Lauren E. Boyd and Letitia Hedges

Abstract

Perioperative hypothermia has the potential for multiple adverse effects and must be taken seriously to effectively treat and prevent harm to surgical patients. This review’s objective focuses primarily on postoperative hypothermia and the specific interventions needed for rewarming or raising core body temperature within the normal range. The literature from 2008-2016 was reviewed to aid in building the knowledge development of postoperative patient temperature monitoring and maintenance in the adult surgical patient. The results demonstrated that active rewarming is superior to the conventional warmed cotton blanket. The literature shows that those patients that received a type of active rewarming in the postoperative period reached normothermia quicker than those in the control group, had higher satisfaction, and lower post anesthesia care unit (PACU) length of stay.

Key words: Postoperative, warming, forced-air warming, radiant heat, hypothermia, rewarming.

Introduction

Postoperative hypothermia is a common occurrence for surgical patients and can be associated with a higher morbidity and mortality rate.

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Current literature recommends a number of rewarming methods to counteract hypothermia in the immediate postoperative period. Hospitals and healthcare providers are incorporating these methods into their practice to improve patient outcomes and to provide safe and effective patient care.

Hypothermia is defined as a core body temperature less than 36°C or 96.8°F. Humans depend on a constant internal body temperature in order to maintain peak function of organs and other body systems. Because operating rooms (OR) are kept below 23°C (73.4°F), up to 20% of patients experience unintended hypothermia in the perioperative period.

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Anesthesia eliminates a patient’s behavior modification and alters thermoregulatory mechanisms that a person would normally use to counteract the cold temperature in the OR.

Background

In the non-anesthetized patient, thermoregulation is a three-phase process involving afferent thermal sensing, central regulation, and efferent responses. Peripheral sensors send messages to the brain via the anterior spinal cord to various regions including the hypothalamus regarding body temperature changes. Normothermia is maintained with behavior modifications including seeking warmth and layer clothing. General anesthesia hinders the patient from normal thermoregulatory mechanisms and thus requires the body to rely solely on autonomic efferent responses to adjust temperature back to normal range, such as shivering, sweating, and vasoconstriction. However, general anesthesia (GA) also inhibits the body’s shivering and vasoconstriction capacity which may compound hypothermia.GA causes peripheral vasodilation which forces the cooler peripheral blood back to the central compartments resulting in a decrease in body temperature.

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The main cause of hypothermia is radiant heat loss, or the transfer of body heat to one’s surroundings. Other causes include evaporation, conduction and convection. There are uncontrollable factors associated with hypothermia and those include a high ASA score, long or involved surgery, combined regional and general anesthetics, and lean body mass (elderly patients). Ultimately temperature monitoring is essential to patient care and the perioperative period. Core body temperature is the most accurate measurement and this entails measurements at the tympanic membrane, distal esophagus, nasopharynx, and pulmonary artery. A core temperature should be the gold standard when referring to a patient’s thermal status.

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Hypothermia can be extremely detrimental to a patient’s well-being and surgical outcomes. It has been estimated that as many as 70 percent of postoperative patients experience hypothermia and up to 90 percent may have experienced adverse outcomes. The risks associated with a core temperature less than 36°C include decreased wound healing with an increased incidence of wound infection, increased blood loss and requirements of blood product administration, increased cardiovascular incidents, increased oxygen consumption, prolongation of certain medications such as muscle relaxants, altered drug effects such as volatile anesthesia agents, increased length of stay in the PACU, increased patient and hospital costs, and decreased patient comfort and satisfaction. There is a well-documented clinical significance of hypothermia and negative patient outcomes. Due to these potential negative outcomes related to postoperatuive hypothermia, it is imperative to prevent perioperative hypothermia in all surgical patients.

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Materials and Methods

A systematic search strategy was used to identify articles pertinent to the literature review. Searches were conducted in health and general science focused information resources, including Medical Literature Analysis and Retrieval System online (MEDLINE), Cumulative Index to Nursing & Allied Health Literature (CINAHL), the Cochrane Database of Systematic Reviews, as well as multidisciplinary resources such as Academic Search Complete. Search keywords (used alone and in combination) included postoperative warming, postoperative rewarming, patient warming, forced-air warming, resistive warming, radiant heat, postoperative hypothermia, postoperative hypothermia prevention, and anesthesia.

The search for evidence yielded 113 possible research articles. Of the 113 articles, 108 articles were eliminated. Inclusion criteria included full-text, English-language articles, and articles published in peer-reviewed journals. Articles written prior to January 2008 and after January 2017 were excluded. Studies that included postoperative rewarming in the pediatric population were excluded. Articles comparing rewarming methods during the pre- and intraoperative period were excluded. The 5 remaining articles included a systematic review with meta-analysis, randomized control studies, a quasi-experiment, and an experimental research design. The review consisted of thermal gowns, forced air warming devices, warmed cotton blankets and sheets, patient controlled warming gowns, radiant warmers, and circulating hot water devices.

Results

Of the 5 articles examined for this review, one was a systematic review with meta-analysis. The meta-analysis compared active warming with a control. To measure treatment effect, dichotomous data were analyzed using risk ratios with 95% confidence intervals. Continuous data were analyzed using mean differences and 95% confidence intervals. Heterogeneity was carried out by assessing the value of the I

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statistic. This estimated the percentage of total variance between studies that was due to heterogeneity rather than chance. Combined estimate included a 95% confidence interval. For the meta-analysis, 11 studies and 699 participants were evaluated. Eight of the studies compared active warming with a control, one compared thermal insulation with a control, one compared active warming with thermal insulation and one compared different methods of active warming. Active warming was shown to reduce the mean time taken to attain normothermia by about 30 minutes in comparison with use of warmed cotton blankets (mean difference (MD) -32.13 minutes, 95% confidence interval (CI) -42.55 to -21.71; moderate-quality evidence) and was found to decrease mean time taken to attain normothermia by almost an hour and a half in comparison with use of unwarmed cotton blankets (MD -88.86 minutes, 95% CI -123.49 to -54.23; moderate-quality evidence). Forced air warming was found to reduce the time taken to attain normothermia by about one hour in comparison to circulating hot water devices (MD=-54.21 minutes 95% CI= -94.95, -13.47).

Four randomized control studies were utilized in this literature review. The first study by Jardeleza et al., (2011) worked to compare the effectiveness of two passive methods of normothermia management in the postanesthesia care unit (PACU). 578 ambulatory surgery patients, 18 years of age or older who were scheduled to undergo surgery in the ambulatory surgery center (ASC) at a level I trauma hospital were included in the study. The control group was given two warm cotton blankets while the treatment group was given a warmed cotton sheet and a cotton blanket. A univariate analysis of temperature was measured using the Student t test. An analysis of covariance (ANCOVA) was used to analyze significant effects on the primary end point (ie, patient temperatures at 30 minutes). It was found that there was a significant difference in 30-minute temperatures and changes in temperatures between the groups. The ANCOVA revealed participants in the treatment group demonstrated higher temperatures (M = 36.71° C, SD = 0.34) than those in the control group (M = 36.59° C, SD = 0.36) 30 minutes after arrival in the PACU.

In the second randomized control study, Benson et al. (2012) looked at the efficacy of a patient-controlled active warming gown in improving patients’ perioperative body temperature and in decreasing postoperative pain after total knee arthroplasty (TKA). Thirty adult patients who would be undergoing TKA at Western Canadian community hospital were included in the study. The control group received standard hospital gowns (n=15) while the treatment group received a patient-controlled, forced-air warming gown (n=15). Patients who received warming gowns had higher temperatures (P < 0.001) in the PACU and reported more satisfaction (P = 0.004) with their thermal comfort than did patients who received standard blankets. These results acknowledge that patient-controlled, forced-air warming gowns can improve perioperative body temperature and enhance patient satisfaction.

Hsiu-Ling Yang et al., (2012) performed a randomized control study that compared the amount of time needed to reach a specified temperature and the effectiveness of warm cotton blankets and a radiant warmer for hypothermia patients in a post anesthetic care unit (PACU) after spinal surgery. 130 adults undergoing spinal surgery (posterior approach only) at a medical referral center in northern Taiwan were included in the study. The experimental group (Group R) was warmed with radiant warmers while the control group (Group B) was warmed by cotton blankets. The radiant warmer device required significantly less time for rewarming and was more efficient in raising body temperature than warm cotton blankets in post-spinal surgery hypothermia patients The time required to reach 36

^â-¦

C ranged from 10 to 120 min (mean 43.54 ± 27.12 min) for group R and 10 to 160 min (mean 76.77 ± 36.19 min) for group B. The time needed to reach a temperature of 36.0

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C was significantly shorter for group R than for group B (t(128) = 5.92, p < .001) The average rate of rewarming to a temperature of 36.0

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C was 1.83

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C/hour and 1.03

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C/hour, respectively, for group R and group B.

Wen-Ping Lee et al., (2015) in a randomized control study examined the effectiveness of the newly designed thermal gown on hypothermic patients after spinal surgery. 100 post-spinal surgery patients in PACU at a medical center were included in the study. The experimental group (N = 50) received the newly designed thermal gown intervention while the control group (N = 50) received the standard postanaesthesia care unit rewarming intervention. The average length of time it took for the thermal gown group patients to reach a body temperature of 36 °C was 49.02 minutes (95% CI: 46.60-51.43), with the median time being 50 minutes, while the cotton blanket group took 93.09 minutes (95% CI: 91.33-94.85), with the median time being 90 minutes. The average percentage for the thermal gown group to reach 36 °C during the first 20 minutes of admission was significantly higher than that of the cotton blanket group (x2 = 12.91, p < 0.001).

D


iscussion

Perioperative hypothermia is a serious concern in regards to patient safety and its prevention should be a goal of all surgical staff. Postoperative decreases in patient temperature can lengthen stay, increase costs, and decrease comfort and satisfaction. Determining the most effective and efficient method for postoperative rewarming was the goal of this literature review.

Five articles met the inclusion criteria and were included in this review. All articles reported that any method of rewarming was superior to the traditional warmed cotton blanket as shown in Table 1. The specific interventions that were examined included radiant warmers, patient controlled warming gowns, thermal gowns, active warming devices, passive warming devices, warmed IV fluids, warmed irrigation, and warmed inhaled inspired gases. As Table 1 shows, all studies found that normothermia was reached quicker and patients in all experimental groups had a higher mean body temperature compared to the control groups with all interventions studied. One article also found the added benefit of increased patient comfort and decreased duration of stay while in the PACU related to the patient-controlled forced air warming gown.

Table 1: Study highlights.

Author, Date, Journal, Design	Population, Sample Size (n)	Type of Postoperative Warming Device & Temperature Measurement Method	Conclusions
Benson et al. (2012) American Journal Of Nursing Randomized Controlled Trial	Adult patients who were scheduled to undergo TKA. N=30	Patient-controlled, forced-air warming gown versus warmed cotton blankets Oral thermometer (Welch Allyn, model 690)	The warming gown group had higher mean oral temperatures in the PACU than the patients in the warm cotton blanket group.
Jardeleza et al. (2011) AORN Journal Unblinded, Prospective, Experimental Design	Adult ambulatory surgery patients who were scheduled to undergo surgery in the ambulatory surgery center (ASC) at a level I trauma center. N=578	Two, warmed cotton blankets versus one warmed cotton sheet and one cotton blanket Temporal artery thermometer	The warm cotton sheet and cotton blanket resulted in a quicker increase in temperature and a significantly higher temperature 30 minutes after arrival to the PACU.
Lee et al. (2015) Journal Of Clinical Nursing Experimental Design	Adult post-spinal surgery patients in PACU. N=100	Thermal gown versus cotton cloth Infrared ear thermometer (OPUS 1000 series)	The thermal gown was shown to be greater than warm cotton cloth in terms of increased patient comfort and the reduction in the duration of a patient’s stay in the PACU.
Warttig et al. (2014) The Cochrane Database Of Systematic Reviews Systematic Review	Adults undergoing routine or emergency surgery under general or regional anesthesia, or both. 11 studies N=699	Any intervention meant to restore normal body temperature during the postoperative period compared with usual care or another intervention. Interventions included: active warming devices, thermal insulation or passive warming devices, warming of IV fluids, warming of irrigation fluids and warming of inspired gases	Active warming, especially forced air warming, presents a clinically significant reduction in mean time taken to achieve normothermia in patients with postoperative hypothermia.
Yang et al . (2012) Journal Of Nursing Scholarship Quasi-Experimental Design	Adults undergoing spinal surgery (posterior approach only). N=130	Radiant warmer versus cotton blankets Infrared ear thermometer (OPUS 1000 series)	The radiant warmer device was quicker and more efficient in raising body temperature than warm cotton blankets in post-spinal surgery hypothermic patients.

The results indicate that active warming reduced the time it takes to achieve normothermia by almost 30 minutes compared to the warmed cotton blankets and by 90 minutes compared to unwarmed cotton blankets. Active warming is also superior to circulating hot water devices by 60 minutes. The radiant warmer device was found to have a mean of 43.54 ± 27.12 minutes to normothermia compared to warmed blankets mean of 76.77 ± 36.19 minutes. Overall, the radiant warmer increased body temperature 1.83â-¦C/hour while the cotton blankets increased it 1.03°C/hour. The median length of time it took for the thermal gown group patients to reach a body temperature of 36 °C was 50 minutes, while the cotton cloth group took 90 minutes. Finally, patients who received two warmed cotton blankets versus an unwarmed blanket and sheet had higher temperatures 30 minutes after arrival to the PACU.

These results make sense in terms of comparing a rewarming device to using the traditional cotton blankets. However, it is hard to state with confidence which rewarming method is superior because no study was found that compared all of these methods together. Our results are consistent with the previous guidelines and research by ASPAN. We agree and conclude that active forced air rewarming continues to be superior when compared to passive methods of rewarming. A new conclusion was found compared to past guidelines that fluid-filled circulating blankets are inferior to active rewarming. Negative pressure rewarming devices were not studied.

Looking forward, these findings can help reduce the incidence of postoperative hypothermia. These results should provide valuable and evidence based knowledge to the postoperative health care team. Moreover, these results make sense. If a patient is normothermic, they will have increased comfort and compliance with health care instructions along with decreased health risks and costs.

A limiting factor for all studies presented was that they all were unblinded as seen in Table 2. Other limitations include lack of ambient temperature monitoring, core temperature was not always the source of data, lack of delivered anesthetic consistency, and extraneous variables were not controlled in every study. Also, two of the five articles found involved spinal anesthesia and these results cannot be generalized to all surgical patients and procedures.

Table 2: Quality & Limitations.

Author, Date, Journal, Design	Quality	Limitations
Benson et al . (2012) American Journal Of Nursing Randomized Controlled Trial	Strength: Level II Quality: Low	Unblinded study No standardization of administration of the anesthetic The temperature of the gowns were controlled by the patient so gown temperatures varied between patients Extraneous variables not controlled: temperature of the warmed blankets taken from the blanket warmer & OR temperatures
Jardeleza et al. (2011) AORN Journal Unblinded, Prospective, Experimental Design	Strength: Level III Quality: Moderate	Unblinded
Lee et al . (2015) Journal Of Clinical Nursing Experimental Design	Strength: Level II Quality: Moderate	Unblinded OR temperature was not controlled. All patients underwent spinal surgery, so the results cannot be generalized.
Warttig et al . (2014) The Cochrane Database Of Systematic Reviews Systematic Review	Strength: Level I Quality: Moderate	Unblinded studies were included Selective reporting
Yang et al . (2012) Journal Of Nursing Scholarship Quasi-Experimental Design	Strength: Level III Quality: Moderate	Unblinded All patients underwent spinal surgery, so the results cannot be generalized.

Further research is still needed to determine the most effective method of rewarming a patient during the postoperative period. It is suggested that studies begin to compare the different types of active warming methods available in the PACU instead of focusing solely on warmed cotton blankets. There is still room for improving patient outcomes, but these findings indicate active rewarming should currently be the gold standard for all surgical patients in the PACU.

Conclusion

The findings of this literature review indicate that the use of an alternative rewarming technique was superior to warmed blankets. The average length of time it took for the patients to achieve a normotherapeutic temperature of 36 °C was approximately 10-45 minutes once in the postanesthesia care unit. Not only was the average temperature achieved quicker but it was also higher with the rewarming devices compared to traditional warmed cotton blankets. It was also found that active warming is always superior to passive warming methods in PACU patients. Increased patient satisfaction was also found while utilizing alternative rewarming methods.

There remains a deficient amount of high quality literature on best practice methods for postoperative hypothermia rewarming. Randomized controlled trials need to be performed comparing all active warming devices so a superior method can be concluded. Future research also needs to focus on consistent control groups and variables along with standard core temperature measurements to increase result accuracy. Finally, varying populations and surgical procedures need to be examined so the results can be generalized to all postoperative surgical patients.

Unfortunately, there is no way to compare all rewarming devices available to the postoperative patients. Therefore, we cannot determine which device provides the highest quality patient care needed to achieve normothermia in the quickest time period. However, it is suggested by the available studies that active rewarming measures and devices should be implemented and incorporated into a standard of care for all postoperative patients to avoid hypothermia and its adverse outcomes.

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