used to obtain rectal, oral, and axillary temperatures. Thermistors within catheters or probes measure rectal, nasopharyngeal, esophageal, bladder, brain, and pulmonary artery temperatures. Infrared thermometers measure tympanic membrane and temporal artery temperatures. Choose the method of temperature monitoring that best meets the patient’s clinical condition. The most accurate temperature monitoring methods are intravascular (e.g., pulmonary artery catheter), esophageal, and bladder, followed by rectal, oral, and tympanic membrane methods.^9,13,21

• Variations in temperatures normally occur in the body (Table 94-2).

• Site choice for temperature monitoring is based on the clinical data needed; the patient’s condition, safety, and comfort; environmental factors (e.g., room temperature); the indication for a catheter or a probe (e.g., pulmonary artery catheter); and the availability of equipment.

• An esophageal temperature probe can be inserted down the esophageal tract. Accurate placement of the temperature probe is necessary to obtain results similar to monitoring the temperature from the pulmonary artery.

• A consistent temperature site must be monitored during the application of warming or cooling therapy.

• Shivering is an involuntary shaking of the body generated to maintain thermal homeostasis. Shivering causes rhythmic tremors that result in skeletal muscle contraction and is a normal physiologic mechanism to generate heat production.^4,11

• Early detection of shivering can be accomplished by palpating the mandible and feeling a humming vibration. Electrocardiographic (ECG) artifact from skeletal muscle is seen on the bedside monitor. If not detected early, shivering can progress from visible twitching of the head or neck to visible twitching of the pectorals or trunk, and then to generalized shaking of the entire body and teeth chattering.

• Shivering may be visible on the Bispectral Index Monitor (BIS) in the form of an increase in EMG activity (see Procedure 86).

• Shivering increases the metabolic rate, carbon dioxide (CO₂) production, oxygen consumption (by 40% to 100%),²² and myocardial work. If cardiopulmonary compensation does not occur to meet these demands, anaerobic metabolism occurs, resulting in acidosis.^11,17,22

• Shivering is counterproductive to strategies intended to lower temperature.

• At a body temperature below 35°C, the basal metabolic rate can no longer supply sufficient body heat and an exogenous source of heat is needed.

• Table 94-3 outlines techniques to increase heat gain.

• Hypothermia may be categorized as mild (32° to 35°C), moderate (28° to 31.9°C), severe (<28°C), or profound (<16.9°C).³ In severe to profound hypothermia, attempts at defibrillation are usually unsuccessful until the core temperature is above 28°C. The American Heart Association recommends only one attempt at defibrillation and then active rewarming should occur before reattempts at defibrillation.¹

• Hypothermia may be caused by an increase in heat loss, a decrease in heat production, an alteration in thermoregulation, and a variety of clinical conditions.

• An increase in heat loss may occur from the following: