Hypothermia

Purpose

All perioperative patients are at risk for developing unplanned hypothermia. This document provides guidance to the perioperative team for measuring the patient’s body temperature, selecting methods for the prevention or treatment of unplanned hypothermia, and implementing the selected warming interventions.

In this document, hypothermia is defined as a core body temperature of < 36° C (< 96.8° F).¹  Although the definition of hypothermia varies among sources, it is frequently stated as body temperature lower than 35° C or 36° C (95° F or 96.8° F).^2,3  Hypothermia may be further delineated as

• mild (32° C to 35.9° C [89.6° F to 96.6° F]),

• moderate (28.1° C to 31.9° C [82.6° F to 89.4° F]), or

• severe (< 28° C [< 82.4° F]).⁴ 

The human body has two thermal compartments: the core and the peripheral.⁵  The core body compartment includes primarily the trunk and head, which have a nearly constant temperature in a wide range of environments and thermoregulatory responses. The peripheral thermal compartment is composed of tissues that are mostly in the arms and legs, and the temperatures in these tissues are typically lower than core body temperature. The core temperature is normally tightly regulated, but the peripheral temperature may vary over a wider range. A change in the core or peripheral temperature normally leads to a behavioral or an autonomic response. A behavioral response to a temperature decrease may be moving into a warmer environment or putting on more clothing. The primary autonomic responses to a decrease in core body temperature are vasoconstriction and shivering. Anesthesia (eg, general, neuraxial, regional) prevents or alters these behavioral and autonomic responses, which results in a decrease in the patient’s core temperature.^5-17 

General anesthesia blocks the autonomic response of vasoconstriction, which causes redistribution of body heat from the core to the periphery.^6,9  This redistribution is responsible for approximately 80% of the decrease in core body temperature during the first hour after initiation of general anesthesia. During this time period, the patient’s core temperature can decrease by 0.5° C to 1.5° C (0.9° F to 2.7° F). General anesthesia also decreases metabolic heat production by about 15% to 30% and slightly increases cutaneous heat loss, which exaggerates temperature loss. After the first hour, the temperature continues to drop, but the rate at which it drops declines. The decrease in temperature during this time is generally caused by the heat loss exceeding metabolic heat production. After a period of time, the autonomic response will be reactivated, resulting in the temperature reaching a plateau. Neuraxial anesthesia (eg, spinal, epidural) also blocks the autonomic responses of shivering and vasoconstriction, but only in the anesthetized part of the body.⁵ 

To a lesser degree, hypothermia is caused by radiant, evaporative, convective, and conductive heat loss. Heat loss occurs because of low perioperative room temperature, the patient wearing little or no clothing, exposure of the patient’s internal organs and tissue to the environment, air moving at a high velocity over the patient, administration of room-temperature IV and irrigation fluids, and the evaporation of skin preparation solutions.^5-17 

Adverse patient outcomes from unplanned hypothermia may include myocardial events, wound infections, poor wound healing, postoperative pain, increased blood loss and need for blood transfusion, reversible coagulopathy (ie, platelet dysfunction), impaired renal function, decreased drug metabolism, increased peripheral vascular resistance, postoperative protein catabolism, altered mental status, pressure ulcers, increased postanesthesia care unit (PACU) length of stay, prolonged hospitalization, and death.^{1,3,4,8,9,13,14,16,18-72}  However, some studies have not found an association between hypothermia and the patient outcomes of surgical site infection, blood loss, wound complications, death, increased length of hospitalization, or increased length of time in the PACU.^{19,27,52,64,73-87}  Further research is needed to determine the association between unplanned patient hypothermia and patient outcomes.

Although thermal comfort is an important patient outcome and is noted in several studies, it has not been found to be an accurate reflection of patient temperature.⁸⁸  Patients who have received regional anesthesia may be hypothermic but report their thermal comfort as “warm.”⁵ 

Shivering, an autonomic thermoregulatory response to hypothermia, is also an important adverse patient outcome. Shivering may exacerbate surgical site pain, raise intracranial and intraocular pressure, and increase oxygen consumption.⁸⁹  Shivering can be minimized by active warming methods and by medications. The selection of medication for treatment of shivering is beyond the scope of this document because it requires an order from a prescriber.

These topics are also beyond the scope of this guideline:

• planned, intentional, or therapeutic hypothermia;

• rewarming after intentionally induced hypothermia;

• pharmacological agents used for prevention of hypothermia (eg, amino acids, fructose, carbohydrates);

• treatment for accidental or extreme hypothermia related to trauma or conditions outside of a health care facility;

• a cost-benefit analysis of treatment methods; and

• care and treatment of patients experiencing a malignant hyperthermia crisis. For guidance in handling malignant hyperthermia, contact the Malignant Hyperthermia Association of the United States (MHAUS) Hotline at (800) 644-9737 or access https://www.mhaus.org.

Figure 1

Flow Diagram of Literature Search Results

Adapted from Moher D, Liberati A, Tetzlaff J, Atman DG; The PRISMA Group. Preferred reporting items for systematic reviews and meta-analyses: The PRISMA Statement. PLoS Med. 2009;6(6):e1000097.

Evidence Review

A medical librarian with a perioperative background conducted a systematic search of the databases Ovid MEDLINE^®, Ovid Embase^®, EBSCO CINAHL^®, and the Cochrane Database of Systematic Reviews. The search was limited to literature published in English from September 2013 through May 2018. At the time of the initial search, weekly alerts were created on the topics included in that search. Results from these alerts were provided to the lead author until September 2018. The lead author requested additional articles that either did not fit the original search criteria or were discovered during the evidence appraisal process. The lead author and the medical librarian also identified relevant guidelines from government agencies, professional organizations, and standards-setting bodies.

Search terms included hypothermia, accidental/unplanned/inadvertent/unintentional/core/redistribution hypothermia, normothermia, shivering, trembling, shaking, shuddering, heat distribution, body temperature regulation, thermal management, thermoregulatory response threshold, thermoregulatory vasoconstriction/vasodilation/vasodilatation, rewarming, heating, thermogenesis, preoperative/intraoperative/comfort warming, warming technique, warming blanket, thermal pad, forced air/convective warming, negative pressure rewarming, circulating-water garment, cutaneous warming system, resistive heating, inspired gas humidification, intravenous infusion warming, warming irrigation, warmed solution, heat production, heat loss, adaptive/nonshivering/facultative thermogenesis, thermoregulation, monitoring (intraoperative), thermography, thermometers, temperature mapping, skin-surface temperature gradient, skin/esophageal/tympanic/temporal artery/oral/axillary/rectal thermometers, skin/esophageal/tympanic/temporal artery/oral/axillary/rectal temperatures, prewarming, temperature reporting, intraoperative care, operating room nursing, perioperative care, surgical procedures (operative), intraoperative/postoperative complications, operative surgical procedures, and operative procedures.

Included were research and non-research literature in English, complete publications, and publications with dates within the time restriction when available. Historical studies were also included. Excluded were non-peer-reviewed publications and older evidence within the time restriction when more recent evidence was available. Editorials, news items, and other brief items were excluded. Low-quality evidence was excluded when higher-quality evidence was available, and literature outside the time restriction was excluded when literature within the time restriction was available (Figure 1).

Articles identified in the search were provided to the project team for evaluation. The team consisted of the lead author and one evidence appraiser. The lead author and the evidence appraiser reviewed and critically appraised each article using the AORN Research or Non-Research Evidence Appraisal Tools as appropriate. A second appraiser was consulted if there was a disagreement between the lead author and the primary evidence appraiser. The literature was independently evaluated and appraised according to the strength and quality of the evidence. Each article was then assigned an appraisal score. The appraisal score is noted in brackets after each reference as applicable.

Each recommendation rating is based on a synthesis of the collective evidence, a benefit-harm assessment, and consideration of resource use. The strength of the recommendation was determined using the AORN Evidence Rating Model and the quality and consistency of the evidence supporting a recommendation. The recommendation strength rating is noted in brackets after each recommendation.

Note: The evidence summary table is available at http://www.aorn.org/evidencetables/.

Editor’s note: MEDLINE is a registered trademark of the US National Library of Medicine’s Medical Literature Analysis and Retrieval System, Bethesda, MD. Embase is a registered trademark of Elsevier B.V., Amsterdam, The Netherlands. CINAHL, Cumulative Index to Nursing and Allied Health Literature, is a registered trademark of EBSCO Industries, Birmingham, AL.