I. INTRODUCTION

A. Temperature abnormalities are frequently encountered postoperatively because anesthetic, surgical, and environmental factors work in concert to disrupt thermostasis. Although normothermia is not a component of frequently utilized postanesthesia care unit (PACU) scoring systems (see Chapter 36), normothermia is often preferred and commonly required prior to discharge from the PACU, especially with ambulatory surgery patients. Evidence has mounted over the past two decades implicating hypothermia as a contributor to numerous adverse outcomes. Not surprisingly, the establishment and maintenance of normothermia has garnered increased regulatory attention.

B. Measurement of temperature in the PACU is a practice standard as outlined by the American Society of Anesthesiologists and must be documented as an element of the postanesthesia evaluation as mandated by the Centers for Medicare & Medicaid Services (CMS).

C. Normal human core temperature is 37°C (98.6°F) and ranges between 36.5°C and 37.5°C. Hypothermia is defined as core temperature less than 36°C and hyperthermia or fever as a temperature greater than 38°C.

II. THE PHYSIOLOGY OF THERMOREGULATION

A. Understanding the mechanisms of heat transfer is critical when conceptualizing both the physiology of thermoregulation and the disruption thereof.

1. Radiation refers to heat emission and absorption by means of electromagnetic waves, which occurs owing to atomic and molecular movement in all matter above absolute zero. Classic whole-body calorimetry studies have suggested that radiation accounts for about two-thirds of heat loss in a cold environment. Accordingly, it is felt to be the primary contributor to perioperative hypothermia.

2. Heat transfer by convection occurs by the movement of fluid or air across a surface and is the second greatest contributor to intraoperative heat loss. The use of surgical drapes is thought to mitigate convective heat loss owing to restricted motion of air over body surfaces.

3. Conduction describes heat transfer between adjacent surfaces in thermal contact and can be mitigated by insulation. The use of foam pads intraoperatively diminishes heat loss due to conduction.

4. The evaporation of water into gas is one example of a matter state change that leads to latent heat loss. The skin and upper airways both serve as barriers to this process. Appreciable skin incisions, bowel exposure, and the introduction of dry air into the respiratory tract during mechanical ventilation have all been postulated to increase evaporative heat loss.

B. For practical purposes, the body can be divided into two thermal compartments: the core and the periphery. Tissues comprising the core are
well perfused, thermally homogeneous, and maintained within a very narrow range of temperatures. In contrast, temperatures across the peripheral compartment are variable. Practically, the core refers to the visceral contents of the head, thorax, and abdomen, whereas the periphery consists of the skin and appendages.

C. Regulatory pathways and responses have evolved to counteract changes in environmental temperature through complex and incompletely understood mechanisms. Despite this tight regulation, thermostasis is readily disrupted by anesthesia.

1. Afferent signals originate from widely distributed peripheral—including cutaneous and visceral—and central thermoreceptors. Although little is known about the nature of visceral thermoreceptors in humans, discrete cold and warm-sensitive cutaneous thermoreceptors have been identified. Electrical conduction is facilitated by means of Aδ and C fibers for cold receptors and warm receptors, respectively. Following some degree of preprocessing and modulation based on the input of spinal thermoreceptors, these signals are transmitted cephalad via the spinothalamic tract.

2. The hypothalamus facilitates central integrative processing of afferent inputs. Lesion studies have demonstrated that the anterior hypothalamic nucleus works to oppose hyperthermia, whereas the posterior nucleus coordinates thermogenesis in response to hypothermia.

3. Efferent responses are variable in their progression and intensity depending on environmental stimuli. They can be broadly categorized as follows:

a. Vasomotor tone can be altered rapidly and at a low energy cost. As such, it is the first response to a change in environmental temperature. The goal of variable vasomotor tone is to shunt blood either toward or away from the core to facilitate the respective preservation or elimination of heat. Notably, cutaneous blood flow can nearly equal resting cardiac output under maximal vasodilatation.

1. Sweating quickly accompanies vasodilatation and is the primary response to an increase in core temperature.

b. Behavioral modifications are extremely effective at counteracting environmental changes but come at a high energy cost.

c. Thermogenesis occurs when other compensatory mechanisms fail to correct hypothermia.

1. Shivering generates heat by means of oscillatory skeletal muscle activity at the cost of increased oxygen consumption and discomfort.

2. Nonshivering thermogenesis that occurs through increased mitochondrial oxidative metabolism within brown adipose tissue is an important thermostatic mechanism in infants and children, because they cannot shiver effectively.

D. Patients at both extremes of age demonstrate impaired thermoregulation.

1. The elderly are prone to hypothermia when subjected to mild environmental or physiologic stress owing to decreased muscle mass with concomitant impaired vasoconstrictive and shivering responses. Additionally, these compensatory mechanisms do not occur until a lower temperature threshold is reached in comparison to younger patients.

2. Infants are prone to hypothermia secondary to a high ratio of body surface area to core mass (radiation), ineffective shivering (impaired thermogenesis), and a thin skin barrier (evaporation). Therefore, nonshivering thermogenesis plays a significant role in the maintenance of normothermia.

III. TEMPERATURE MONITORING SITES AND MODALITIES

A. Assessment of core temperature should be the goal of any measurement modality. Although heat is distributed heterogeneously throughout the body, perturbations in the comparatively stable core compartment provide the best insight into total body thermal status. Core temperature can be measured directly at the tympanic membrane, nasopharynx, pulmonary artery, or (distal) esophagus. Although these sites are frequently chosen intraoperatively or in critical care environments, they are too invasive for routine postoperative monitoring of extubated patients in the PACU.

B. A more practical approach to temperature monitoring in the PACU relies on the high degree of concordance between less invasive sites and core temperature.

1. Bladder temperature, as measured by a Foley catheter, has long been excluded from the list of core temperature sites because it can lag behind rapid changes in total body heat, such as during cardiopulmonary bypass and deliberate hypothermia. However, under most every other condition, bladder temperature very closely approximates core temperature. For patients who require a Foley catheter, placement of one with an integrated thermistor or thermocouple is a practical approach to facilitate accurate and continuous postoperative temperature assessment. For these reasons, bladder temperature is often chosen as the investigational standard against which other modalities are compared for postoperative patients.

2. Oral temperature measurement, although not practical for intubated patients, is a low-cost and reliable modality in the PACU. Postoperatively, it has demonstrated a high degree of accuracy compared to bladder temperature measurement. Avoiding contemporaneous per os intake, keeping the mouth closed during measurement, and placement of the thermometer deep in the rear sublingual pocket yield the most accurate readings.

a. Axillary measurement should be considered only when oral measurement is not feasible. Accuracy can be increased by positioning the thermometer near the axillary artery and keeping the arm adducted.

3. Skin temperature measurement, although convenient, should be considered an option of last resort given the heterogeneous nature of heat distribution in the periphery. Of all sites, the forehead with its thin skin and relatively high vascularity is the most commonly utilized.

4. Rectal temperature, more so than bladder temperature, lags behind core temperature during periods of rapid flux. Furthermore, its measurement in the PACU is often impractical.

C. As no single thermometry modality or monitoring site is without its caveats, using a single method of temperature monitoring in the perioperative period can allow for more accurate comparison of readings. Clinicians should always be suspicious of extreme temperature values, and in such instances a second modality can be useful.

IV. HYPOTHERMIA

A. Hypothermia is defined as a core body temperature less than 36°C (96.8°F) and is the most common perioperative temperature derangement. In the recent past, it was common practice to permit a modest degree of intraoperative hypothermia. Although hypothermia does confer benefits in a few discrete instances, its strong association with numerous adverse outcomes has drawn clinical and regulatory attention to the avoidance and treatment of hypothermia.

1. The incidence of unintended postoperative hypothermia was estimated at over 60% in the era before forced air warming, and more recently ranges between 5% and 20% of patients. Variable patient populations, anesthetic techniques, surgical procedures, temperature measurement modalities, and attentiveness to intraoperative normothermia cloud the true incidence.

2. Benefits to mild hypothermia include a reduction in the cerebral metabolic rate of oxygen (˜7%/°C), improved neurologic outcomes following cardiac arrest, and a potential, although controversial, protective role in patients with traumatic brain injury.

3. The risks of hypothermia mount with even a 1°C drop in core temperature.

a. Cardiac morbidity has been associated with hypothermia. In one study, normothermia conferred a 55% risk reduction in the pooled incidence of ischemia, infarction, and arrest.

b. Coagulopathy is an inevitable complication of hypothermia that begins with platelet dysfunction and progresses to overt dysfunction of the coagulation cascade. Increased blood loss and transfusion requirements, especially in orthopedic surgery, have been consistently associated with hypothermia.

c. Drug metabolism is impaired because hypothermia reduces both hepatic and renal blood flow. The effects of multiple drug classes are prolonged, including neuromuscular blockers, intravenous anesthetics, and volatile agents.

d. Discomfort is an obvious but underappreciated effect of hypothermia that leads to both increased circulating catecholamines and poor patient satisfaction.

e. Length of stay in the PACU is prolonged, which contributes to increased healthcare costs and compromised operating room workflow. Furthermore, hypothermia is an important contributor to delayed emergence.

f. Surgical site infection is one of the most costly and potentially devastating consequences of hypothermia. The risk appears to be highest in patients undergoing abdominal surgery.

4. As healthcare delivery becomes increasingly focused on quality of care, the avoidance of hypothermia has become one of the first metrics that directly affects anesthesiologists. The first such impetus came from the CMS Surgical Care Improvement Project (SICP), which tracked compliance with either the achievement of postoperative normothermia or use of forced air warming. Although this metric is no longer tracked as of 2015 owing to high compliance rates, normothermia remains an element of the CMS Physician Quality Reporting Initiative.

B. Unintended or inadvertent perioperative hypothermia (IPH) is, by far, the most common cause of hypothermia on admission to the PACU. Given its deleterious effects and relatively high incidence as outlined earlier, all PACU practitioners must be familiar with the treatment of hypothermia.

1. The etiology of IPH is multifactorial and stems from increased heat loss. General anesthesia affects all efferent compensatory mechanisms in the setting of hypothermia. As such, patients will invariably develop IPH when subjected to a sufficiently lengthy anesthetic in the absence of any active warming efforts. Intraoperative core temperature trends have been described as triphasic with steep initial decline because heat is redistributed from the core compartment to the periphery. Heat loss continues over the next 2 to 4 hours, in the
second phase, owing to blunted compensatory mechanisms and environmental exposure until a final steady state is reached around 33°C to 35°C, which is often sufficient to prompt an increase in vasomotor tone despite anesthetics.

a. Neuraxial and regional techniques disrupt thermoregulation in a similar fashion and also block peripheral thermoreceptor input, which can further contribute to hypothermia even in awake or lightly sedated patients.

2. Several treatment modalities can be employed in the PACU to correct IPH. Notably, the cessation of general anesthesia gradually restores many of the body’s compensatory mechanisms. Vasoconstriction, although normally helpful, slows the rewarming of postoperative patients due to shunting of blood away from the periphery, which is the site of surface warming modalities. As such, the prevention of IPH intraoperatively is easier than postoperative rewarming.

a. Prewarming of patients prior to general anesthesia not only improves patient comfort but avoids peripheral vasoconstriction and can thus blunt the magnitude of heat redistribution that accompanies induction. Attention to temperature preoperatively and the prompt implementation of warming strategies even before induction can aid in this effort. Disposable gowns with integrated forced air warming channels can facilitate warming before, during, and after procedures.

b. Surface warming is the cornerstone of temperature management. As a general principle, efforts should be made to warm as much of the patient’s surface area as can be practically accomplished. Even thin stretcher mattresses are sufficient to insulate against dorsal heat loss; therefore, efforts to warm the dorsum should be pursued only when ventral warming efforts prove inadequate.

1. Forced air warming is the most common, effective, and comfortable way to warm patients. The disposable plastic covers not only act as a conduit for convective warming but also help to reduce radiant heat loss. The covers are safe and most effective when used in direct contact with skin. However, the heated air output of the combined heater/blower unit should never be directed onto patients or used with paired patient covers from another manufacturer. Both of these practices have resulted in thermal injury.

a. Resistive heating with electric blankets has largely been replaced by forced air warming over concerns for sterility. Although resistive heating is highly effective, older devices require heightened vigilance to avoid thermal injury. A new offering combines resistive polymer in several topical and wraparound configurations with a control unit that monitors a temperature feedback loop to reduce the likelihood of thermal injury.

2. Circulating water devices come in many variations including mattresses, blankets, and pads with or without gel coatings. Postoperatively, the application of these devices on top of patients is preferable because they are more safe, effective, and comfortable in this configuration. (The placement of these devices under patients intraoperatively represents a trade-off between easy facilitation of surgical exposure and efficacy.) Wraparound circulating water garments and pads have recently become available, but remain expensive. Regardless of application, water temperature should never exceed 40°C.

3. Use of passive insulation, such as blankets, helps to minimize heat loss, but will not effectively treat hypothermia in the absence of active warming. The use of modest insulation on top of warming devices can help to increase their efficacy.

4. Ambient temperature can be increased to aid in the maintenance of normothermia, but is typically impractical in a large PACU. Furthermore, ambient temperatures high enough to actively rewarm patients will be uncomfortable for staff with associated reduced performance and vigilance.

c. Fluid warming alone is insufficient to actively warm patients, but plays an important role in the avoidance of hypothermia. Patients undergoing aggressive fluid resuscitation or transfusion of cold blood products should receive warmed fluids. Furthermore, fluid warming should be implemented in conjunction with other warming strategies in the face of persistent hypothermia.