Background

In humans, core temperature is maintained within a narrow range of 36.5 to 37.5 °C, even under adverse environmental conditions, through a combination of behavioral and autonomic responses. General anesthesia suppresses all behavioral responses and has the potential to disrupt autonomic responses (Sessler 1997). Consequently, inadvertent perioperative hypothermia (IPH) is common. Inadvertent perioperative hypothermia (IPH) defined as core temperature below 36.0 °C is a common complication of general anesthesia (GA) with prevalence up to 70% (Harper et al. 2008).

Perioperative thermal discomfort is often cited by patients as the worst aspect of their postoperative experience. Hypothermia develops because the operating room environment is typically cold; however, the alteration in thermoregulatory responses induced by anesthesia is the most important factor. Redistribution of core body heat to the peripheries contributes more to central hypothermia than actual heat loss in most patients (Sessler 2016).

Primary outcome studies have shown that mild hypothermia prolongs postanesthesia recovery time and hospital stay (Leslie and Sessler 2003; Lenhardt et al. 1997; Buggy and Crossley 2000; Kurz et al. 1996), triples the rate of surgical site infections (Kurz et al. 1996), triples the rate of adverse cardiac events (Valeri et al. 1992), and increases surgical blood loss and the need for blood transfusions (Rajagopalan et al. 2008).

IPH develops due to decreased metabolic heat production, heat loss in the cold operating room environment, intravenous fluids and irrigating fluids, derangement of thermoregulation, and redistribution of heat from the core to the peripheries due to anesthesia. Heat redistribution occurs after induction of anesthesia, which explains the drop in core temperature to 1.6 °C (Sessler and Todd 2000). During general anesthesia, the metabolic rate is reduced by 15–40%. Although intraoperative forced-air rewarming can effectively restore core temperatures within 2 h, the physiology of heat redistribution makes them unsuitable for short-term procedures (Matsukawa et al. 1995). However, warming peripheral tissues prior to induction of anesthesia reduce the central to peripheral temperature gradient, thereby minimizing central heat loss due to heat redistribution (Hynson and Sessler 1992).

A reduction in drop in core temperature has been demonstrated in several prewarming studies (Kurz et al. 1993). But the optimal time of effective preheating is unknown. Most studies on prewarming practised longer duration of active prewarming, ranging from 60 to 120 min (Chan and Venus 2016; Sessler et al. 1995; Ziolkowski et al. 2017). Sessler and colleagues estimate that 30 to 60 min is sufficient, using the FAW device (Sessler et al. 1995). The efficacy of a shorter prewarming time of 30 min before induction was therefore examined in our study.

This study aimed to evaluate the outcome of reducing the temperature gradient from the periphery to the core by pre-induction heating using forced-air heaters (FAW), on post-induction core temperatures, and the occurrence of inadvertent hypothermia resulting in postoperative shivering.

Methods

After getting approval of the hospital institutional review board (IRB no.: 10/2015/05), all patients in the study were briefed on the procedure, and written informed consent was obtained for the same. Patients under general anesthesia for elective surgeries lasting less than 3 h, aged 18 to 65 years, were included in the study. The study was conducted over a period of 6 months from January 2017 to June 2017. Patients with American Society of Anesthesiologists (ASA PS) grades 3 and 4 and/or who have received drugs that interfere with intraoperative thermoregulation, namely clonidine, tramadol, pethidine, dexmedetomidine, and those who underwent surgery lasting more than 3 h were excluded from the study (Fig. 1). On admission of patients to the preoperative holding zone, pre-induction warming was initiated with forced-air warmer (Covidien™ Warm Touch Convective warming unit) set at 38 °C for 30 min. Management of induction of anesthesia, maintenance, and reversal of anesthesia were done at the discretion of the attending anesthesiologist. Forced-air warming was maintained throughout the surgical period at 38 °C. The FAW blanket was removed after extubation immediately before transferring the patient to the PACU.

Fig. 1
figure 1

Flowchart for patient recruitment

Full size image

The baseline core temperature and the core temperature at induction were recorded. Core temperatures were also recorded over a 20-min period using a nasopharyngeal thermistor (Philips Medizin Systeme). The thermistor was inserted nasally at a length equal to the distance between the alae of the nose and the tragus. These patients were also observed in the recovery room for 30 min (10 min apart) for the development of postoperative shivering as graded on the PAS scale described by Crossley and Mahajan (Table 1). The data obtained above were compared with data obtained from 60 patients who were observed and evaluated for similar complications after routine GA.

Table 1 PAS scale by Crossley and Mahajan
Full size table

Based on the study by Andrezejwoski et al. (2008), corresponding to the fall in core temperature at 40 min, with an 80% power and 95% confidence interval, the minimum sample required for the present study was 54 in each group. By considering additional dropouts of 10%, the sample size was fixed at 120 (60 in each group).

Data was entered in Microsoft Excel, and Statistical Package for the Social Sciences (SPSS; Windows ver. 28.0, SPSS Inc., Chicago, IL, USA) was used for statistical analysis. Quantitative data was described as mean and standard deviation and qualitative data by frequency distribution. Normality was assessed using Kolmogorov–Smirnov one-sample test. Comparison of qualitative variables was assessed and analyzed by chi-square test. Comparison of quantitative variables was done by Students’ t-test. Paired t-test was done for pairwise comparison for normally distributed variables and Wilcoxon signed rank test for non-normal variables. Results were considered statistically significant when P-value of < 0.05 was obtained.

Results

The two groups were found to be comparable in terms of mean baseline temperature measured immediately after patient admission in the preoperative patient waiting area with 36.91 °C in the unwarmed group and 36.95 °C in the prewarmed group. IPH, defined as core temperature below 36.0 °C, was significantly higher in the unwarmed group than in the actively warmed group of patients. A total of 31.70% in the unwarmed group developed IPH compared with one patient (1.7%) in the prewarmed group shortly after onset (Table 2, Fig. 2). While comparing core temperatures at the end of surgery during reversal, 43.30% (26 patients) developed hypothermia in the unwarmed group, requiring active warming in the recovery room compared with one patient alone (1.7%) in the prewarmed group (Table 3).

Table 2 Demographic data of two groups
Full size table
Fig. 2
figure 2

Comparison of the sample based on the frequency of occurrence of inadvertent perioperative hypothermia (core temperature < 36 °C)

Full size image
Table 3 Comparison of the sample based on the frequency of occurrence of inadvertent perioperative hypothermia (core temperature < 36 °C)
Full size table

There was a statistically significant decrease in temperature from baseline up to 100 min after induction in the unwarmed group, while in the prewarmed group the temperature drop was remedied within 40 min after induction (Table 4, Table 5).

Table 4 Mean change in intraoperative core temperature in 20-min interval post induction in comparison with baseline temperature in not-warmed groups
Full size table
Table 5 Mean change in intraoperative core temperature in 20-min interval post induction in comparison with baseline temperature-prewarmed group
Full size table

In the unwarmed group, 46% of the patients had shivering with visible muscle activity, of which 22% required drug treatment. In the preheated group, no patient exhibited muscle activity, although grade 1 shivering was observed in 3 patients (5.3%) (Fig. 3).

Fig. 3
figure 3

Comparison of sample based on postanesthetic shivering at 10 min in PACU

Full size image

Discussion

One of the major causes of discomfort in patients recovering from general anesthesia is postanesthesia shivering (Crossley and Mahajan 1994). Mild perioperative hypothermia does not necessarily precede the onset of postanesthesia shivering, but it does tend to predispose to it, and the more severe the hypothermia, the higher the incidence of postanesthesia shivering. Many studies show that during the first hour after anesthesia, a significant drop in body temperature leads to IPH in 48 to 70% of surgical patients receiving general anesthesia (Chan and Venus 2016). After induction of anesthesia, thermal redistribution occurs and causes a drop in core temperature up to 1.6 °C (Matsukawa et al. 1995). Preheating reduces the center to periphery temperature gradient, thereby minimizing central heat loss due to heat redistribution. The optimal time of effective preheating is unknown. Sessler and colleagues estimate that 30 to 60 min is sufficient, using the FAW device (Sessler et al. 1995). Effective prewarming of 30 min before induction was conducted in our study. Most studies on prewarming practised longer duration of active prewarming, ranging from 60 to 120 min (Chan and Venus 2016; Sessler et al. 1995; Ziolkowski et al. 2017). The efficacy of a shorter warm-up time was examined in this study. The mean baseline core temperature at 20-min intervals was also significantly higher at 20 min, 40 min, and 80 min after induction among patients who were actively prewarmed during the pre-anesthesia period, substantiating the role of prewarming in preventing redistribution hypothermia when compared to intraoperative warming. The temperature drop recorded at subsequent time intervals was smaller than in the unwarmed subjects (0.6 difference). However, this result lacked statistical significance and was caused by thermal recovery and restoration of core body temperature due to prolonged surgery, accompanied by intraoperative warming. Observations support the results of studies performed by (Just et al. 1993; Andrzejowski et al. 2008). In the aforementioned studies, significantly longer prewarming (1 h) was achieved. The prolonged warming time employed may be due to the lower ambient temperature of the study environment. These studies took place in temperate climates, where the average ambient temperature is much cooler than in the tropics.

Limitations of the study

Our study was limited by the lack of standardization of anesthesia care in the two groups.

The choice of drugs and their dosage for induction, maintenance, and reversal of cases was left to the discretion of the respective attending anesthesiologists, which could have influenced pain scores and could be considered as a limitation of the study. Another limitation of the study was the failure to account for comorbidities that might interfere with thermoregulation. More research is recommended to determine the optimal prewarming time. Other possible consequences of hypothermia, such as increased blood loss, transfusion requirements, length of hospital stay, incidence of surgical site infections, or cardiovascular adverse events, were not observed or taken into account.

Further randomized studies are needed to ratify the beneficial effects of prewarming.

Conclusions

It is vital that surgical patients receiving general anesthesia are maintained normothermic since inadvertent perioperative hypothermia and postoperative shivering can lead to many unfavorable complications. Preoperative warming is significantly diminishing the drop in perioperative core temperature, thereby reducing the chances of IPH and PAS.