Prevention of inadvertent hypothermia and the ability to reduce temperature-related perioperative morbidity and mortality have attracted significant attention within the anesthesiology community and the potential patient safety implications have been highlighted in presentations and publications.

In reviewing the relevant literature, it is important to acknowledge that these findings have primarily been restricted to specific patient cohorts. In general, from an evidence-based approach, randomized clinical trials have strong internal validity, i.e. they demonstrate effectiveness or lack of effectiveness of a given intervention in a defined cohort, but have a weaker external validity, i.e. they may not be applicable to all patient groups. Therefore, the importance of preventing inadvertent hypothermia depends upon the patient population of interest. Because of the nature of the surgical procedure and duration of surgery, many patients are at extremely low risk of developing complications from hypothermia. Previous research from our group was unable to detect any difference in PACU length of stay or patient satisfaction.⁶ These were not significantly different between patients who were allowed to become hypothermic versus those who received forced air warming. Importantly, these patients were relatively healthy without coronary disease and were undergoing operations with low probability of receiving a transfusion. Therefore, it is important to determine which patient populations are most at risk for morbidity from inadvertent hypothermia in order to optimize patient safety in a cost effective manner. Otherwise, we may focus attention and resources away from areas where they may be of greater patient benefit.

In determining for which patients there is sufficient evidence to advocate the use of new technologies to maintain normothermia, it is important to review the few randomized trials evaluating effectiveness, all of which have been published in the last few years. In interpreting randomized clinical trials, key characteristics are that they have strong internal validity, they demonstrate the effect of an intervention on an outcome, but they have weaker external validity in that the results may not be generalizable or applicable to real-world conditions. When deciding whether to adopt a new therapy, it is important to determine if the study’s patient population and conditions apply to one’s individual practice.

There are three major outcomes that have been associated with mild perioperative hypothermia. A core temperature of < 35.0-35.5°C increases the risk of 1) wound infection, 2) bleeding, and 3) cardiac complications, in patients at risk for these specific adverse events. The same degree of hypothermia may also be associated with two other issues, 1) longer duration of PACU stay and, 2) thermal discomfort, outcomes which, however, have more impact on logistics/economics and patient satisfaction than on patient safety.

Kurz et al.¹ identified a three-fold increase in the incidence of postoperative wound infection in a prospective, randomized, blinded study in patients who developed hypothermia (average core temperature of 34.7°C) during colon resection despite routine antibiotic administration. These wound infections have been attributed to decreased tissue oxygen delivery resulting from thermoregulatory vasoconstriction as well as hypothermia-induced impairment of white blood cell function. Note that the rate of infection was greater than reported in other studies, which limits the generalizability of their findings. Schmeid et al.² demonstrated a > 20% increase in intraoperative and postoperative blood loss in hypothermic patients (average core temperature of 35.0°C) in a prospective, randomized nonblinded trial in patients undergoing total hip arthroplasty. These mildly hypothermic patients also required more transfusion of allogeneic blood. Since the transfusion of blood products continues to be associated with risk, avoidance of unnecessary transfusions should increase patient safety. Frank et al.³ identified a 55% reduction in relative risk of a postoperative morbid cardiac event in a prospective, randomized, blinded trial in high-risk nonvascular and vascular surgical patients with an average core temperature of 35.3°C. The proposed mechanism for cold-induced cardiac morbidity is the adrenergic response (primarily norepinephrine) that is triggered by mild hypothermia in awake postoperative patients.⁴

Length of PACU stay is a difficult outcome to measure because many factors determine when patients are discharged from the PACU. Leinhardt et al.⁵ showed a > 40 minute average reduction in time required for readiness for PACU discharge in patients undergoing major abdominal surgery when they were admitted with an average core temperature of 34.8°C in a randomized, blinded study. The longer PACU stay may be explained by a slower metabolism of anesthetic drugs and/or a greater sensitivity to the residual anesthetic (reduced MAC) in colder patients. Hypothermia (core = 35.4°C), however, was not associated with a longer duration of PACU stay in another randomized blinded study by Fleisher et al.⁶ for patients undergoing gynecologic procedures. The differences in these two studies may be in the methods used to assess readiness for discharge or in the magnitude of hypothermia.

The last outcome to consider is thermal discomfort. This is perhaps one of the most common and most unpleasant memories of the postoperative period for many patients, but is more an issue of patient satisfaction than safety and morbidity. Nonetheless, two prospective, randomized, nonblinded studies by Kurz et al.⁷ and Krenzischeck et al.⁸ have shown that cutaneous warming either intraoperatively, or postoperatively is associated with patients’ increased thermal comfort.

Where does all this leave us? The only thing clear is that the cost, risks, and benefits of active warming should be considered for all patients individually, factoring in their preexisting co-morbidity and the surgical procedure. Body temperature is, simply stated, a vital sign. It might be convenient and even reassuring to have published guidelines that specifically state how to monitor and treat body temperature, perhaps even with defined limits within which body temperature should be maintained. There are, however, no such analogous guidelines for the other vital signs (heart rate, blood pressure, or respiratory rate).

From the perspective of patient safety, the literature would suggest that for patients at risk for wound infection, bleeding, and cardiac morbidity, monitoring temperature and maintaining normothermia will help improve outcome. For other patients, monitoring temperature and maintaining normothermia may have logistic/economic and quality of care benefits, but further research will be required to determine specific additional patient populations for whom patient safety can be enhanced with these strategies.

Drs. Frank and Fleisher are from the Department of Anesthesiology and Critical Care Medicine, The Johns Hopkins University, Baltimore.

References

1. Kurz A, et al.: Perioperative normothermia to reduce the incidence of surgical-wound infection and shorten hospitalization. New Eng J Med, 1996. 334:1209-1215.
2. Schmeid H, et al.: Mild hypothermia increases blood loss and transfusion requirements during total hip arthroplasty. Lancet, 1996. 347:289-292.
3. Frank SM, et al.: Perioperative maintenance of normothermia reduces the incidence of morbid cardiac events: A randomized trial. JAMA, 1997. 277:1127-1134.
4. Frank SM, et al.: The catecholamine, cortisol, and hemodynamic responses to mild perioperative hypothermia: A randomized clinical trial. Anesthesiology, 1995. 82:83-93.
5. Leinhardt R, et al.: Mild intraoperative hypothermia prolongs postanesthetic recovery. Anesthesiology, 1997. 87:1318-1323.
6. Fleisher LA, et al.: Perioperative cost-finding analysis of the routine use of intraoperative forced-air warming during general anesthesia. Anesthesiology, 1998. 88: 1357-64.
7. Kurz A, et al.: Postoperative hemodynamic and thermoregulatory consequences of intraoperative core hypothermia. J Clin Anesth, 1995. 7:359-366.
8. Krenzischeck DA, et al.: Forced-air skin-surface warming vs. routine thermal care and core temperature monitoring sites. J Postanesth Nurs, 1995. 10:69-78.

Table. Summary of Randomized Trials of Thermal Management and Outcome

Publication	Surgical Procedure	Outcome	Average T (core) vs. normothermia	Outcome difference
Kurz A, et al.1	Colon resection	Wound infection	34.7	3-fold
Schmeid H, et al.2	Hip arthroplasty	Bleeding	35.0	20% increase
Frank SM, et al.3	Abdominal, thoracic with cardiac risk factors, and vascular	Cardiac morbidity	35.3	55% relative risk reduction
Leinhardt R, et al.5	Major abdominal	PACU length of stay	34.8	40 min increase
Fleisher LA, et al.6	Gynecologic	PACU length of stay	35.4	No difference
Kurz A, et al.7	Major abdominal	Thermal comfort	34.4	40 points on a 100 point scale
Krenzischeck DA, et al.8	Abdominal, thoracic, vascular	Thermal comfort	35.3	2 points on a 10 point scale