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Relative efficiency of two warming devices during laparoscopic cholecystectomy.

There is variability among anaesthetists with regard to monitoring temperature during anaesthesia. Sessler proposed guidelines and recommended temperature monitoring should be performed routinely during anaesthesia, even if the procedure lasts for 30 minutes (1).

The human temperature typically ranges between 36.5 and 37[degrees]C: values less than 36[degrees]C indicate loss of control on thermoregulatory defences. General anaesthesia impairs normal autonomic thermoregulation and consequently the interthreshold range increases 10-fold, to approximately 2 to 4[degrees]C (2). Laparoscopic surgery may further exacerbate hypothermia (3,4). There are conflicting reports of heat loss during laparoscopy and open cholecystectomy (3-5). Laparoscopic insufflations lasting more than an hour have been reported to drop the abdominal gas and core temperature by 1[degrees]C even when warming methods were used (5).

The forced air warming device (Bair Hugger[TM], Augustine, USA) has been shown to be effective in maintaining body temperature during anaesthesia (6). As opposed to the use of warmed air, radiant warmers use infrared B energy to warm patients. There is limited data on use of the radiant warming device (Sun-touch[TM], Fisher and Paykel, New Zealand) during laparoscopic surgery. In 2004 Wong et al compared a radiant warmer (Sun-touch) and forced air warming device (Bair Hugger) in 42 patients during laparoscopic surgery lasting one hour and showed that both were effective in preventing heat loss (7). In the same year, Lee et al compared the above devices in 60 general and spinal surgery patients lasting over two hours and found the radiant warmer ineffective in maintaining core temperature (8). There appear to be no other studies reporting Sun-touch radiant warmer use under general anaesthesia. The aim of the current study was to compare the efficacy of the Sun-touch radiant warmer to a forced air warming device in preventing hypothermia in patients undergoing laparoscopic cholecystectomy lasting for more than one hour.

MATERIAL AND METHODS

The study was approved by The Queen Elizabeth Hospital Ethics Committee.

Inclusion criteria

All patients, aged 18 to 75 years and presenting for elective laparoscopic cholecystectomy, were considered for entry into the study. They were seen preoperatively by their anaesthetist, given an information sheet before participation in the study and written informed consent was obtained. The surgical procedure was expected to take >60 minutes, as previous review of such procedures had at our hospital indicated a training registrar operating time of 60 to 90 minutes.

Patients requiring emergency or open cholecystectomy and who were on antipyretic medication were excluded. Patients with a history of malignant hyperthermia or preoperative temperature of either >37.5[degrees]C or <35.5[degrees]C were also excluded. Perioperative hypothermia was considered as a temperature below 36[degrees]C. The postoperative temperature was measured on immediate arrival in the recovery room.

After consent in the holding bay, randomisation was performed via a closed, opaque envelope system and numbered cards (one or two) indicating to which of the two groups the patient was to be assigned. These were: group 1, Warm-touch[TM] (Patient warming system, Tyco Health Care, Mallinckrodt Medical, USA) and group 2, Suntouch radiant warmer model PW820 AEA (Fisher and Paykel, New Zealand). Initial temperature (T0) was taken preinduction using a tympanic probe (Genius First temp model 3000A, Sherwood Medical, St Louis, USA). If the temperature was between 35.5[degrees]C and 37.5[degrees]C, the patient entered the study. Patients were not informed about what device would be used. None of the patients had preoperative warming. Intravenous fluids were warmed in all groups via a Ranger blood/fluid warming system model 245 (Augustine Medical Inc, Mo, USA) set at 41[degrees]C. An oesophageal temperature probe (Thermistor 400 Series Mon-a-therm 9 Fr; Mallinckrodt, Ireland) was utilised to measure the ambient theatre temperature after anaesthesia was induced. After induction, an oesophageal temperature probe was introduced nasally to reach the lower oesophagus at the 30 cm mark. Initial core temperature was recorded before commencement of surgery at T15 and thereafter measured every 15 minutes till the end of procedure. The position of the patients was supine and upper limbs were at right angles to the table and laid in soft gel gutters. The eyes were taped to avoid a direct heat effect of the radiant warmer.

In group 1, the wrap was placed on the upper part of the body just below the nipple and fixed in position with adhesive tape, and the Warm-touch was switched to the highest level (46[degrees]C). In group 2, the Sun-touch was used after induction. The overhead warmer had a robust stand to facilitate easy manoeuvring to the required direction. The device was placed 40 cm above the head before the power was turned on. The heat was assessed after turning on the sensor using the dorsum of the hand. The sensor height and position lights were adjusted so that the red light position converged on the patient's forehead. The warmer alarms if too close to the skin. A skin temperature sensor was placed on the patient's forehead near to the red positioning light and secured to the skin with tape. The warmer was set at 41[degrees]C as per the manufacturer's recommendations for adults. The warmer reduces its power once the set skin temperature is reached.

Premedication was given in the induction room using midazolam 1.5 mg IV and standard monitoring for general anaesthesia was instituted. General anaesthesia was standardised for induction using propofol 2 mg/kg, fentanyl 2 [micro]g/kg followed by rocuronium 0.6 mg/kg. After intubation, ventilation was maintained to achieve an end-tidal C[O.sub.2] of 35 to 45 mmHg using a disposable closed circuit and airway filter (Portex Thermovent Hepa, Smith Medical, USA). Maintainence of anaesthesia was by sevoflurane, [O.sub.2] and air to maintain an inspired [O.sub.2] concentration of 50%. The total gas fresh flow was limited to 1.0 l/min. The amount of fluids administered was recorded. Reversal agents were standardised as neostigmine (2.5 mg) and glycopyrrolate 0.4 mg. On extubation, the temperature of the Warm-touch was recorded by inserting the oesophageal probe into the blanket. In the recovery ward, patient temperature was recorded on arrival and any untoward events were observed.

Statistical analysis

Continuous variables were reported as mean (SD) or as indicated. Differences between continuous variables were assessed by the t-test (assuming unequal variances) or as indicated, and categorical variables by Fisher's exact test. Due to the unbalanced data set (not all patients had the same length of surgery), group and over-time oesophageal temperature differences (and their interaction) were assessed using a linear mixed model (individual subjects [intercepts] considered as random effects with indivdual time slopes: random coefficient model) and an unstructured covariance matrix (the "xtmixed" Stata[TM] module), as recently recommended by Mallinckrodt et al (9). Stata[TM] statistical software, V10 2007, was used (College Station, TX, USA).

RESULTS

Between 2004 and 2006, 30 consecutive patients presenting for elective laparoscopic cholecystectomy were enrolled; one patient from group 2 withdrew in the preinduction room, leaving 14 patients in group 1 and 15 patients in group 2. No significant differences in baseline characteristics were found (Table 1); parenteral fluid administration (at a mean of 1.8 l) did not differ between the groups and there was no significant overall difference between procedural times (sign rank test, P >0.99). Figure 1 displays individual patient oesophageal temperatures from T15. With few exceptions, recorded temperatures showed minimal over-time difference.

No between group (P=0.74), over-time (P=0.62) or group*time interaction (P=0.91) temperature changes were found; albeit the coefficient point estimates suggested a uniform negative effect (Table 2). The (patient-level) intra-class correlation was 0.896.

[FIGURE 1 OMITTED]

Recorded postoperative complications are shown in Table 3. No difference in the incidence of postoperative hypothermia was found and a modest increase in the incidence of headache was observed in group 2.

DISCUSSION

Our study demonstrates that the Sun-touch device has similar efficacy to the Warm-touch in maintaining intraoperative patient temperatures and avoiding perioperative hypothermia. This was based on the use of oesophageal temperature, which is accepted as the most accurate measurement of core temperature (10).

Wong et al (7) demonstrated the efficiency of the Sun-touch warmer with respect to a forced air warming device (Bair Hugger) by attaining oesophageal temperatures of 36 and 36.2[degrees]C respectively by the end of the surgery. Both the current study and that of Wong et al used similar standardised surgical procedures, albeit surgery in the latter study lasted only up to one hour. In contrast, Lee et al reported that final core temperatures at the end of surgery were 36[degrees]C in the Sun-touch and 36.4[degrees]C in the forced air warming device group (Bair Hugger), with a difference of 0.4[degrees]C. The conclusion was that the former device was not effective (8), albeit the surgical procedures were not standardised. Torrie et al found postoperative (rectal) temperatures in both Sun-touch and Bair Hugger warming patient groups were below 36[degrees]C. The procedures were done under spinal anaesthesia and it was concluded that neither of these devices were effective in preventing hypothermia11. The average procedure time was less than one hour. In the current study the median duration of surgery was about 1.5 hours and there was a possible added impact on temperature maintenance due to the laparoscopy. In laparoscopic insufflations there may be a drop in abdominal gas temperature from 36 to 27.7[degrees]C, eventually decreasing the overall core temperature by up to 1[degrees]C (5). Lee et al and Torrie et al both used the Sun-touch warmer on the hand. Our study and that of Wong et al used the warmer directed onto the face and this site may be more effective (7). Wong did not comment on any radiant warmer effect upon postoperative headache, the latter being a statistically significant complication of warming in our study. The mechanism of action may involve dilation of facial vessels leading to increasing blood volume to the head.

To transfer heat the Sun-touch warmer has to heat the subdermal vasculature in an area of high concentration of arteriovenous anastomoses, such as in the face, hands or feet (12). The face was the ideal site in our study. The infrared heat source penetrates the skin and heats arteriovenous anastomoses and further dilates it 40 times resulting in increased blood flow in the skin (13). This heated blood is returned to the heart via the venous system and then redistributed throughout thus increasing the core temperature.

Apart from possible increase in incidence of headache, compared with the Warm-touch, other disadvantages to the Sun-touch warmer are sensor position changes with table height alteration and the unsuitability for use in peripheral vascular disease due to decreased transfer of heat. Positional adjustment during the cholangiogram caused shifting of the red spot position and required vigilance to prevent the red spot focussing on the eyes. Although the eyes were covered by tape during anaesthesia for safety, long exposure may go unnoticed with possible deleterious effects. With respect to cost, the Warm-touch costs about A$1800 and each disposable paper wrap is A$14. The one-time cost of Sun-touch is A$4500 and its use in high volume environments will have negligible cost implications. other advantages of the Sun-touch are that it consumes less power compared with Warm-touch and is suitable when Warm-touch is contraindicated. As the warmer occupies a small area of the face of the patient and it is away from the surgical site, there is less concern of infection (7).

Although we did not find an efficacy difference between the two devices, our sample number was small and the ability to detect a difference ([beta] error) was constrained. Using a linear mixed model (power simulations were performed using PASS 2008 software) (9) for a minimal detectable difference between groups of 0.6[degrees]C and 0.1[degrees]C within subjects, we had a power of 84% (95% confidence interval 74 to 92%) and 82% (95% confidence interval 71 to 91%) respectively. For a minimal detectable difference between groups of 0.5[degrees]C and 0.1[degrees]C within subjects there was a power of 70% (95% confidence interval 57 to 80%) and 83% (95% confidence interval 72 to 91%). Thus our ability to detect a between-group difference of 0.5[degrees]C, which we feel is a reasonable minimally important clinical difference, was marginal. The starting temperatures (recorded at T15 minutes) in our study were 36.2[degrees]C, consistent with other referenced studies and is expected during the first hour of general anaesthesia. These temperatures were maintained over time during anaesthesia (Figure 1), but a minority of patients had recorded postoperative hypothermia (Table 3).

In conclusion, we found no difference in the efficacy of the two warming devices. Sun-touch may have an advantage in intermediate surgery such as laparoscopic cholecystectomy, but not on the basis of efficacy. Postoperative headache has not been previously documented with use of the Sun-touch. This complication should be further investigated before its routine use on the face can be recommended.

ACKNOWLEDGEMENT

We thank the anaesthetic nurses at the Queen Elizabeth Hospital who were very supportive during the study.

Accepted for publication on December 4, 2008.

REFERENCES

(1.) Sessler DI. A proposal for new temperature monitoring and thermal management guidelines. Anesthesiology 1998; 89:1298-1300.

(2.) Sessler DI. Temperature monitoring and perioperative thermoregulation. Anesthesiology 2008; 109:318-338.

(3.) Berber E, String A, Garland A, Engle KL, Kim KM, Ituarte P et al. Intraoperative thermal regulation in patients undergoing laparoscopic vs open surgical procedures. Surg Endosc 2001; 15:281-285.

(4.) Luck AJ, Moyes D, Maddern GJ, Hewett PJ. Core temperature changes during open and laparoscopic colorectal surgery. Surg Endosc 1999; 13:480-483.

(5.) Jacobs VR, Morrison JE Jr, Mundhenke C, Golombeck K, Jonat W. Intraoperative evaluation of laparoscopic insufflation technique for quality control in the oR. JSLS 2000; 4:189-195.

(6.) Kurz A, Kurz M, Poeschl G, Faryniak B, Redl G, Hackl W. Forced-air warming maintains intraoperative normothermia better than circulating-water mattresses. Anesth Analg 1993; 77:89-95.

(7.) Wong A, Walker S, Bradley M. Comparison of a radiant patient warming device with forced air warming during laparoscopic cholecystectomy. Anaesth Intensive Care 2004; 32:93-99.

(8.) Lee L, Leslie K, Kayak E, Myles PS. Intraoperative patient warming using radiant warming or forced-air warming during long operations. Anaesth Intensive Care 2004; 32:358-361.

(9.) Mallinckrodt CH, Lane PW, Schnell D, Peng y, Mancuso JP. Recommendations for the primary analysis of continuous endpoints in longitudinal clinical trials. Drug Inf J 2008; 42:303-321.

(10.) Cork RC, Vaughan RW, Humphrey LS. Precision and accuracy of intraoperative temperature monitoring. Anesth Analg 1983; 62:211-214.

(11.) Torrie JJ, Yip P, Robinson E. Comparison of forced-air warming and radiant heating during transurethral prostatic resection under spinal anaesthesia. Anaesth Intensive Care 2005; 33:733-738.

(12.) Bergersen TK. A search for arteriovenous anastomoses in human skin using ultrasound Doppler. Acta Physiol Scand 1993; 147:195-201.

(13.) Thoresen M, Walloe L. Skin blood flow in humans as a function of environmental temperature measured by ultrasound. Acta Physiol Scand 1980; 109:333-341.

V. RAO KADAM *, D. MOYES ([dagger]), J. L. MORAN ([double dagger])

Department of Anaesthesia, The Queen Elizabeth Hospital, Woodville, South Australia, Australia

* M.B., B.S., M.D., D.N.B., F.A.N.Z.C.A., Senior Consultant.

([dagger]) M.B., B.Ch., M.R.C.S., L.R.C.P., F.F.A.R.C.S., F.R.C.A., F.A.N.Z.C.A., Professor and Head of Critical Care.

([double dagger]) M.B., B.S., F.R.A.C.P., F.J.F.I.C.M., M.D., Senior Consultant, Intensive Care Unit, The Queen Elizabeth Hospital.

Address for reprints: Dr V. Rao Kadam, Department of Anaesthesia, The Queen Elizabeth Hospital, 28 Woodville Rd, Woodville, SA 5011.
TABLE 1
Demographic analysis

Age (y) Warm-touch

Gender (M:F) 40.9 [+ or -] 15.0
Body mass index (kg/[m.sup.2]) 7:7 (14)
Ambient temperature ([degrees]C) 30.9 [+ or -] 5.2
Oesophageal temperature 20.7 [+ or -] 1.9
([degrees]C) (T15) 36.2 (0.3)
Parenteral fluids (l) 1.80 (0.68)
Surgical time (mins) 90 (60-180)

Age (y) Sun-touch

Gender (M:F) 39.0 [+ or -] 10.1, P=0.69
Body mass index (kg/[m.sup.2]) 9:6 (15), P=0.72
Ambient temperature ([degrees]C) 28.7 [+ or -] 6.2, P=0.31
Oesophageal temperature 19.9 [+ or -] 1.7, P=0.31
([degrees]C) (T15) 36.2 (0.6), P=0.89
Parenteral fluids (l) 1.86 (0.54), P=0.78
Surgical time (mins) 90 (90-150), P=1.0

T15=temperature recorded at 15 minutes after beginning of
anaesthesia.

TABLE 2
Linear mixed model analysis; variable estimates

Variable Coefficient P value 95% confidence
 interval

Sun-touch -0.073 0.74 -0.500, 0.354
Time -0.018 0.62 -0.090, 0.054
Sun-touch *time -0.006 0.91 -0.109, 0.097

Base comparison category: Warm-touch.

TABLE 3
Postoperative complications

Group Postoperative Postoperative headache
 hypothermia

Warm-touch 2/13 (7.14%) 0/15
Sun-touch 3/10 (23.08%) 4/10
P value 0.64 0.04
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Author:Kadam, V. Rao; Moyes, D.; Moran, J.L.
Publication:Anaesthesia and Intensive Care
Article Type:Report
Geographic Code:1USA
Date:May 1, 2009
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