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REFLECTIVE BLANKETS USED FOR REDUCTION OF HEAT LOSS DURING REGIONAL ANAESTHESIA
British Journal of Anaesthesia 1992; 68: 531-533
REFLECTIVE BLANKETS USED FOR REDUCTION OF HEAT LOSS DURING REGIONAL ANAESTHESIA K. B. HINDSHOLM, C. BREDAHL, P. HERLEVSEN AND P. K. KRUH0FFER
SUMMARY
KEY WORDS Temperature: body. Anaesthetic techniques: extradural, spinal. Equipment: reflective blanket. Surgery: hip arthroplasty.
Loss of body heat during both general and regional anaesthesia is a common problem. When heat loss exceeds metabolic heat production, the patient may become hypothermic (core temperature < 36 °C). The adverse effects and risks of allowing an anaesthetized patient to cool have been emphasized recently by Imrie and Hall [1]. There is some controversy regarding the heat conserving capacity of reflective blankets during general anaesthesia for neurosurgical procedures [2-A\. In only one of these studies was heat balance rather than core temperature considered [4]. In this study, a reduction in net heat loss was found in patients wrapped in reflecting blankets. The ability of reflecting blankets to reduce net loss of body heat has not previously been evaluated during regional anaesthesia. Although central thermoregulation remains intact during regional anaesthesia, abnormal heat loss occurs because of redistribution of heat from the core to the periphery, increased skin blood flow resulting from sympathetic block, reduced hormonal response to surgical stimulation and impairment of shivering in the area of the block [1]. The present study was designed to assess the heat conserving capacity of reflecting blankets during regional anaesthesia for total hip arthroplasty.
We studied 30 patients (13 female), ASA class I or II, undergoing elective total hip arthroplasty for osteoarthritis. Informed consent was obtained from each patient and the study was approved by the District Ethics Committee. Patients were allocated randomly to either the study group (insulated with reflective blankets as detailed below) or the control group (no reflective blankets). The two groups were comparable in age (range 43-82 yr), weight (range 45-90 kg), height (range 150-183 cm), and rectal temperature on the morning of operation (range 36.5—37.4 °C). Surgical draping was identical in both groups. In addition, all patients wore a cotton gown and were covered as completely as possible with a standard operating room draping—three weave cotton blankets. In the anaesthetic room, patients in the study group were wrapped additionally in reflecting blankets (SunFlex aluminized plastic sheetings) leaving only the head, fingers and operating field uncovered. All patients received the same anaesthetic technique. Premedication consisted of diazepam 5-20 mg orally according to age. On arrival in the anaesthetic room, the patient received 500 ml of isotonic sodium chloride (37 °C) i.v. Subsequently, combined spinal—extradural analgesia was performed, using a "single space technique". Subarachnoid puncture was performed via the L2-3 or L3-4 interspace and 0.5 % plain bupivacaine 3.0 ml was injected. The level of sensory block was assessed 15 min later. The analgesic level was denned as the upper dermatomal level with diminished sensation to pinprick. Extradural 2 % lignocaine 4.0 ml was administered 90 min after the induction of spinal analgesia and repeated every 30 min for the remainder of the operation period. All patients underwent surgery in the same operating theatre, where the ambient temperature was maintained at 21 °C with air renewal 20 times per hour. All blood and i.v. fluid infusions were heated to 37 °C. Throughout operation, dry oxygen 3 litre min"1 was given by nasal catheter. K. B. HlNDSHOLM*, M.D. ; C. BREDAHL, M.D.; P. HERLEVSEN, M.D.; P. K. KRUH0FFER, M.D.; Department of Anaesthesia, Aalborg City
and County Hospital, DK-9000 Aalborg, Denmark. Accepted for Publication: December 9, 1991. * Present address: Department of Anaesthesiology, Aarhus University Hospital, DK-8000 Aarhus, Denmark.
Downloaded from http://bja.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on April 19, 2015
We have studied the ability of reflective blankets to reduce net loss of body heat during regional anaesthesia for total hip arthroplasty. Thirty patients were allocated randomly to either the study group (insulated with reflective blankets) or the control group (no reflective blankets). Surgical and operation room draping, theatre temperature and i.v. fluid administration were standardized for all patients. Total body heat was deduced from core temperature (aural canal) and mean skin temperature (four measuring sites). After 2 h of surgery, loss of body heat was reduced significantly in patients wrapped in reflective blankets (26 kJ) compared with those in the control group (95 kJ).
METHODS AND RESULTS
532
BRITISH JOURNAL OF ANAESTHESIA
Core temperature was measured in the aural canal. Peripheral temperature was measured on four sites: lateral upper arm (Ta), nipple (Tn), anterior midthigh (Tt) and lateral mid-calf (Tc). Central and peripheral temperatures were recorded every 15 min during surgery using an electronic thermometer (type CTD-25 Ellab A/S, Copenhagen). All probes were fast reacting (type MEA-22130-A and MHF18058-A Ellab A/S, Copenhagen) and had been calibrated previously against a mercury-in-glass thermometer in a water bath. Accuracy was +0.1 °C over the temperature range studied. An estimate of the mean skin temperature (MST) was calculated using the four-skin-points formula: MST = 0.3(Tn + Ta) + 0.2(Tt + Tc). Even though the mean skin temperature is most accurately determined using 10 or more skin temperature sites, for practical reasons the four-site system chosen was the most appropriate for this study (Holdcroft and Hall compared the 15-site method of Mitchell and Wyndham with the four-point method during both general and regional anaesthesia and found good agreement [5]). Mean body temperature (MBT) was determined from aural temperature (Tau) and MST according to the equation: MBT = 0.66 x Tau + 0.34 x MST. Total body heat (TBH) in kj was derived using the formula: TBH = MBT x 3.47 x body weight (kg) where 3.47 is the specific heat of the body in kj "C"1 kg"1. Different aspects of these methods of measuring net changes in body heat have been discussed in detail elsewhere [1,5].
37 - ,
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—
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33
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c H.
75 50 25
0 2 -25 -
B
B -75 -100 -125 t
0
15
i
r
30 45 60 75 90 105 120 Duration of surgery (min)
FIG. 1. Aural temperatures, mean skin temperatures and changes in body heat during surgery (median values, lower and upper quartiles). • = Study group; V = control group.
received one dose of ephedrine 5 mg i.v. and one patient (control group) received two doses of ephedrine 5 mg i.v. The first temperature measurement did not differ between the two groups in either aural canal temperature or MST. After 2 h of surgery, the aural temperature had decreased significantly in both groups; however, the decrease was significantly less in the study group compared with the control group. The MST increased significantly in both groups, but no difference was found between groups in the course of the MST. In both groups there was a tendency towards a decrease in TBH which became significant 105 min after the initial temperature measurement. This decrease was reduced significantly in the study group compared with that in the control group (fig.l). After 2 h of surgery, median total body heat had decreased by 26 kj in the study group and 95 kj in the control group (P < 0.05). COMMENT
In this study, we found a decrease in core temperature throughout the whole period, but there was no change in total body heat during the first 90 min of surgery. This is a result of the associated increase in MST caused by a distribution of heat from the core to the periphery.
Downloaded from http://bja.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on April 19, 2015
For calculation of the required number of patients, the smallest difference between the groups not to be overlooked with regard to changes in core temperature was estimated to 1.00 °C and the SD to 0.75 °C. Type I error was set at 5 % and type II error at 10%. With these assumptions, 11 patients were required in each group. The data are presented using median values and 25—75 percentiles. Comparison of the groups with regard to' patient characteristics, blood loss, infusion requirements and level of sensory block after spinal analgesia was made using the Fisher exact test and Mann-Whitney rank-sum test. Changes in temperature and body heat in each group and between the groups were analysed using non-parametric two-way analysis of variance. Changes within groups at different times were analysed using the multiple comparison procedure based on the Friedmann test. The statistical computer program MEDSTAT version 2.1 (Astra, Copenhagen 1989) was used for calculations. P < 0.05 was considered statistically significant. Statistical analysis of the data was confined to the first 2 h of surgery because of an inadequate number of patients beyond this interval (10 patients). The two groups were comparable for blood loss (range 300-2600 ml) and infusion requirements: isotonic sodium chloride (range 500-3000 ml), isotonic sodium-glucose (500-3000 ml), Macrodex (range 350-1000 ml) and blood transfusions (range 0—1400 ml). There was no difference between the groups in the level of sensory block after spinal analgesia (range T12-T8). Hypotension (mean arterial pressure < 66 % of the original value) was observed in two patients: one patient (study group)
o
CHANGES IN BODY HEAT
the protein breakdown occurs in skeletal muscles, and as protein loss may be expected to delay recovery after operation, every effort should be made to minimize thermal stress during anaesthesia. REFERENCES 1. Imrie MM, Hall GM. Body temperature and anaesthesia. British Journal of Anaesthesia 1990; 64: 346-354. 2. Rfldford P, Thurlow AC. Metallized plastic sheeting in the prevention of hypothermia during neurosurgery. British Journal of Anaesthesia 1979; 51: 237-354. 3. Bourke DL, Wurm H, Rosenberg M, Russel J. Intraoperative heat conservation using a reflective blanket. Anesthesiology 1984; 60: 151-154. 4. Brunton JLAT, Thorns GMM, Blair I. Reduction of heat loss in neurosurgical patients using metallized plastic sheeting. British Journal of Anaesthesia 1982; 54: 1201-1204. 5. Holdcroft A, Hall GM. Heat loss during anaesthesia. British Journal of Anaesthesia 1978; 50: 157-164. 6. Carli F, Emery PW, Freemantle CAJ. Effect of peroperative normothermia on postoperative metabolism in elderly patients undergoing hip arthroplasty. British Journal of Anaesthesia 1989; 63: 276-282.
Downloaded from http://bja.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on April 19, 2015
When the outer region of a body becomes warmer, heat loss obviously occurs more easily. To prevent this, adequate covering is necessary. The conservation of body heat in the control group was impressive, and demonstrates the importance of insulation with cotton blankets. Our results demonstrate also the additional effect of reflective blankets. These blankets comprise a metallized plastic sheeting which acts primarily by reducing the heat loss caused by radiation, but also by reducing convectional loss by minimizing draughts. At rest, 75 % of basal heat production is lost by radiation and convection from the body surface [1]. Although additional covering with reflecting blankets did not totally prevent net loss of body heat during the whole period of our study, it was reduced significantly compared with the control group. Carli, Emery and Freemantle have shown recently that the maintenance of normothermia during elective hip arthroplasty appears to attenuate protein breakdown and nitrogen loss after surgery [6]. As a major part of
533
REFLECTIVE BLANKETS USED FOR REDUCTION OF HEAT LOSS DURING REGIONAL ANAESTHESIA K. B. HINDSHOLM, C. BREDAHL, P. HERLEVSEN AND P. K. KRUH0FFER
SUMMARY
KEY WORDS Temperature: body. Anaesthetic techniques: extradural, spinal. Equipment: reflective blanket. Surgery: hip arthroplasty.
Loss of body heat during both general and regional anaesthesia is a common problem. When heat loss exceeds metabolic heat production, the patient may become hypothermic (core temperature < 36 °C). The adverse effects and risks of allowing an anaesthetized patient to cool have been emphasized recently by Imrie and Hall [1]. There is some controversy regarding the heat conserving capacity of reflective blankets during general anaesthesia for neurosurgical procedures [2-A\. In only one of these studies was heat balance rather than core temperature considered [4]. In this study, a reduction in net heat loss was found in patients wrapped in reflecting blankets. The ability of reflecting blankets to reduce net loss of body heat has not previously been evaluated during regional anaesthesia. Although central thermoregulation remains intact during regional anaesthesia, abnormal heat loss occurs because of redistribution of heat from the core to the periphery, increased skin blood flow resulting from sympathetic block, reduced hormonal response to surgical stimulation and impairment of shivering in the area of the block [1]. The present study was designed to assess the heat conserving capacity of reflecting blankets during regional anaesthesia for total hip arthroplasty.
We studied 30 patients (13 female), ASA class I or II, undergoing elective total hip arthroplasty for osteoarthritis. Informed consent was obtained from each patient and the study was approved by the District Ethics Committee. Patients were allocated randomly to either the study group (insulated with reflective blankets as detailed below) or the control group (no reflective blankets). The two groups were comparable in age (range 43-82 yr), weight (range 45-90 kg), height (range 150-183 cm), and rectal temperature on the morning of operation (range 36.5—37.4 °C). Surgical draping was identical in both groups. In addition, all patients wore a cotton gown and were covered as completely as possible with a standard operating room draping—three weave cotton blankets. In the anaesthetic room, patients in the study group were wrapped additionally in reflecting blankets (SunFlex aluminized plastic sheetings) leaving only the head, fingers and operating field uncovered. All patients received the same anaesthetic technique. Premedication consisted of diazepam 5-20 mg orally according to age. On arrival in the anaesthetic room, the patient received 500 ml of isotonic sodium chloride (37 °C) i.v. Subsequently, combined spinal—extradural analgesia was performed, using a "single space technique". Subarachnoid puncture was performed via the L2-3 or L3-4 interspace and 0.5 % plain bupivacaine 3.0 ml was injected. The level of sensory block was assessed 15 min later. The analgesic level was denned as the upper dermatomal level with diminished sensation to pinprick. Extradural 2 % lignocaine 4.0 ml was administered 90 min after the induction of spinal analgesia and repeated every 30 min for the remainder of the operation period. All patients underwent surgery in the same operating theatre, where the ambient temperature was maintained at 21 °C with air renewal 20 times per hour. All blood and i.v. fluid infusions were heated to 37 °C. Throughout operation, dry oxygen 3 litre min"1 was given by nasal catheter. K. B. HlNDSHOLM*, M.D. ; C. BREDAHL, M.D.; P. HERLEVSEN, M.D.; P. K. KRUH0FFER, M.D.; Department of Anaesthesia, Aalborg City
and County Hospital, DK-9000 Aalborg, Denmark. Accepted for Publication: December 9, 1991. * Present address: Department of Anaesthesiology, Aarhus University Hospital, DK-8000 Aarhus, Denmark.
Downloaded from http://bja.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on April 19, 2015
We have studied the ability of reflective blankets to reduce net loss of body heat during regional anaesthesia for total hip arthroplasty. Thirty patients were allocated randomly to either the study group (insulated with reflective blankets) or the control group (no reflective blankets). Surgical and operation room draping, theatre temperature and i.v. fluid administration were standardized for all patients. Total body heat was deduced from core temperature (aural canal) and mean skin temperature (four measuring sites). After 2 h of surgery, loss of body heat was reduced significantly in patients wrapped in reflective blankets (26 kJ) compared with those in the control group (95 kJ).
METHODS AND RESULTS
532
BRITISH JOURNAL OF ANAESTHESIA
Core temperature was measured in the aural canal. Peripheral temperature was measured on four sites: lateral upper arm (Ta), nipple (Tn), anterior midthigh (Tt) and lateral mid-calf (Tc). Central and peripheral temperatures were recorded every 15 min during surgery using an electronic thermometer (type CTD-25 Ellab A/S, Copenhagen). All probes were fast reacting (type MEA-22130-A and MHF18058-A Ellab A/S, Copenhagen) and had been calibrated previously against a mercury-in-glass thermometer in a water bath. Accuracy was +0.1 °C over the temperature range studied. An estimate of the mean skin temperature (MST) was calculated using the four-skin-points formula: MST = 0.3(Tn + Ta) + 0.2(Tt + Tc). Even though the mean skin temperature is most accurately determined using 10 or more skin temperature sites, for practical reasons the four-site system chosen was the most appropriate for this study (Holdcroft and Hall compared the 15-site method of Mitchell and Wyndham with the four-point method during both general and regional anaesthesia and found good agreement [5]). Mean body temperature (MBT) was determined from aural temperature (Tau) and MST according to the equation: MBT = 0.66 x Tau + 0.34 x MST. Total body heat (TBH) in kj was derived using the formula: TBH = MBT x 3.47 x body weight (kg) where 3.47 is the specific heat of the body in kj "C"1 kg"1. Different aspects of these methods of measuring net changes in body heat have been discussed in detail elsewhere [1,5].
37 - ,
o
—
36 -
5
35-
a E a>
34 35 -i
34
•go C Q. to E"
a E 5 S
33
32 -I
c H.
75 50 25
0 2 -25 -
B
B -75 -100 -125 t
0
15
i
r
30 45 60 75 90 105 120 Duration of surgery (min)
FIG. 1. Aural temperatures, mean skin temperatures and changes in body heat during surgery (median values, lower and upper quartiles). • = Study group; V = control group.
received one dose of ephedrine 5 mg i.v. and one patient (control group) received two doses of ephedrine 5 mg i.v. The first temperature measurement did not differ between the two groups in either aural canal temperature or MST. After 2 h of surgery, the aural temperature had decreased significantly in both groups; however, the decrease was significantly less in the study group compared with the control group. The MST increased significantly in both groups, but no difference was found between groups in the course of the MST. In both groups there was a tendency towards a decrease in TBH which became significant 105 min after the initial temperature measurement. This decrease was reduced significantly in the study group compared with that in the control group (fig.l). After 2 h of surgery, median total body heat had decreased by 26 kj in the study group and 95 kj in the control group (P < 0.05). COMMENT
In this study, we found a decrease in core temperature throughout the whole period, but there was no change in total body heat during the first 90 min of surgery. This is a result of the associated increase in MST caused by a distribution of heat from the core to the periphery.
Downloaded from http://bja.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on April 19, 2015
For calculation of the required number of patients, the smallest difference between the groups not to be overlooked with regard to changes in core temperature was estimated to 1.00 °C and the SD to 0.75 °C. Type I error was set at 5 % and type II error at 10%. With these assumptions, 11 patients were required in each group. The data are presented using median values and 25—75 percentiles. Comparison of the groups with regard to' patient characteristics, blood loss, infusion requirements and level of sensory block after spinal analgesia was made using the Fisher exact test and Mann-Whitney rank-sum test. Changes in temperature and body heat in each group and between the groups were analysed using non-parametric two-way analysis of variance. Changes within groups at different times were analysed using the multiple comparison procedure based on the Friedmann test. The statistical computer program MEDSTAT version 2.1 (Astra, Copenhagen 1989) was used for calculations. P < 0.05 was considered statistically significant. Statistical analysis of the data was confined to the first 2 h of surgery because of an inadequate number of patients beyond this interval (10 patients). The two groups were comparable for blood loss (range 300-2600 ml) and infusion requirements: isotonic sodium chloride (range 500-3000 ml), isotonic sodium-glucose (500-3000 ml), Macrodex (range 350-1000 ml) and blood transfusions (range 0—1400 ml). There was no difference between the groups in the level of sensory block after spinal analgesia (range T12-T8). Hypotension (mean arterial pressure < 66 % of the original value) was observed in two patients: one patient (study group)
o
CHANGES IN BODY HEAT
the protein breakdown occurs in skeletal muscles, and as protein loss may be expected to delay recovery after operation, every effort should be made to minimize thermal stress during anaesthesia. REFERENCES 1. Imrie MM, Hall GM. Body temperature and anaesthesia. British Journal of Anaesthesia 1990; 64: 346-354. 2. Rfldford P, Thurlow AC. Metallized plastic sheeting in the prevention of hypothermia during neurosurgery. British Journal of Anaesthesia 1979; 51: 237-354. 3. Bourke DL, Wurm H, Rosenberg M, Russel J. Intraoperative heat conservation using a reflective blanket. Anesthesiology 1984; 60: 151-154. 4. Brunton JLAT, Thorns GMM, Blair I. Reduction of heat loss in neurosurgical patients using metallized plastic sheeting. British Journal of Anaesthesia 1982; 54: 1201-1204. 5. Holdcroft A, Hall GM. Heat loss during anaesthesia. British Journal of Anaesthesia 1978; 50: 157-164. 6. Carli F, Emery PW, Freemantle CAJ. Effect of peroperative normothermia on postoperative metabolism in elderly patients undergoing hip arthroplasty. British Journal of Anaesthesia 1989; 63: 276-282.
Downloaded from http://bja.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on April 19, 2015
When the outer region of a body becomes warmer, heat loss obviously occurs more easily. To prevent this, adequate covering is necessary. The conservation of body heat in the control group was impressive, and demonstrates the importance of insulation with cotton blankets. Our results demonstrate also the additional effect of reflective blankets. These blankets comprise a metallized plastic sheeting which acts primarily by reducing the heat loss caused by radiation, but also by reducing convectional loss by minimizing draughts. At rest, 75 % of basal heat production is lost by radiation and convection from the body surface [1]. Although additional covering with reflecting blankets did not totally prevent net loss of body heat during the whole period of our study, it was reduced significantly compared with the control group. Carli, Emery and Freemantle have shown recently that the maintenance of normothermia during elective hip arthroplasty appears to attenuate protein breakdown and nitrogen loss after surgery [6]. As a major part of
533