Effect of preheating on the maintenance of body temperature in surgical patients: a randomized clinical trial*

ABSTRACT Objective: to evaluate the effect of preheating on the maintenance of body temperature of patients submitted to elective gynecological surgeries. Method: eighty-six patients were randomized, without blinding, to receive usual care (heating with a cotton sheet and blanket) or preheating with a forced air system for 20 minutes (38°C). All patients were actively heated during the intraoperative period. Data were collected from admission of the patient in the surgical center until the end of the surgery. Body temperature was measured during the preoperative and intraoperative periods with an infrared tympanic thermometer. A thermo-hygrometer was used to monitor air temperature and humidity of the operating room. Results: data indicated homogeneity between the groups investigated. There was no statistically significant difference between groups after preheating (p = 0.27). At the end of the surgery, the mean temperature of the groups studied was the same (36.8°C), with a statistically non-significant difference (p = 0.66). Conclusion: preheating with the heated forced air system had a similar effect to the usual care in the body temperature of patients submitted to elective gynecological surgeries. ClinicalTrials.gov n. NCT02422758. CAAE, n. 38320814.2.0000.5393.

gradient, which lessens the redistribution of body heat during the surgical anesthetic procedure (2,(4)(5) . The difference between temperature in core and peripheral compartments becomes small, even in adverse conditions, and this can be decisive to maintaining the surgical patient's body temperature.
It should be emphasized that passive heating, usually obtained with use of cotton bed sheets and blankets, is a conventional method adopted in clinical practice due to lack of resources or lack of knowledge on the part of the health team. There is evidence in the literature that active heating methods are more effective than passive methods to prevent perioperative hypothermia (2) . However, the use of a single layer of passive heating, i.e., the use of a cotton bed sheet can reduce the loss of body heat by around 30%, what may be clinically important (2) .
Research in the literature on preheating is a fruitful theme in the literature, addressing different aspects such as clinical characteristics of patients, type of surgery, type of anesthesia, body temperature measurement technique, intervention choice, duration and moment of preheating (6)(7)(8)(9)(10)(11)(12) .
Regarding the procedure itself, preheating time is not consensus: it ranges from 10 minutes to two hours of intervention (3,(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16) . Another important point is that there is no information in the studies was about the presence Perioperative hypothermia is an event that can be prevented (2,17) and nursing plays a fundamental role in the planning of care for surgical patients in all perioperative phases, contributing to the early detection of risks and clinical alterations such as hypothermia (17) , as well as to the creation and implementation of protocols for the management of care, permanent education, insertion of quality indicators of clinical practice, and most importantly improving the outcomes of the care provided.
The maintenance of body temperature in the perioperative period, has an important clinical impact besides the patient comfort, as it can reduce the morbidity associated with hypothermia, reducing bleeding during surgery and incidence of surgical site infection, and consequently hospital stay and health care costs for services (1)(2) .
The results of the research can guide actions to improve nursing care for surgical patients, as well as increase the knowledge and discussion about perioperative hypothermia.
Fuganti CCT, Martinez EZ, Galvão CM. Patients' selection and recruitment occurred in the nursing ward of the hospital on the same day of the surgery or one day before the procedure. Participants were randomly assigned to two groups: control and experimental. The randomization procedure was carried out through a list generated by a computer program. The randomization strategy was in blocks, being prepared eight blocks of 10 patients and one of six. The preparation of the list of allocation of participants in the blocks and the preparation of the sealed and opaque envelopes were procedures performed by a person who was not part of the study (Figure 1).
A two-part instrument was developed to achieve the The primary endpoint of the study was the body temperature variation evaluated by means of the tympanic membrane during the surgical anesthetic procedure, before and after preheating (T01 and T02, respectively), at the beginning of the surgery (T03), every thirty minutes during the surgical procedure (T030, T060, T090, T120, T150, T180, T210) and at the end of the procedure (TFINAL).
For this purpose an infrared electronic thermometer model GENIUS 2, brand Tyco/Kendall was used. This thermometer measures body temperatures between 33°C and 42°C with accuracy of ± 0.1°C.
Data were collected by one of the authors of the study and occurred as follows: after the patient's reception in the surgical center, the admission procedure was performed, i.e., checking the identification data and the preoperative preparation, as well as checking of vital signs. At that moment, the body temperature in the tympanic membrane was measured (T01). Then, the sealed and opaque envelope was opened to determine the allocation of the participant.
Patients allocated to the experimental group were preheated for 20 minutes with a heated forced air system on medium power (38°C), with a thermal blanket placed over the entire body, while the participants allocated in the control group were covered with cotton bed sheets and blankets (usual care) for 20 minutes. Body temperature was measured after intervention in both groups (T02).
After applying the tested intervention (experimental group), the heated forced air system was shut down and a thermal blanket was kept on the patient until transference to the operating room, and in the control group, the participants remained covered with cotton bed sheets and blankets. All patients waited the moment the surgical room was released for surgery and were, therefore, only passively heated.
After the patient entered the operating room, the tympanic body temperature was measured again. In the operating room before surgery, all participants were covered with a cotton bed sheet and blanket, following the routine of the sector, regardless of the group to which they had been assigned. Venous access and standard monitoring For all analyses, the significance level adopted was α = 0.05.

Results
Clinical characteristics of the patients (Table 1) and surgical anesthetic procedures (Table 2) were compared between groups. The results showed no statistically significant differences.
The results of the mixed effect linear regression did not identify a statistically significant difference in the patients' mean body temperature between the studied groups, at the different moments evaluated during the research. There was difference only between the mean temperatures of the control and experimental groups in the T150 measurement (p = 0.01) ( Table 3).   mean body temperature after preheating; ||T03 -mean body temperature at the beginning of the surgery; ¶T030 -mean body temperature thirty minutes after the start of the surgery; **T060 -mean body temperature sixty minutes after the start of the surgery; † †T090 -mean body temperature ninety minutes after the start of the surgery; ‡ ‡T120 -mean body temperature one hundred and twenty minutes after the start of the surgery; § §150 -mean body temperature one hundred and fifty minutes after the start of the surgery; ||||p < 0.05; ¶ ¶T180 -mean body temperature one hundred and eighty minutes after the start of the surgery; ***T210 -mean body temperature two hundred and ten minutes after the start of the surgery; † † †Tf -mean body temperature at the end of the surgery.
Rev. Latino-Am. Enfermagem 2018;26:e3057. At the end of surgery, the mean temperature of the operating room was 18.9°C and the mean air humidity was 55% in the control group, and 19.5°C and 52% in the experimental group.
The mean temperature of the operating room in the different periods measured was not significantly different between the studied groups. As for air humidity, only in the T120 period the results showed a statistically significant difference between groups (p = 0.03), but this difference did not remain in the moments evaluated afterwards.

Discussion
Due to the different complications resulting from perioperative hypothermia, the maintenance of body temperature became indicative of the quality standard of patient care provided in the surgical center. Preheating is an intervention that may help reduce perioperative hypothermia (18) . However, in the present study, the results did not show statistically significant differences in the maintenance of body temperature among patients in the experimental group (active heating with the heated forced air system) and in the control group (passive heating).
Clinical trials are found in the literature, and their results are in agreement with the findings of the present study (3,7,9,11) .
In a randomized clinical trial, the authors tested the effectiveness of preheating in 27 patients for a period of 30 minutes, randomized into three groups, namely: no preheating (control group); preheating with heated forced air system (experimental group 1); and preheating with carbon fiber electric cover system (experimental group 2). Both equipment sets were turned on at 42°C. The results indicated the carbon fiber electric cover system as the most effective in maintaining body temperature, and there was no statistically significant difference in the body temperature variation between the experimental group 1 (heated forced air system) and the control group (7) .
In another randomized clinical trial, the effect of preheating was analyzed in 66 patients undergoing colorectal surgery, randomized into two groups. In the control group, the participants were covered with a cotton bed sheet, and in the experimental group, the patients were heated for 30 minutes with heated forced air system. Although the preheating time was planned for 30 minutes, it averaged 75 minutes. The author identified similar proportions of hypothermic patients in both study groups, showing that preheating did not result in less hypothermia among patients (9) .
Participants in a randomized clinical trial (n = 50 elderly patients undergoing transurethral resection surgery) were randomized into two groups: patients not preheated and patients preheated for 20 minutes with heated forced air system (38°C). In both groups there was a decline in body temperature during the intraoperative period (p < 0.001), with a statistically non-significant difference between groups (p = 0.763).
The authors concluded that preheating did not prevent perioperative hypothermia but decreased its severity (3) .
Preheating was studied in another clinical trial, but the authors investigated its effect on blood pressure The patients' core body temperature was higher in the group receiving preheating (p < 0.004), but over time, that temperature changed, with a statistically non-significant difference between groups (p=0.06).
The authors concluded that preheating increased core body temperature prior to anesthesia induction, but this did not lead to increased blood pressure or reduced hypotension (11) .
On the other hand there are clinical trials in the literature whose results demonstrated the positive effects of preheating in the reduction of perioperative hypothermia (6,8,10,12) .
The results of a randomized clinical trial showed that preheating with a heated forced air system attenuates hypothermia through redistribution. The Fuganti CCT, Martinez EZ, Galvão CM.
sample consisted of 68 adult participants. In the experimental group, patients were preheated with a heated forced air system for 60 minutes (38°C), and compared to the control group (without preheating).
All patients were heated with heated forced air system during the intraoperative period. The results showed that the preheated group had a higher core body temperature than the control group (p < 0.005), and patients in the experimental group maintained normothermia more often than patients in the control group (p < 0.05) (6) .
In The results showed that there was no statistically significant difference between the active preheating groups (p=0.54), but there was a statistically significant difference between the passive preheating group and the three active groups (p < 0.00001) (8) .
In hypothermic. In the group where active preheating was performed 15 minutes after epidural anesthesia, the incidence of hypothermia was reduced by 6%.
The authors concluded that pre-heating the patient 15 minutes before and after epidural anesthesia is effective in preventing perioperative hypothermia (12) . is that active preheating showed greater effectiveness in preventing hypothermia (10) .
In this study, the preheating time was 20 minutes.
The results showed a statistically significant difference where the intervention was effective to maintain body temperature, the preheating time was up to 30 minutes (7)(8)12) . In a recent review of the literature aimed at evaluating the best preheating method and time, the authors stated that the heated forced air system is effective for prevention of perioperative hypothermia. The time of 30 minutes was found to be the suggested average time for preheating, and 10 minutes was the minimum time suggested as significant to reduce hypothermia rates (19) . Results of other studies demonstrated the effectiveness of the intervention with a longer preheating time (6,10) .
The environment temperature influences the rate of metabolic heat that is lost from the skin to the environment through radiation, convection and evaporation (17) . Regarding the influence of ambient temperature on body temperature, two studies presented similar results. A prospective cohort study was developed to identify the incidence and magnitude of hypothermia in a heated operating room (26°C) and age-related thermoregulatory response in this circumstance. The participants were divided into groups of age, namely: age between 20 and 40 years, and age between 60 and 75 years. The results showed that heating the operating room had a significant effect in maintaining the body temperature of adult and elderly patients (14) .  The authors concluded that a slight increase in the temperature of the operating room reduced the rate of neonatal and maternal hypothermia (15) .
On the other hand, in a clinical trial, in which the effect of preheating the operating room on the body temperature of patients submitted to knee and hip surgery was evaluated, the results indicated that there was no statistically significant difference between the experimental group and the control group in the last measurement performed. The sample consisted of 66 patients, divided into three strata according to BMI, and then randomized into two groups: patients placed in a surgical room with a standard temperature (17°C) and patients placed in a surgical room preheated at 24°C before the patient entered (16) .
In the present study, the two groups presented Thus, the moment without maintenance of preheating may also have contributed to the non-effectiveness of the intervention, because the heat provided by the intervention may have been loss, equaling the groups or even eliminating the preheating effect.
In the analyzed literature, only in two studies (3,12) the authors described the time elapsed between preheating and the beginning of the surgery. In these investigations, the surgery started immediately after preheating, and in only one of them (12) , the results were positive with respect to maintaining patients' body temperature. In the other studies analyzed, there was no description of the time between preheating and the beginning of the surgery, or of the place where the intervention was conducted (5)(6)(7)(8)(9)(10)(11)(13)(14)(15)(16) .
Regarding the temperature of the system used for preheating, in two studies (3,6) , the selected temperature was the same as that of the present research (38°C), and only in one study (6) the results showed maintenance of patients' body temperature. In the other studies, the system temperature varied from 42°C to 46°C, with positive (8) and negative (7,11) results in body temperature conservation. The authors of the other studies did not present clearly the temperature of the system adopted in the preheating.
The study presented some limitations, namely: the room temperature where the preheating was performed was not measured; blinding, which is advised for clinical trials, was not possible due to the type of equipment used; and the time elapsed between the end of the intervention and the start of the surgery.
We recommended therefore for future research the application of preheating inside the operating room, as well as using the heated forced air system at a temperature higher than 38°C (medium power of the equipment).

Conclusion
The results of the randomized clinical trial showed that preheating with heated forced air system had a similar effect to the usual care in the body temperature of patients undergoing elective gynecological surgeries.