Efficacy and Safety of Forced-Air Warming System versus Passive Warming Measures in Major Surgeries: A Systematic Review

ASPAN: American Society of Peri-Anesthesia Nurses

BH: Bair Hugger™

CI: Confidence interval

CNKI: China National Knowledge Infrastructure

FAW: Forced-air warming system

NICE: National Institute for Health and Care Excellence

RD: Risk difference

MD: Mean difference

Body temperature is an important vital sign of the human body. In normal conditions, body temperature is regulated by nerves-body fluids and maintained at approximately 37^°C to ensure the stability of physiological functions [1]. However, during surgical operation, patients are prone to hypothermic events (core temperature < 36^°C) due to the influence of various factors such as anesthesia, operating room temperature, and warming measure, intraoperative transfusion and infusion, and patients themselves, leading to intraoperative and postoperative rehabilitation risks [2,3]. In 2014-2015, Yi, et al., conducted a nationwide epidemiological survey in China, which showed that the incidence of intraoperative hypothermia was approximately 44.3% in this country [4]. 

The primary harm of intraoperative hypothermia is a resultant poor prognosis. Existing studies have shown that the occurrence of intraoperative hypothermia can affect blood coagulation function in patients and lead to increased bleeding amount and prolonged operation time; increase wound infection rate; slow down bodily and drug metabolism; impair the function of respiratory system; and increase the possibility of shivering and cardiovascular complications, causing a decline in patient satisfaction with treatment and quality of life [5,6], 

Currently, the methods of intraoperative body temperature protection can be divided into passive warming and active warming. The former method achieves warming by increasing insulating material to reduce heat dissipation, such as covering a quilt, multilayer surgical drapes and insulating blanket. The latter method actively provides conductive, convective, or radiative heat energy, including warmed intravenous fluid and intraabdominal irrigation fluid for internal use, and circulating water mattress with alternating temperature and Forced-Air Warming System (FAW) for external use. It has gradually become a consensus in medical practice across various countries to apply prewarming before surgery and take active warming measures in a timely manner for patients presenting with preoperative or intraoperative hypothermia [4,7]. However, with regard to clinical practice, intraoperative body temperature protection is still primarily limited to passive warming for patients in China [2]. Yi, et al., showed that active warming devices were used in only 10.7% of Chinese patients, while the application of warming devices with higher efficiency has not yet been popularized [8]. 

Existing reviews on the effects of warming measures have mainly focused on the change of body temperature and the incidence of hypothermia, while lacking exploration into a series of adverse events caused by intraoperative hypothermia. In the present study, we selected the Bair Hugger™ (BH), a FAW blanket that performs well in active warming, to systematically evaluate its clinical effects versus passive warming measures. The results of this study could provide more sufficient clinical evidence for the selection of suitable warming method in the Perioperative period.

Search strategy 

Computer searches were performed in the Pubmed, Cochrane Library, Clinical Trials.Gov, and China National Knowledge Infrastructure (CNKI) databases to retrieve studies related to the clinical effects of BH. The time limit of the search was set to January 2019. Search with “Bair Hugger” as the key word. The citations and grey literature were searched manually. 

Inclusion and exclusion criteria 

Inclusion criteria: The inclusion criteria were formulated according to the principle of PICOS. Target population: patients undergoing major surgery (with high complexity and high risk); intervention method: BH FAW blanket; control group: passive warming measures, such as covering a cotton blanket; outcome indicators: relevant clinical effects, such as infection and shivering; and study design: randomized clinical trial.

Exclusion criteria: 1. Articles that did not meet the inclusion criteria; 2. not published in Chinese or English; 3 incomplete outcome indicators, for example, not reporting the parameter estimation range; 4. Repeated publication; and 5. not intraoperative warming. 

Literature selection, data extraction, and quality evaluation 

Two researchers independently carried out literature selection, data extraction and quality evaluation. Divergence of opinions was resolved by discussion or assistance from a third party. Data extraction information included author, year of publication, country, sample size, basic patient information, type of surgery, intervention measures for the experimental group/control group, and outcome indicators. The quality of the included studies was evaluated using the modified Jadad scale [9,10]. The evaluation items comprised random sequence generation, randomization concealment, blinding method, withdrawal and dropout. The scoring criteria are listed in table 1. The quality of literature was indicated by the score: 1-3 points for low quality and 4-7 points for high quality.

Table 1: Evaluation items and scoring criteria of literature quality. 

Statistical analysis: The effect size of count data and measurement data was estimated using Risk Difference (RD) and Mean Difference (MD), respectively. The 2 test was used to evaluate the heterogeneity of included study results, and I2 was used to quantitatively determine the level of heterogeneity. If there was low statistical heterogeneity between the results of different studies, the fixed-effects model was used for meta-analysis; if there was high statistical heterogeneity, the source of heterogeneity was further analyzed, and if there was evident clinical heterogeneity, subgroup analysis or sensitivity analysis was performed, or only qualitative description was used; otherwise, the random-effects model was adopted for meta-analysis. For all tests the level of significance was set at α = 0.05.

Literature retrieval results 

A total of 289 relevant articles were retrieved through the preliminary search, and 27 articles were finally included after stepwise selection. The procedure of literature selection is shown in figure 1.
 

Figure 1: Flowchart of literature retrieval.

*English databases (PubMed, Cochrane Library, and Clinical Trials.Gov) = 126; Chinese database CNKI = 13 

Basic characteristics of included studies 

The basic characteristics of the included studies are summarized in table 2. 

Table 2: Basic characteristics of included studies.

Quality evaluation of included studies 

Among the 27 articles included, six (22.22%) were high-quality articles and 21 (77.78%) were low-quality articles. Ten of the low-quality articles scored 3 points (47.62%; Table 3). Overall, the quality of the included studies was not high. 

Table 3: Quality evaluation of included studies.

Meta-analysis results 

Compared with the passive warming group, the patients using BH had lower incidence of hypothermia [RD = –0.42, 95% confidence interval (CI: –0.68, –0.16)], lower incidence of shivering [RD = –0.28, 95% CI (–0.43, –0.13)], shorter anesthesia recovery time [MD = –8.27, 95% CI (-13.49, -3.05)], and shorter hospital stay [MD = –1.27, 95% CI (–2.05, –0.48)]. The BH group also showed advantages in terms of other indicators including postoperative incision infection rate [RD = –0.15, 95% CI (–0.40, 0.11)], intraoperative blood volume [MD = –16.88, 95% CI (–34.73, 0.96)], intraoperative blood transfusion volume [MD = –41.49, 95% CI (–108.36, 25.38)], pain [RD = –0.02, 95% CI (–0.08, 0.03)], and other complications [RD = –0.13, 95% CI (–0.39, 0.12)], albeit not significant (Figure 2).

 Figure 2: Meta-analysis results.

Note: On the left, from top to bottom are the results of incidence of hypothermia (%), incision infection rate (%), intraoperative blood loss volume (ml) and blood transfusion volume (ml); on the right, from top to bottom are the results of shivering (%), anesthesia recovery time (min), hospital stay (d), pain (%) and other complications (%). 

Subgroup analysis results 

To reduce the heterogeneity of the study results and explore the clinical effects of BH in more scenarios, subgroup analysis was carried out on the type of surgery (laparoscopy vs. non-laparoscopy), duration of surgery (with a 2-h boundary), and their combination. The heterogeneity was reduced, while the number of outcome indicators showing significant advantages in the BH group was increased. The number and heterogeneity of the included articles are listed in, and the results of the subgroup analysis are summarized in (Table 4 and 5).

Table 4: Number and heterogeneity of articles included in the subgroup analysis. 

Table 5: Subgroup analysis results.

Note: Bold text indicates a statistically significant difference. ^a: There was only one article.

This study conducted a relatively comprehensive exploration into the indicators of the clinical effects of different warming measures. The results indicate that compared with passive warming measures, BH has remarkable advantages in reducing the risk related to hypothermia, shivering, and hospital stay in major surgeries. The results of the basic analysis are further supported by the subgroup analysis with more detailed subgroups. 

In the case of high heterogeneity when combining some of the clinical evidence, it is impossible to perform subgroup discussions on the surgical site-a more critical factor-due to limited quantity of literature. Therefore, we conducted a subgroup analysis with regard to the characteristics of treatment regimen (laparoscopy/non-laparoscopy; surgical duration). We also performed a subgroup analysis based on the quality of literature. However, none of these analyses helped reduce the heterogeneity. This result might be related to factors such as the type of surgery and the year/country of publication that corresponded to the literature, and the long chain of evidence that led to the clinical results. 

To date, intraoperative body temperature protection and monitoring have become a consensus in medical practice across various countries: Institutions such as the American Society of Peri-Anesthesia Nurses (ASPAN), National Institute for Health and Care Excellence (NICE), and Chinese Society of Anesthesia all recommend prewarming before surgery, continuous monitoring of body temperature and keeping the patient warm during surgery, and taking immediate active warming measures for patients with preoperative/intraoperative hypothermia [38]. However, the utilization rate of active warming devices in China is still not high, which would increase the risk of intraoperative hypothermia and other related events in patients, further causing a greater economic burden [8]. 

In summary, considering the difference of patient protection by various warming measures and the relatively low utilization rate of active warming devices in China, it is necessary to strengthen the awareness of patients and medical staff on body temperature protection, standardizes the medical behavior, and improve the application popularity of active warming measures.

This study was supported by project of Postgraduate Education Reform under “double first-class” capability construction from China Pharmaceutical University (Grant NO: 3151920118), provided to WT.

WT conceptualized the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. Study design: AM, WT. Literature review: HX, YL, WT, XG. Data analysis: HX, XG, YM, YL. Drafting of the manuscript: HX, WT.

We thank Qiang Liu, the former advanced health economist engineer in Health Care Business Group Medical and Clinical Affairs, 3M China R&D Center, for providing some articles about BH.