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Pediatric Anesthesiology: Research Report

Forced-Air Warming During Pediatric Surgery

A Randomized Comparison of a Compressible with a Noncompressible Warming System

Triffterer, Lydia MD; Marhofer, Peter MD; Sulyok, Irene MD; Keplinger, Maya MD; Mair, Stefan MD; Steinberger, Markus MD; Klug, Wolfgang MD; Kimberger, Oliver MD

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doi: 10.1213/ANE.0000000000001036
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Inadvertent perioperative hypothermia is a common problem challenging the anesthesiologist. It is caused by the inhibition of thermoregulation induced by anesthesia, redistribution of body heat from the core to the periphery, and the exposure of patients’ skin and tissues to a cold environment.1

Hypothermia has short-, mid-, and long-term consequences for patient outcome, which have been demonstrated mostly in adult patient studies. Inadvertent hypothermia has been shown to triple the incidence of postoperative wound infection2 and increase bleeding and blood transfusion requirements.3 Inadvertent perioperative hypothermia also increases the incidence of morbid cardiac complications and ventricular tachycardia.4 The metabolism of most anesthetic drugs, including muscle relaxants and propofol, is significantly decreased,5 and postoperative recovery period and hospital stay are prolonged.6

The only efficient measure to avoid unintentional perioperative hypothermia is active patient warming, typically performed by skin warming with forced warm air. Forced-air warming generally is described as the most cost-effective method of patient warming (e.g., see National Institute for Health and Clinical Excellence [NICE] Guideline CG65a), and there are numerous forced-air patient-warming systems on the market.

Pediatric patients particularly are prone to unintentional hypothermia,7–9 and adequate warming-preserving normothermia and avoiding both hypo- and hyperthermia can be challenging. Characteristically, the only surface for warming available is the pediatric patient’s back. However, because airflow of an underbody forced-air mattress can be blocked by the weight of the patient, heat transfer can be limited. A forced-air mattress with a noncompressible core might thus distribute forced air more evenly and efficiently. Such a system was recently tested in a nonrandomized, observational study, displaying satisfactory performance.10 This report was discussed in a subsequent editorial where the lack of a control group was highlighted.11

We hypothesized that, in pediatric patients, the noncompressible underbody warming mattress would be superior to the compressible underbody warming mattress as regards core temperature warming performance and would maintain greater intraoperative temperature stability, as indicated by fewer temperature adjustments of the device.


After approval of the IRB/Independent Ethics Committee of the Medical University of Vienna, 80 pediatric patients (<2 years) undergoing pediatric surgery were studied. This trial was registered at the German Clinical Trials Register (DRKS; with the DRKS-ID: DRKS00004200 and was conducted with written informed consent from the parents or legal guardians.

The patients were randomly assigned via a computer-generated randomization list to treatment with either noncompressible or routine forced-air warming. Before the sedated patient was carried into the operating room (OR), an opaque envelope containing the computer-generated randomization was opened to determine the assigned warming method. Anesthesia and fluid management were administered as desired by the attending anesthesiologist. Perioperative blinding for the anesthesiologist was not possible, as the difference between the devices was too obvious; however, patients were blinded to group assignment and data analysis was also performed with blinded group assignments. After the sedated patient arrived in the OR, standard monitoring was applied and the skin temperature probes were taped to the patient’s skin. Induction of anesthesia was performed and the rectal probe was placed, temperature measurement was initialized, and warming devices were started.

Study Groups

Figure 1:
Patient-warming devices. Left: Pediatric Underbody Blanket no. 555, 3M, St. Paul, MN, using the forced-air blower Bair Hugger model no. 750, 3M, St. Paul, MN; right: Baby/Kleinkinddecke of MoeckWarmingSystems, art. 902, Moeck & Moeck GmbH, Hamburg, Germany, using Moeck und Moeck Twinwarm forced-air blower, Moeck & Moeck GmbH, Hamburg, Germany.

The devices are displayed in Figure 1.

Group BH

Warming with Pediatric Underbody Blanket no. 555, 3M, St. Paul, MN, using the forced-air blower Bair Hugger model no. 750, 3M, St. Paul, MN, initial temperature setting 43°C (no specific lower recommendation for pediatric temperature setting by manufacturer); standard device.

Group MM

Warming with Baby/Kleinkinddecke of MoeckWarming Systems, art. 902, Moeck & Moeck GmbH, Hamburg, Germany, using Moeck und Moeck Twinwarm forced-air blower, Moeck & Moeck GmbH, pediatric temperature setting as recommended by the manufacturer (40°C, air power 3; as recommended by the manufacturer). No plastic drapes covered the patients and only the standard surgical draping was used.

If one of the randomized warming mattresses was torn or otherwise damaged, it was immediately exchanged before use. The mattress was used for patient transfer and postsurgery in the recovery room, as needed, to preserve normothermia. However, data acquisition ended immediately after surgery.


A probe (Mallinckrodt Anesthesiology Products, Inc., St. Louis, MO) was introduced into the rectum to measure core temperature. Before warming in the OR, 4 temperature probes were attached with tape to the upper arm, thigh, abdomen, and lower back of the patients. Treatment with BH or MM mattress was started as soon as possible. Primary outcomes were the differences between slopes (°C/min) and incidences of necessary temperature downregulation of devices. Skin temperatures were secondary outcomes.

Other measurements included fluid balance (crystalloid and colloid in mL, normalized by body weight), estimated blood loss (normalized by body weight), OR temperature, duration of intervention, demographic and morphometric baseline parameters (age, weight, height, sex, and type of surgical intervention), use of additional warming methods (e.g., fluid warming), and skin reactions to warming. Neither crystalloid nor colloid fluids were warmed on a routine basis.

The measurements were recorded every 5 minutes until the end of surgery, when intraoperative warming was stopped. Anesthesia was administered according to the discretion of the attending anesthesiologist.

All devices were set on the highest heat setting allowed in the manufacturer’s manual for pediatric use (43°C for Bair Hugger, 40°C for Moeck und Moeck Twinwarm)—if core temperature ≥37.5°C occurred in any patient, forced-air temperature was reduced. If after 10 minutes Tcore remained >37.5°C or would rise higher, the device was stopped for 10 minutes and afterward continued on the second highest setting. After 10 minutes, the temperature was reassessed and warming was either continued or discontinued as described earlier. All incidences of reduction or stopping of warming devices were recorded.

Statistical Methods

From previous experience, the slope of forced-air warming with the Bair Hugger device was typically approximately 0.28 ± 0.35°C/h. A change to 0.50°C/h may be considered clinically relevant. With α = 0.05 and power (1 − β) of 0.80, a sample size of 40 patients per group was calculated for a 2-sided Student t test with G*Power 3.1 for the first primary outcome, the temperature slope.12

Assumption of normality of temperature data was assessed and confirmed with QQ-plots. The group difference between core temperature slopes was analyzed by comparing the slopes (°C/time) using a 2-tailed unpaired Student t test. The incidence of downregulation of devices and the distribution of surgeries in the 2 study groups were compared with a χ2 test. As 2 primary outcomes were assessed, the hypotheses were hierarchically ordered. Only if the first null hypothesis (no difference between core temperature slopes) was rejected was the second null hypothesis (no differences in incidence of downregulation) analyzed with a P of 0.05 considered significant. Incidences of hypothermia (Tcore <35.5°C) were recorded and compared with χ2 test between the groups. Differences of sex, incidence of hypothermia, and frequency of use of additional warming methods between the groups were calculated with the Fisher exact test.

All results are expressed as means ± SDs or as median (interquartile range), as applicable. R 3.1.2 (R Foundation for Statistical Computing, Vienna, Austria) was used for analysis, and GraphPad Prism 5.0b (La Jolla, CA) was used for figures.


All consented patients (n = 80) completed the study period. Figure 2 displays the CONSORT Flow Diagram for the present study. Table 1 shows the patients’ morphometric baseline parameters (age, sex, weight, and height) and type of surgical intervention.

Table 1:
Morphometric Baseline Parameters and Type of Intervention
Figure 2:
CONSORT flow diagram.

There was a distinct difference of temperature slope between the 2 groups (Fig. 3). Core temperatures of patients in the group MM remained relatively stable. Mean of the core temperatures of patients in the group BH increased significantly (difference: +1.48°C/h; 95% confidence interval [CI], 0.82–2.15°C/h; P = 0.0001). The need for temperature downregulation to avoid hyperthermia, as defined in the protocol, occurred more often in the BH group, with 22 vs 7 incidences (RR, 3.14; 95% CI, 1.52–6.52; P = 0.0006).

Figure 3:
Core temperature (rectal, line: mean, bars: SD).
Table 2:
Fluid Balance, Average OR Temperature, Duration of Intervention, and Incidence of Use of Any Additional Warming Methods (e.g., Fluid Warming), Incidence of Hypothermia (Tcore <35.5°C at Least for One Measurement) and Incidence of Skin Reactions to Warming, No Differences Between the Groups for Any Parameter
Figure 4:
Skin temperature back (line: mean, bars: SD).
Figure 5:
Skin temperature chest (line: mean; bars: SD).
Figure 6:
Skin temperature upper arm (line: mean; bars: SD).
Figure 7:
Skin temperature thigh (line: mean; bars: SD).

Table 2 shows fluid administration, average OR temperature, duration of intervention, use of any additional warming methods (e.g., fluid warming), incidence of hypothermia, and incidence of skin reactions to warming. There were 4 patients with brief episodes of hypothermia in the MM group and 6 patients in the BH group (RR, 0.67; 95% CI, 0.20–2.19; P = 0.74). Application of additional warming methods (i.e., fluid warming) was used for one patient in the MM group, and no skin reactions to warming were observed in either group. Figures 4 to 7 display the different skin temperature courses between the groups, with all skin temperatures remaining consistently higher in comparison with the MM group, including skin temperatures on the patients’ back.


In the present study, we compared a standard, compressible pediatric patient forced-air mattress warming system with an alternative system for distributing forced air via a noncompressible mattress.

Our results are in accordance with the results by Witt et al.10 In their multicenter, observational study, the same noncompressible mattress system was used in patients perioperatively and also during diagnostic interventions. However, the researchers did not limit the forced-air temperature, as in the present study, to the manufacturer-recommended 40°C and used forced-air temperatures of up to 43°C. Notably, the researchers encountered both inadvertent hypothermia and hyperthermia in their patients. In 8 of the 119 included children, a temperature >38°C was registered.

There are several other technical options in use for pediatric warming. Warming can also be performed via conductive warming13 and infrared heater. Loss of body heat can be ameliorated via warmed fluid14 and increased environmental temperature. There are numerous studies comparing different patient-warming techniques and devices, mostly conducted in adult patients15–18 or in pediatric mannequins19–22 with some pediatric patient studies conducted as observational studies.10,23,24

In previous studies, differences in performance of forced-air devices were observed.25,26 In a study looking at the factors influencing heat transfer of forced-air warming system combinations,27 the authors noted that blanket design was a major determinant of forced-air warming system efficacy. However, in the present study, not just 2 mattress designs but 2 completely different forced-air warming systems were compared. These systems differed not only in their mattress design but also had different forced-air blowers and recommended blower settings. Although this might be considered a limitation of the study, it is in our opinion not possible to choose any other temperature setting than the one recommended by the manufacturer, because thermal burns caused by forced-air warming devices are an actual, documented risk.28,29

In the present study, we encountered a more stable temperature course in the MM group with lower forced-air temperature, and the necessity to adjust forced-air temperature was significantly reduced. This finding is of particular importance because pediatric patients are not only prone to hypothermia but may also easily become hyperthermic. Hyperthermia may especially occur, when no, or inadequate, core temperature measurement techniques (e.g., ear-infrared thermometry) are used30 or if the temperature probe dislodges perioperatively. For the present study’s patient population, the MM device thus showed—using the pediatric setting—a “flat-line” core temperature course with almost no user intervention necessary to adjust forced-air temperature. Obviously, it is valid to hypothesize that changing forced-air temperature to a lower setting for the BH group might have had a similar effect on the core temperature course.

One of the claimed benefits of the noncompressible mattress is that, underneath the patient, warm air can circulate in the integrated mattress supporting structure, whereas in the forced-air mattress the air canals are simply compressed and thus blocked. Surprisingly, an analysis of the lower back temperatures in our pediatric setting could not identify any superiority for the noncompressible mattress system. In the present investigation, only patients aged <2 years were studied, although there are no strict age restrictions for the warming mattresses. This patient group was chosen to keep the study groups as homogeneous as possible and because the tolerance for hypothermia in these young patients is particularly limited.

In summary, we found that both the compressible and the noncompressible forced-air mattress systems are feasible choices for active, perioperative pediatric patient warming. The increase of core temperature was steeper in the compressible mattress group (BH), whereas in the noncompressible mattress group (MM), with lower, pediatric forced-air temperature settings, a more stable core temperature course was observed and the need for temperature downregulation to avoid hyperthermia was reduced. E


Name: Lydia Triffterer, MD.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Conflicts: Lydia Triffterer reported no conflicts of interest.

Attestation: Lydia Triffterer has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.

Name: Peter Marhofer, MD.

Contribution: This author helped analyze the data and write the manuscript.

Conflicts: Peter Marhofer reported no conflicts of interest.

Attestation: Peter Marhofer has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Irene Sulyok, MD.

Contribution: This author helped analyze the data and write the manuscript.

Conflicts: Irene Sulyok received research funding from Moeck & Moeck.

Attestation: Irene Sulyok has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Maya Keplinger, MD.

Contribution: This author helped conduct the study.

Conflicts: Maya Keplinger reported no conflicts of interest.

Attestation: Maya Keplinger has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Stefan Mair, MD.

Contribution: This author helped conduct the study.

Conflicts: Stefan Mair received salary via research funded by Moeck & Moeck.

Attestation: Stefan Mair has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Markus Steinberger, MD.

Contribution: This author helped conduct the study.

Conflicts: Markus Steinberger reported no conflicts of interest.

Attestation: Markus Steinberger has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Wolfgang Klug, MD.

Contribution: This author helped conduct the study.

Conflicts: Wolfgang Klug reported no conflicts of interest.

Attestation: Wolfgang Klug has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Oliver Kimberger, MD.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Conflicts: Oliver Kimberger received research funding from Moeck & Moeck and was funded by 3M for a lecture.

Attestation: Oliver Kimberger has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

This manuscript was handled by: James A. DiNardo, MD.


a NICE Clinical Guideline 29. London: National Institute for Health and Clinical Excellence; 2008. Perioperative hypothermia (inadvertent): the management of inadvertent perioperative hypothermia in adults. Available at: Accessed March 1, 2015.
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