Comparison Between Forced Air and Intravenous Fluid Warmers in Gynecologic Laparoscopic Surgery
Phase: NA
Conditions: Hypothermia; Anesthesia
Sponsor: Siriraj Hospital

 
 
 
Comparison between Forced Air and Intravenous Fluid 
Warmer   in Gynecologic Laparoscopic Surgery:  
A Randomized Trial 
                  
Warunee Boayam, B.NS*, Phongthara Vichitvejpaisal MD, PhD*$, Pawan Suton, B.NS*,  
Sarisa Tapala , B.NS** 
 
* Department of Anesthesiology 
** Department of Perioperative Nursing 
Faculty of Medicine Siriraj Hospital, Mahidol University,  
Bangkok 10700, THAILAND 
 
 
 
 Correspondence to:  
Name 
 
     Professor Phongthara Vichitvejpaisal MD, PhD. 
Department    
     Department of Anesthesiology 
Institution  
     Faculty of Medicine Siriraj Hospital,   
 
 
     Mahidol University, Bangkok 10700, THAILAND  
Mailing address   2 Prannok Rd., Bangkoknoi, Bangkok 10700, THAILAND 
Phone 
 
    +66 (2) 4197978 
Fax 
 
    +66 (2) 4113256 
Email 
           phongthara@gmail.com  
 

 
 
 
Comparison between Forced Air and Intravenous Fluid 
Warmer   in Gynecologic Laparoscopic Surgery:  
A Randomized Trial 
                  
Warunee Boayam, B.NS*, Phongthara Vichitvejpaisal MD, PhD*$, Pawan Suton, B.NS*,  
Sarisa Tapala , B.NS** 
 
* Department of Anesthesiology ** Department of Perioperative Nursing, Faculty of Medicine 
Siriraj Hospital, Mahidol University, Bangkok 10700, THAILAND 
 Corresponding author: E-mail address- phongthara@gmail.com 
Abstract 
Background: 
Peri-operative hypothermia is a common problem in anesthesia. Investigators would like 
to compare the difference between core and room temperature in patients undergoing 
gynecologic laparoscopic surgery by using forced air and intravenous fluid warmer. 
Material and Method:  
After IRB approval COA: Si201/2016, the study has been registered at ClinicalTrials.gov 
NCT02990429. A prospective experimental study was conducted with 90 patients. All 
participants were randomized into two groups: A receiving intra-operative forced air warming 
and B having intra-operative intravenous fluid via a fluid warmer. Core and room temperatures 
were measured at 15-minute interval until the end of surgery. The data was expressed as means 
and standard deviation. The p-value less than 0.05 was considered statistical significance at 95% 
Confidence Interval. 
 
 
 

 
 
 
Results: 
 
The 86 patients accomplished the trial. Temperature of both groups showed to decrease 
insignificantly after induction, but it appeared a slightly lower in group B as compared to group 
A. In addition, group A showed a little higher temperature than that of group B in the recovery 
room.  
Discussions: 
Bair Hugger seemed to be an effective warming technique. However, the forced air 
warmer yielded a circulating flow only on the upper part of patient’s torso. As a result, the body 
heat could easily loss from its remaining part. On the other hand, as a patient was in lithotomy 
position with both arms laid aside the trunk and the whole body was draped over at the beginning 
of surgery, an anesthetist had to use a long extension for intravenous fluid management. Thus, 
the warmed fluid lost warmth en route through the patient. Therefore, the distance between 
source of fluid and site of needle cannulation should be short as much as possible. Consequently, 
this generated a troublesome in intra-operative fluid administration.  In addition, the fluid flow 
rate played a role in bodily heat control; as the slower the rate was, the lower the temperature 
would be.  
Conclusion: 
The forced air warmer was clinically effective than the fluid warmer in gynecologic 
laparoscopic surgery.  
 
Keywords: 
hypothermia, gynecologic laparoscopic surgery, general anesthesia.  
 
 
 
 
 
 
 

 
 
 
Introduction 
Peri-operative hypothermia is a common problem in anesthesia. It has been defined as a 
core temperature below 36ºC(1). The reasons why patient undergoing gynecologic laparoscopic 
surgery has this adverse event, are reduced metabolic heat production, heat redistribution from 
the core to the periphery, impaired thermoregulation, cool carbon dioxide gas insufflation, 
surgical irrigation solution, and cool environment(2). Consequently, the sequelae are myocardial 
ischemia as hypothermia increases plasma catecholamine, surgical site infection as hypothermia 
diminishes wound tissue O2 tension and coagulopathy as hypothermia impairs platelet function(3-
6).  
Studies claim that peri-operative heat loss occurs by radiation (60%), convection (25%) 
and evaporation (10%)(7). These are due to the difference between peripheral body and ambient 
temperature, air circulation around the body and vasodilatation. 
In daily practice, most anesthesia personnel warm patient peri-operatively by using force 
air warmer and/or intravenous fluid warmer. Thus, investigators would like to compare the 
difference between core and room temperature in patients undergoing gynecologic laparoscopic 
surgery by using forced air and intravenous fluid warmer. 
Materials and Methods 
The Siriraj Institutional Review Board approved the study COA: Si201/2016, and written 
informed consent was obtained from all subjects. Study setting was registered at 
ClinicalTrials.gov NCT02990429. The study was conducted at the Department of Obstetrics and 
Gynecology, Siriraj Hospital. 
A total of 90 patients were enrolled in the study: 84 patients for calculated sample size, 
and 6 patients for dropout purpose. All patients underwent general anesthesia for elective 
gynecologic laparoscopic surgery. Inclusion criteria were patients aged between 18 and 65, 
elective case, ASA physical status class I--III, BMI 25-30 kg/sq.m., surgical time > 90 minutes. 
Exclusion criteria were the core temperature less than 35ºC or more than 38ºC. Withdrawal or 
termination criterion was the change of laparoscopic surgery to exploratory laparotomy. 

 
 
 
On the day of surgery, participants signed the informed consent and were randomized 
equally into two groups: A = 45, receiving intra-operative forced air warming (Bair Hugger, 
Arizant Healthcare Inc., St. Eden Prairie, USA) and B =45, having intra-operative intravenous 
fluid via a fluid warmer (Ranger Warmer, Augustin Medical, Inc. Prairie, USA).  
After application of standard monitors, anesthesia was induced intravenously with 
fentanyl 1-2 mcg/kg or morphine 0.1-0.2 mg/kg, propofol 1.5-2.5 mg/kg, cisatracurium 1-1.5 
mg/kg or atracurium 0.6 mg/kg. Anesthesia was maintained with sevoflurane, air, O2 
supplemented with fentanyl or morphine. Core temperatures were measured with an electronic 
thermometer via tympanic membrane.  
Intra-operatively, core temperatures and room temperatures were measured at 15-minute 
interval until the end of surgery. 
Postoperative data were measured at 15-minute interval at the recovery room. Data 
consisted of vital signs, core temperature, room temperature, shivering, medication requirements 
and use of heating devices. 
Group A, the warming blanket was applied on the upper part of body after induction of 
anesthesia. The forced air was delivered at the high setting of 43ºC. At the end of anesthesia, the 
blanket was removed and the patient was delivered to the recovery room with standard care. 
Group B, patients received intravenous fluid via a fluid warmer after induction of anesthesia. The 
device automatically heated fluid up to 41ºC as set point. At the end of procedure, the fluid 
warmer was disconnected and the patient was transferred to the recovery room with intravenous 
fluid administered at room temperature. 
Patient developing intra-operative core temperature greater than 37.5ºC had to quit all 
devices to avoid hyperthermia. 
 
 
 
 

 
 
 
Statistical analysis: 
Using non-dependent t-test and Chi-square test compared parametric and nonparametric 
data respectively. The parametric ones expressed as means and standard deviation. The p-value 
less than 0.05 was considered statistical significance at 95% Confidence Interval. 
Results 
 
The 86 patients accomplished the trial, while 4 patients were excluded from the study 
because of the failure of room temperature device.   
 
Both groups (A = 44, B = 42) were insignificant difference with respect to age, body 
mass index, ASA status, fluid balance, irrigation fluid, type and duration of surgery, core and 
operating room temperature (Table 1).  
 
 
 
Temperature of both groups showed to decrease insignificantly after induction (Figure 1), 
but it appeared a slightly lower in group B as compared to group A. In addition, group A had 
insignificantly higher temperature at 15, 30, 45, 60, 75, and 90 min interval than that of group B 
(Table 2). 
 
After 15 minutes of arrival in the recovery room, group A showed a little higher 
temperature than that of group B (Figure 2). Afterwards, there were no differences in 
temperature between the two groups (Table 3). 
 
Four patients in group B and one patient in group A had mild shivering without the needs 
of any treatment such as opioids or warmed blankets (Table 4).  
 
 
 
 
 
 
 

 
 
 
Table 1. Patients’ characteristics (group A: Bair hugger, group B: Ranger warmer) 
 
 
Group A  
(N = 44) 
                         
Group B  
(N = 42)           
p 
Age   ( yrs) 
46.1 + 6.2 
46.0 + 6.2 
0.946 
BMI  ( kg/m2) 
27.0 + 1.7 
26.8+ 1.8 
0.678 
Type of surgery: 
 
 
 
TLH, BSO 
15 (34.1%) 
10 (23.8%) 
- 
TLH 
18 (40.9%) 
24 (57.2%) 
- 
Lap.Myomectory 
3 (6.8%) 
4 (9.5%) 
- 
Oter  
8 (18.2%) 
4 (9.5%) 
- 
ASA physical status:     
 
 
 
| 
23 (52.3%) 
22 (52.4%) 
- 
|| 
20 (45.4%) 
18 (42.9%) 
- 
||| 
1 (2.3%) 
2 (4.7%) 
- 
Core Temperature  
preoperative (ºC) 
36.6 + 0.4 
36.6 + 0.4 
0.895 
Core temperature  
pre - induction (ºC) 
35.8 + 0.6 
35.9 + 0.6 
0.833 
Operating theatre 
temperature (ºC) 
23.5 + 1.1 
23.5 + 1.2 
0.704 
Surgery time (min) 
166.0 + 57.1 
170.7 + 49.0 
0.684 
Fluid balance (ml) 
984.1 + 505.4 
957.1 + 382.3 
0.782 
Irrigate water (ml) 
756.8+ 552.1 
833.3 + 633.0 
0.551 
 
 
 

 
 
 
 
 
Figure 1. Intra-operative mean temperature in patients receiving forced air warming (Bair 
hugger) and intravenous fluid warmer (Ranger warmer). 
 
A  : Forced air warming (Bair hugger)   
B  : Intravenous fluid warmer (Ranger warmer). 
 
 
 
 
 
 
 
 
 
 
33.6
34.0
34.4
34.8
35.2
35.6
36.0
36.4
pre-induction
15 
45 
75 
// 
Temperature (ºC) 
Time ( min ) 
  A 
B 

 
 
 
 
Table 2. Intra-operative core and room temperature between the two groups (mean + SD) 
 
 
 
 
 
 
 
 
 
Room temperature 
Group A 
Group B 
  p 
0  minute 
23.0 + 1.1 
23.0 + 1.2 
0.998 
15 minute 
22.9 + 1.0 
22.8 + 1.3 
0.717 
30 minute 
22.7 + 1.1 
22.7 + 1.2 
0.803 
45 minute 
22.6 + 1.0 
22.6 + 1.2 
0.838 
60 minute 
22.5 + 1.0 
22.4 +1.3 
0.673 
75 minute 
22.4 +1.0 
22.3 + 1.3 
0.748 
90 minute 
22.3 + 1.0 
22.3 + 1.1 
0.817 
 
 
 
 
Core temperature 
Group A 
Group B 
p 
0  minute 
35.1 + 0.8 
35.1 + 1.2 
0.905 
15 minute 
34.9 + 1.0 
34.8 + 1.1 
0.689 
30 minute 
34.8 + 1.0 
34.8 + 1.0 
0.692 
45 minute 
34.7 + 1.1 
34.8 + 1.0 
0.826 
60 minute 
34.8 + 1.1 
34.7 + 1.0 
0.561 
75 minute 
34.7 + 1.0 
34.6 + 1.0 
0.483 
90 minute 
34.8 + 1.0 
34.6 + 1.0 
0.333 

 
 
 
Figure 2.  Post-operative mean temperature at the recover room in patients receiving forced air 
warming (Bair hugger) and intravenous fluid warmer (Ranger warmer).  
A  : Forced air warming (Bair hugger). 
B  : Intravenous fluid warmer (Ranger warmer). 
Table 3. Post-operative core temperature between the two groups (mean + SD) 
Core temperature 
Group A 
Group B 
p 
0  minute 
35.3 + 0.7 
35.0 + 0.9 
0.108 
15 minute 
35.4 + 0.7 
35.2 + 0.8 
0.240 
30 minute 
35.5 + 0.7 
35.2 + 0.8 
0.109 
45 minute 
35.6 + 0.7 
35.4 + 0.7 
0.284 
60 minute 
35.6 + 0.7 
35.5 + 0.7 
0.612 
75 minute 
35.7 + 0.7 
35.6 + 0.7 
0.486 
90 minute 
35.8 + 0.7 
35.7 + 0.6 
0.475 
 
34.6
34.7
34.8
34.9
35.0
35.1
35.2
35.3
35.4
35.5
35.6
35.7
35.8
35.9
0 
15 
30 
45 
60 
75 
90 
Temperature (ºC) 
Time (min) 
A 
B 

 
 
 
 
  Table 4. Post-operative treatment  for shivering. 
Recovery room 
Group A 
Group B 
Warm blankets 
1 (2.3%) 
5 (11.9%) 
Pethidine 
0 
0 
 
Discussion 
 
The demographic characteristics of both groups were similar. After 15 minutes of 
induction, group A and B insignificantly showed to decrease in temperature and appeared to 
maintain at steady stage throughout the procedure, with a slightly higher in group A.  
The forced air warmer seemed to be an effective warming technique. Feroe, et al. 
supported this in a study on effectiveness of convective warming therapies in the PACU(8). In 
addition, Adriani, et al. claimed that conventional intra-operative forced-air warming still offered 
benefit(9). However, the decreasing of temperature might due to the warmer at the high setting of 
43ºC, yielded a circulating flow only on the upper part of patient’s torso. As a result, the body 
heat could easily loss from its remaining part. This also mentioned in the study performed by 
Wagner, et al(10). 
Therefore, if the forced air warmer used to apply all over patient’s body, it could have 
alleviated heat loss efficiently. Apparently, Bernthal, et al. and many other researchers confirmed 
that the device was the most effective means to prevent hypothermia and suggested to use it 
routinely(11-13). 
On the other hand, as a patient was in lithotomy position with both arms laid aside the 
trunk and the whole body was draped over at the beginning of laparoscopic procedure; an 
anesthetist had to use a long extension tube for intravenous fluid management. Thus, the warmed 
fluid at the set point of 41ºC, lost warmth en route through the patient. Therefore, in a cool 

 
 
 
operating theatre, the distance between source of fluid and site of needle cannulation should be 
short as much as possible.  
Consequently, this generated a troublesome in intra-operative fluid administration.  In 
addition, the fluid flow rate played a role in bodily heat control; as the slower the rate was, the 
lower the temperature would be. These findings agreed with Faires, et al. who performed a study 
on the relationship between temperature to distance and flow rate of warmed intravenous fluids 
in pediatrics(14) and Person, et al. who concerned about the large volume of fluid relative to 
patient size and suggested a slow rate infusion(15). Moreover, Rein, et al. stated that warmed 
water with pulsating negative pressure was better than forced air warmer(16) and Turner, et al. in 
a study on simulated clinical evaluation of fluid warming devices, revealed that in order to 
achieve temperature close to 37ºC, the flow rate at 150 ml/min was needed(17).  
Interestingly, Seo, et al. suggested that the deceasing rate of temperature was related 
inversely to the flow rate and directly to the catheter length. Accordingly, it might need a rapid 
infusion pump with adequate heating system at a high flow rate and locate the warmer close to 
patient for reserving a heating effect(18). 
In the recovery room, all participants were covered by warmed blanket; while, the core 
temperature of both groups showed insignificant difference. However, the forced air warmer 
seemed to keep body warmed longer than that of the fluid warmer. This agreed with Patel, et al. 
who reported that patients receiving convective warming were more likely to leave the operating 
room normothermic, and had higher central temperature during the first 30 min in the recovery 
room(19).   
Conclusion 
The forced air warmer was clinically effective than the fluid warmer in gynecologic 
laparoscopic surgery.  
 
What is already known on this topic? 
Currently, most anesthesia personnel prevent patients from hypothermia by using forced 
air and/or intravenous fluid warmer during gynecologic laparoscopic surgery. Forced air 

 
 
 
warming system are claimed to be effective in preventing peri-operative hypothermia and 
shivering. However, the increasing number of intravenous fluid warmer usage is still 
questionable on its effectiveness.  
Suggestion of further study 
Regarding the effectiveness of intravenous fluid warmer, the distance between the fluid 
and the site of intravenous cannulation are of concern, other than the rate of fluid administration. 
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