A Summary of fewer than 150 words should state the purpose of the study or investigation, basic procedures, main findings (giving actual results not just a broad description) and their statistical significance (using actual p values), and principal conclusions. The Summary should not be structured nor in note or abbreviated form. It should not state that ‘the results are discussed’ or that ‘work is presented’. Abbreviations should not be used except for units of measurement. Use the same order when discussing the methods and results as in the main body of the text, and always mention the groups in the same order.

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Introduction:
Perioperative hypothermia, defined as a core temperature below 36°C, is still one of the most common side effects of general anaesthesia (1, 12) and results from low preoperative core temperatures (19), anaesthetic-induced inhibition of thermoregulatory defenses with redistribution of heat after induction of anaesthesia combined with a cold surgical environment, administration of unwarmed intravenous fluids, and evaporation from surgical incisions (25).
Several prospective, randomized trials and retrospective studies have shown that perioperative hypothermia is associated with numerous adverse effects and outcomes (24). Following head and neck surgery perioperative hypothermia can cause delayed extubation, the development of early perioperative wound complications e.g. neck seromas, and flap dehiscence (2, 26). Although the authors of these studies recommend active warming for patients at risk for intraoperative hypothermia (2, 26) most patients are not actively warmed during head and neck surgery.
The purpose of this prospective, randomized, controlled study was to test the hypothesis that the use of a new conductive warming system (PerfecTempâ„¢, The Laryngeal Mask Company Limited, St. Helier, Jersey) in combination with insulation is superior to reduce the incidence of intraoperative and postoperative hypothermia during head and neck surgery compared to insulation only.
Methods:
After approval of the protocol by our local hospital ethics committee, 40 patients were recruited. Written, informed consent was obtained from all patients on the day prior to anaesthesia and surgery. All patients in the study were required to be adults between 18 and 75 yrs, to have American Society of Anesthesiology physical status I-III and to undergo elective, head or neck surgery that was scheduled to last between 90 min and 180 min.
The exclusion criteria were: age > 75 yr; body mass index 30 kg/m²; preoperative temperature > 38°C or 180 min.
All patients were premedicated with 7.5 mg oral midazolam. General anaesthesia was induced with propofol (2 to 2.5 mg per kg of body weight) and remifentanil (0.2-0.5µg/kg) followed by rocuronium (0.4-0.6 mg/kg) to facilitate tracheal intubation. Anaesthesia was maintained with infusions of remifentanil and propofol titrated to maintain adequate anaesthetic depth and hemodynamic stability.
The ambient temperature of the O.R. was 19°C. Sublingual temperatures were measured preoperatively with an electronic thermometer (Geratherm rapid, Geratherm Medical AG, Geschwenda, Germany). During all measurements, sublingual placement and mouth closure was carried out by member of the study team (A.R.) experienced in the use of this device. Following induction, until the end of surgery, oesophageal temperatures were measured every 15 minutes using a temperature probe (TEMPRECISE #4-1512-A, Arizant International Corp. Eden Prairie, MN, USA) inserted 30 to 35 cm into the distal oesophageus.
All patients were identified through the daily surgical schedule. A computer generated randomisation list with four blocks of ten patients was used to allocate patients to either the treatment group (conductive warming and insulation) or control group (insulation only).
In the treatment group the patients were positioned supine on the conductive warming mattress (190.5 cm x 50.8 cm) (LMA PerfecTempâ„¢, The Laryngeal Mask Company Limited, St. Helier, Jersey) placed on the operating table, as suggested by the manufacturer. Then the patients were immediately insulated with a standard hospital duvet (188 cm x 122 cm), filled with Trevira (100% polyester) (Brinkhaus GmbH & Co. KG, Warendorf, Germany) with an insulation value of 1.29 clo (6). The conductive patient warming system was set to a temperature of 40.5°C throughout the study and warming was stopped when the oesophageal temperature was > 37.5°C.
Patients of the control group were positioned supine on the operating table and were immediately insulated with the standard hospital duvet.
All intravenous fluids were infused at room temperature. The duration of anaesthesia and surgery (time from skin incision to last suture) were recorded.
Power analysis, assuming a clinically important reduction in the incidence of intraoperative and postoperative hypothermia from 50 % to 90% suggested that eleven patients were required in each group (α = 0.05; β = 0.2). To compensate for unexpected dropout of patients with a shorter or longer duration of surgery than planned the initial total number of recruited patients was increased to 20 patients in each group.
Comparisons of nominal data were made using the Fisher’s exact test. A Kolmogorov-Smirnov test was used prior to parametric testing to ascertain that values came from a Gaussian distribution. Comparisons of normally distributed data were made using the Student’s t-test. Comparisons of not normally distributed data were made using the Mann-Whitney-U test. Time-dependent changes of core temperature were evaluated using repeated-measures analysis of variance (ANOVA) and post hoc Scheffé’s test. Results are expressed as means ± SD or as median and interquantil range as appropriate. A value for p Results
A total of 86 patients were assessed for eligibility. 25 patients could not be asked to participate, because they came to the hospital on the day of the operation. 21 patients refused to participate. Of the 40 patients recruited, 10 patients had to be excluded because of an operating time below 60 minutes (five patients in the treatment and four in the control group) or above 180 minutes (one patient).
Figure 1: Flow diagram of the study
In three patients the conductive warming mattress did not fully heat up to 40.5°C for unknown technical reasons. These patients were still included in the data analyses. Data were therefore complete for 15 patients in each group. Patient characteristics, ambient temperature of the O.R., core temperatures before induction of anaesthesia and duration of surgery were not different (table 1).
Table 1 Patient characteristics and perioperative variables. Values are presented as mean values ± SD, median and interquantil range [IQR] or numbers of patients.
Variable
Treatment group (n = 15)
Control group (n = 15)
P-value
Age [yr]
51±18
51±15
0.99
Sex [m/f]
7/8
10/5
0.46
Height [cm]
173±11
175±10
0.64
Weight [kg]
74±16
80±9
0.21
Temperature of the O.R [°C]
19±1
19±1
0.3
Core temperature before induction of anaesthesia [°C]
36.1±0.4
35.9±0.5
0.33
Duration from positioning on the conductive warming mattress to induction of anaesthesia [min]
7 [IQR: 5-9]
–
–
Duration of anaesthesia [min]
118±28
122±38
0.74
Duration of surgery [min]
97±25
103±37
0.61
The ANOVA identified a significantly higher core temperature in the treatment group at 45, 60, 75, 90, 105 and 120 min (Figure 2). Further testing was futile as there were only three patients with a longer duration of surgery included.
Figure 2 Mean pre- and intraoperative temperatures of the treatment group and control group. Error bars represent SD. In each group data were complete for at least sixty minutes.
Furthermore, Fishers’s exact test confirmed a lower incidence of intraoperative (3 vs. 9 patients; p = 0.03) and postoperative hypothermia (0 vs. 6 patients; p = 0.008) in the treatment group. However, the mean duration of hypothermia was not significantly shorter in the treatment group (55±17 min vs. 80±51 min; p = 0.42). No adverse effects could be observed.
Discussion:
This prospective, randomized, controlled study demonstrates that, during head and neck surgery under general anaesthesia, a conductive warming mattress combined with insulation significantly reduces the incidence of intraoperative and postoperative hypothermia compared to insulation only. With this approach the incidence of intraoperative and postoperative hypothermia could be reduced significantly. However, the mean intraoperative duration of mild hypothermia could not be reduced significantly.
Redistribution of body heat from the core to the periphery was unusually small in this study and similar in both groups as core temperature decreased only 0.1°C in the control group and 0.2°C in the study group. In most clinical studies redistribution of heat after induction of anaesthesia leads to a reduction in core temperature of about 0.3°C to 0.8 °C (3, 4, 8, 28) in the first hour whereas under experimental conditions it can reach up to 1.7°C (17). This small decrease in core temperature may be explained by the fact that patients were kept comfortably warm during the whole preoperative period (ward, transport to the O.R. and induction of anaesthesia) with the same good insulating hospital blanket as used intraoperatively. This approach refers to the recent NICE guideline “Inadvertent perioperative hypothermia. The management of inadvertent perioperative hypothermia in adults” (22).
Patients during head and neck surgery are often thought to have a relatively low risk for perioperative hypothermia because in most cases no body cavity is opened, the surgical incisions as well as blood losses are small. This is probably why there are almost no studies about perioperative hypothermia and its prevention during head and neck surgery. However, many patients undergoing head and neck surgery are prone to hypothermia by advanced age (2, 14, 27) and cancer with associated malnutrition and low body weight (2, 16). According to their preoperative risk profile (e.g. ischemic heart disease, diabetes mellitus, chronic obstructive pulmonary disease, preoperative radiotherapy, preoperative chemotherapy) (20, 26) they are often vulnerable to hypothermia associated complications. These complications include an increasing incidence of myocardial ischemia (10, 11, 11) which is also a relevant complication after reconstructive head and neck surgery (7), augmenting blood loss (23), decreasing resistance to surgical wound infections or increasing local wound complications (2, 15, 18, 26), thus prolonging hospitalization.
The few existing studies were particularly focused on longer operations like parotidectomies, neck dissections (2) and reconstructive surgery with free tissue or regional flaps (13, 26). In the study of Agrawal et al. (2) the incidence of perioperative hypothermia was 65% in the unwarmed group showing clearly the high risk of perioperative hypothermia in patients during head and neck surgery. In our study with relatively short operations we observed an incidence of perioperative hypothermia of 40% in the control group. In contrast to the study of Agrawal et al. (2) we used a high insulation of 1.29 clo for these patients which is much more than the insulation value of most commercially available materials designed for use in the operating room. With this insulation heat losses from the covered skin can be reduced about 70%. (6). In most of our patients this insulation was able to maintain a stable thermal steady state with a relative constant core temperature. However, this thermal steady state was at a core temperature of about 36.0°C with many patients being hypothermic.
In general the efficacy of posterior patient-warming systems is limited (5, 9, 13, 21). These devices have the disadvantage that warming the back of the patient in the supine position is suboptimal. During surgery, little heat is lost from the back (9) and heat gain via the back is also limited, resulting in a small change in heat balance. However, in this special setting the additional heat generated by the conductive warming system leads to a positive thermal balance and an increasing core temperature after 30 minutes. In contrast to conventional circulating water mattresses the new conductive system is made of thick viscoelastic foam. This material enhances contact between the mattress and the back, thereby reducing thermal contact resistance and increasing the efficacy of heat exchange.
In contrast to forced-air warming the combination of good insulation and conductive warming has several advantages. There are no expensive disposables elements, low costs for maintenance, low power consumption and no relevant noise emission (28). Another advantage is that is very easy to use the system for prewarming as soon as the patient can be placed on the operating table when the controller unit is mounted at the operating table.
Our study has several limitations. First, two different anatomic locations were used to measure core temperature (oral temperature before induction of anaesthesia and oesophageal during general anaesthesia). However, both methods are reasonable methods for core temperature measurements and we could record the first reliable oesophageal temperature 5 minutes after induction of anaesthesia so that this temperature can serve as a reliable starting temperature.
Second, five patients per group had to be excluded from data analyses because the operation time was shorter or longer than planned. Nevertheless, we had to exclude these patients because it is not advisable to compare operations with durations of 30 minutes with operations of more than 3 hours.
Finally we did not fully take advantage of the possibility to prewarm our patients with the conductive system. On average time from the beginning of warming to induction of anaesthesia was only seven minutes. It seems to be likely that longer prewarming periods would enhance the efficacy of the conductive warming mattress.
Conclusion
The combination of good thermal insulation and conductive warming is effective to prevent perioperative hypothermia during head and neck surgery. In contrast to other warming methods there are no expensive disposables, low costs for maintenance, low power consumption and no relevant noise emssion.