1 Department of Anesthesiology, Emergency and Intensive Care Medicine, University of Göttingen, Robert-Koch-Str. 40, D-37075 Göttingen, Germany. 2 Department of Anaesthesia, Montreal General Hospital and McGill University, Montreal, Canada. 3 Department of Anaesthesia and Intensive Care Medicine, Evangelisches Bethesda-Krankenhaus, Essen, Germany
*Corresponding author. Email: abraeue{at}gwdg.de
Accepted for publication: February 1, 2004
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Abstract |
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Methods. The following insulation materials were tested using a validated manikin: cotton surgical drape tested in two and four layers; Allegiance drape; 3M Steri-Drape; metallized plastic sheet; ThermadrapeTM; Barkey thermcare 1 tested in one and two layers; hospital duvet tested in one and two layers. Heat loss from the surface of the manikin can be described as: Q·=h·T·A where Q· is heat flux, h is the heat exchange coefficient,
T is the temperature gradient between the environment and surface and A is the area covered. The heat flux per unit area (Q·A1) and surface temperature were measured with nine calibrated heat-flux transducers. The environmental temperature was measured using a thermoanemometer.
T was varied and h was determined by linear regression analysis as the slope of
T vs Q·A1. The reciprocal of h defines the insulation.
Results. The insulation value of air was 0.61 Clo. The insulation values of the materials varied between 0.17 Clo (two layers of cotton surgical drapes) to 2.79 Clo (two layers of hospital duvet).
Conclusions. There are relevant differences between various insulating materials. The best commercially available material designed for use in the operating room (Barkey thermcare 1) can reduce heat loss from the covered area by 45% when used in two layers. Given the range of insulating materials available for outdoor activities, significant improvement in insulation of patients in the operating room is both possible and desirable.
Br J Anaesth 2004; 92: 83640
Keywords: complications, hypothermia; equipment, insulation; equipment, manikin; heat loss; measurement, heat flux
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Introduction |
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During long surgical operations, perioperative hypothermia can be avoided only if the patients heat loss is offset by an equal heat gain, either from metabolic heat production or from an external heat source. It is usual to exploit a combination of measures to maintain normothermia. Heat gain from metabolic heat production can be augmented by the infusion of amino acids,11 while external heat can be applied by conductive warming methods12 or forced air warming.4 5 10 Heat losses from the airways can be reduced by the use of heat and moisture exchangers13 and heat losses from skin that can not be warmed actively can be reduced by insulation.14 15 Although insulation is invariably used to reduce heat loss from the skin, there is little information about the physical properties of various insulating materials used in the operating room. Therefore the following study compared the efficacy of seven insulating materials using a validated copper manikin to simulate the human body.
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Methods |
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Q·=h·T·A
where Q· is heat flux (W), h is the heat exchange coefficient (W m2 °C1), T is the temperature gradient between the environment and surface (°C) and A is the area covered (m2).
Q· per unit area (Q·A1) can be measured directly with heat-flux transducers, and temperatures can be measured with standard thermometric techniques. From these data, h, which defines the efficiency of heat exchange, can be calculated. Covering a surface with insulation decreases heat flow to the environment, therefore lowering h. The reciprocal of h defines the resistance to heat exchange, or the insulation. Insulation values can be expressed in SI units (°C m2 W1), togs (0.1°C m2 W1) or Clo units. One Clo unit is equivalent to the insulation required to keep a seated subject comfortable at an air temperature of 21°C in an air movement of 0.1 m s1. Such insulation is provided by an ordinary suit, with shirt, trousers etc.16 One Clo is equivalent to 0.155°C m2 W1.
The following insulation materials were tested: (i) cotton surgical drape TB 202 B (160 cm x 140 cm) (Karl Dieckhoff GmbH & Co. KG, Wuppertal, Germany), tested in two and four layers, because surgical cotton drapes are rarely used in one layer; (ii) Allegiance beach chair shoulder drape (262 cm x 411 cm) (Allegiance Healthcare Corporation, McGaw Park, IL, USA); (iii) 3M Steri-Drape adhesive split sheet No. 9045 (228 cm x 260 cm) (3M Health Care, St Paul, MN, USA); (iv) metallized plastic sheet (140 cm x 220 cm) (VauDe, Norderstedt, Germany), tested with the silver side facing the manikin; (v) ThermadrapeTM Blanket T2000 (120 cm x 120 cm) (OR Concepts, Roanoke, TX, USA); (vi) Barkey thermcare 1 whole body blanket for adults (220 cm x 140 cm) (Barkey GmbH & Co. KG, Leopoldshöhe, Germany) which consists of cotton, polyester and polyester with polyurethane coating; it was tested in one and two layers; (vii) hospital duvet (188 cm x 122 cm) (Brinkhaus GmbH & Co. KG, Warendorf, Germany), filled with Trevira (100% polyester). The hospital duvet was tested together with its covering (140 cm x 200 cm) (Karl Dieckhoff GmbH & Co. KG, Wuppertal, Germany) made of 50% polyester and 50% cotton and was tested as one and two duvets (one and two layers).
The manikin
The manikin consists of six copper tubes painted matt black. Two tubes serve as arms, two as legs, one as the head and one as the trunk. The total surface area of all tubes is 1.98 m2. In order to set surface temperature and achieve steady-state conditions, water mattresses (Maxi-Therm®, Cincinnati Sub-Zero Products Inc., Cincinnati, OH, USA) are bonded to the inner surface of the copper tubes. The circulating water is warmed and cooled by a hypo-hyperthermia system (Hico-Variotherm 530, Hirtz & Co. Hospitalwerk, Cologne, Germany).
Measurement of environmental conditions
Air humidity and velocity were measured using a gauged thermoanemometer (Velocicalc plus TSI® Model 8388-M-D, TSI Incorporated, St Paul, MN, USA).
Measurement of heat exchange at the manikin
We measured Q·A1 between the environment and the manikin with nine calibrated heat-flux transducers (Heat Flow Sensor Model FR-025-TH44033-F16, Concept Engineering, Old Saybrook, CT, USA) distributed equally over the trunk of the manikin.
Measurement of temperature gradient
The temperature gradient was defined as the difference between the environmental temperature and the surface temperature of the manikin underneath the heat-flux transducer. The environmental temperature was measured in the middle of the room and near the wall using the thermoanemometer. The surface temperature of the manikin was measured with calibrated thermistors incorporated into the heat-flux transducers.
Data sampling
Heat-flux signals were measured and digitized using a Dash TC AD converter (Keithley Instruments Inc., Taunton, MA, USA). The thermistors incorporated into the heat-flux transducers for measurement of the manikin surface temperature were connected to Hellige Servomed 236039 monitors (Hellige, Freiburg, Germany). The signal of these monitors was digitized on a Dash 1402 A/D board (Keithley Instruments Inc., Taunton, MA, USA). All data were sampled synchronously in 10 s intervals on a computer, averaged over 1 min and written to a hard disk.
Determination of the heat exchange coefficient
The trunk of the manikin was completely covered with the insulation material, which was smoothed flat to exclude any obvious trapped air. To determine h, Q·A1 and T were measured simultaneously over a range of temperature differences. Six tests were created by setting six different surface temperatures of the manikin (22, 26, 30, 34, 38 and 42 °C). Each test consisted of a 60 min preparation period to achieve steady-state conditions followed by a 20 min measurement period. The collected data were averaged for the single measurement period. Each test was repeated three times. There were nine sites, six tests and three repetitions, so that h was calculated from 162 results for Q·A1 and the corresponding temperature gradients. h was calculated by linear regression analysis as the slope of Q·A1 as a function of the temperature gradient. Heat flux from the manikin to the environment was called heat loss and was assigned a negative value.
Calculation of the insulation values of the tested materials
The insulation of the trunk of the manikin when covered with an insulation material represents the total insulation provided by the insulation material and the insulation of air. Therefore the insulation of air was determined by exposing the manikin, using only air as the insulating material. Subtracting the insulation of air from the total insulation gave the insulation value of the tested material.
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Results |
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Insulation value of air
h for the trunk of the manikin was 10.6 W m2 °C1 (Fig. 1) The reciprocal of h defines the resistance to heat exchange, or the insulation. This resistance is 1/h=1/10.6 W m2 °C1 = 0.09 °C m2 W1 or 0.9 tog or 0.61 Clo.
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Discussion |
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Different types of thermal insulators
Thermal insulators can be divided into two different types. The majority consist of mass insulators (cotton surgical drape, Allegiance drape, 3M Steri-Drape, Barkey thermcare 1 whole body blanket, hospital duvet). These insulators entrap air within a fibre matrix. This entrapped air does not move and is called still air. Still air is a very effective insulator, with an insulation value of 1.8 Clo cm1.17 Therefore the insulation value of these insulators is proportional to the thickness of the still air enclosed. The kind of fibre used to trap the air is of little importance.16 18
The second type of insulator is the radiant insulator (ThermadrapeTM blanket, metallized plastic sheet) which reflects radiant heat back to the radiating surface and emits little radiant heat to the exterior. To provide a significant effect, the radiant insulator should have a distance of about 1 cm from the radiating surface.17 This distance to the radiating surface should consist of air. If this distance is filled by loose material of low bulk density, the effect of the radiant insulator is reduced.17
Simulation of heat loss and the influence of insulation by the manikin
Thermal manikins are used extensively in environmental physiology16 19 and are a useful and valuable complement to direct experiments with human volunteers. The main application areas of thermal manikins are relevant simulation of human whole body and local heat exchange. Clothing insulation in particular has been extensively studied in heated thermal manikins and this work forms the basis of American and European standards.19
The heat exchanging properties of our manikin have been validated.20 h for the whole manikin is 11 W m2 °C1. In this study, we used only the trunk and found a heat exchange coefficient of 10.6 W m2 °C1. This corresponds very well with the heat exchange coefficient of 10.8 W m2 °C1 we found in human volunteers.20
The correct determination of insulation values for different insulating materials is complicated by the fact that, while the air trapped within a mass insulator determines that insulators characteristics, the variable amount of air trapped beneath the insulator will increase its apparent insulation effectiveness. For this reason we excluded any obvious trapped air between the insulating material and the trunk of the manikin by smoothing flat all the test materials. In clinical practice there will be more trapped air under the insulation material and therefore the practical insulation of the materials will be slightly higher.
Insulation values of the insulation materials
The insulation value of air was 0.61 Clo, which means that air is a better insulator than the materials found in an operating room. A value of 0.61 Clo compares well with insulation values of air given by Burton and Edholm.17 The insulation materials had insulation values between 0.17 Clo and 2.79 Clo. This result is different from the results of a study by Sessler and colleagues,14 who concluded that there were only minor important differences among the thermal barriers. The reason for this is that we included effective insulating materials that are not commonly used in the operating room (e.g. hospital duvet). However, if we compare similar materials in both studies we find very similar results. We have also found that disposable covers are more effective than a cloth surgical drape, but they are less effective than a reflective material (e.g. Thermadrape). Adding additional layers of the insulating material increases the efficacy. This result is also comparable to a volunteer study.15 However, the exact influence of more layers on the reduction of heat loss is still to be determined.
In contrast to many clinical studies,2123 the results of the radiant insulators were better than many other insulating materials. Possibly the efficacy of these materials is lowered in clinical practice by placing additional sterile drapes on them. This consideration is confirmed by studies that have found no improvement of thermal insulation by adding radiant insulators sandwiched into insulating materials.24 25 However, this problem requires further detailed analysis.
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Conclusion |
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Acknowledgement |
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References |
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