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* From the Department of Anesthesiology, Emergency and Intensive Care Medicine, University of Göttingen, Germany; the
Department of Anesthesia, Montreal General Hospital and McGill University, Montreal, Quebec, Canada; and the
Department of Anesthesia and Intensive Care Medicine, Evangelisches Bethesda-Krankenhaus Essen, Germany.
Address correspondence to: PD Dr. med. Anselm Bräuer, Department of Anesthesiology, Emergency and Intensive Care Medicine, University of Göttingen, Robert-Koch-Str. 40, D-37075 Göttingen, Germany. Phone: ++49-551-39-8826; Fax: ++49-551-39-8676; E-mail: abraeue{at}gwdg.de
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Abstract |
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Methods: The following systems were tested: 1) Bair Hugger® 505; 2) Bair Hugger® 750; 3) Life-Air 1000 S; 4) Snuggle Warm®; 5) Thermacare®; 6) Thermacare® with reusable Optisan® blanket; 7) WarmAir®; 8) Warm-Gard®; 9) Warm-Gard® and reusable blanket; 10) WarmTouch®; and 11) WarmTouch® and reusable blanket. Heat transfer of forced-air warmers can be described as follows: 
= h · T · A. Where = heat flux (W), h = heat exchange coefficient (W·m–2·°C–1), 
T = temperature gradient between blanket and manikin surface (°C), A = covered area (m2). Heat flux per unit area and surface temperature were measured with 16 heat flux transducers. Blanket temperature was measured using 16 thermocouples. The temperature gradient between blanket and surface (T) was varied and h was determined by linear regression analysis. Mean T was determined for surface temperatures between 32°C and 38°C. The covered area was estimated to be 1.21 m2.
Results: For the 11 devices, heat transfers of 30.7 W to 77.3 W were observed for surface temperatures of 32°C, and between –8.8 W to 29.6 W for surface temperatures of 38°C.
Conclusion: There are clinically relevant differences between the tested forced-air warming systems with full body blankets. Several systems were unable to transfer heat to the manikin at a surface temperature of 38°C.
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Introduction |
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The following study compared the efficacy of 11 forced-air warming systems with full body blankets using a validated copper manikin of the human body. The primary outcome variable was the heat transfer at surface temperatures of 32oC to 38oC.
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Measurement of environmental conditions |
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The manikin
The manikin consisted of six copper tubes painted matt-black. Two tubes served as arms (circumference 330 mm, length 705 mm), two as legs (circumference 485 mm, length 750 mm), one as the head (circumference 500 mm, length 330 mm) and one as the trunk (circumference 840 mm, length 740 mm). The total surface area of all tubes was 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) were bonded to the inner surface of the copper tubes. The circulating water was warmed and cooled by a hypohyperthermia system (Hico-Variotherm 530, Hirtz & Co. Hospitalwerk, Cologne, Germany).
Heat flow delivered to the blanket
Forced-air warming systems consist of a power unit incorporating an electrical heater and a fan to generate an air flow that is delivered downstream to a blanket. Each manufacturer’s heater was connected to the corresponding full body blanket. Temperature control was set to the highest temperature, with exception of the Thermacare® power unit, where the highest temperature recommended for anesthetized patients was used. This temperature setting was used throughout the study and was called the "maximum temperature". The air flow control of the Warm-Gard® power unit was set to "high". All other power units have only one flow rate. The air delivery hose from the power unit to the blanket was fully extended. The blankets were then positioned on the manikin and covered with two layers of cotton sheets.
Measurement of nozzle temperature and air velocity at the nozzle
Nozzle temperature and air velocity at the nozzle were measured using a thermoanemometer (VELOCICALC PLUS TSI® Model 8388-M-D, TSI Inc., St. Paul, MN, USA). Both parameters were measured directly at an adapter which connected the nozzle to the blanket and which contained a mesh (mesh size 1 mm x 1 mm) to create laminar air flow. Air temperature and air velocity were measured at three defined positions evenly distributed on the diameter. The average of these three measurements was taken as the average air temperature and air velocity. Air velocity was multiplied by the area of the adapter to calculate air flow.
Heat flow produced by the power units was calculated as follows:
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where:
= heat flow (W)
F = air flow (L·sec–1)
T = temperature gradient between the nozzle and the room (°C)
c = specific heat capacity of air (J·g–1 °C–1)
= density of air at the nozzle temperature (g·L–1)
The values of the specific heat capacity of air and the density of air at the nozzle temperature were taken from standard tables.9
Heat exchange at the manikin
The basic equation for temperature-dependent heat transfer is:
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where:
= heat flow (W)
h = heat exchange coefficient (W·m–2·°C–1)
T = temperature gradient (°C)
A = area (m2)
This equation can be applied to describe the heat exchange process between a forced-air warming blanket and the manikin. The heat exchange coefficient h defines the efficacy of all the heat exchange mechanisms (radiation, convection, and conduction) between the blanket and the manikin, whereas the temperature gradient T is the driving force of this heat exchange.
Measurement of heat exchange at the manikin
We measured heat flow per unit area between blanket surface and the manikin with 16 calibrated heat flux transducers (Heat Flow Sensor Model FR-025- TH44033-F16, Concept Engineering, Old Saybrook, CT, USA).
Measurement of temperature gradient
The temperature gradient was defined as the difference between the surface temperature of the manikin underneath the heat flux transducer and the temperature 1 cm above (blanket temperature). Surface temperature of the manikin was measured with thermistors incorporated into the heat flux sensors. To determine the blanket temperature thermocouple needles (MAT Myocardial sensor 18 mm, Mallinckrodt Medical Inc., St. Louis, MO, USA) were used, so that they made direct contact with the surface of the blanket. Both the thermistors and the thermocouples were calibrated before the procedure.
Distribution of measurement sites
Sixteen measurement sites were distributed on the manikin as follows: two sites were placed on each arm, three sites were placed on each leg and six sites were placed on the trunk (Figure 1
). On each measurement site heat exchange at the manikin, manikin surface temperature and blanket temperature were measured.
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Determination of h
To determine the heat exchange coefficient, heat flux per unit area and temperature differences were measured simultaneously over a range of temperature differences. Eight tests were created by using four different surface temperatures of the manikin (27°C, 32°C, 37°C and 42°C), combined with two different blower temperatures (maximum and room temperature). In this way temperature differences of approximately –10 to +10°C were produced. Each test consisted of a 30-min preparation period to achieve steady state conditions followed by a five-minute measurement period. The collected data were averaged for the single measurement period. In order to randomize the position of blanket perforations in relation to the heat flux transducers each test was repeated three times, each time using a new blanket. The heat exchange coefficient was calculated by linear regression analysis as the slope of heat exchange per unit area as a function of the blanket-surface temperature gradient. The regression line was forced through zero. Heat flux from the blanket to the manikin was called heat gain and was assigned a positive value.
Calculation of mean T for defined surface temperatures
The mean T is dependent on the surface temperature of the manikin and the efficacy of each single system. To compare the different systems it was necessary to derive a mean T for a defined range of manikin surface temperatures. In post-cardiac surgical patients, the mean skin temperature under a forced-air warming blanket ranges between 32°C and almost 38°C.5,10 Therefore, these surface temperatures were chosen to determine the corresponding mean T. To calculate mean T, the temperature difference between the blanket and the manikin surface was plotted as a function of the temperature of the manikin surface and a regression line was calculated to define their relationship. The equation of this regression line was used to derive mean T for surface temperatures of 32°C and 38°C.
Determination of the covered area
The area covered by the full body blanket was considered to be the same for all systems. Approximately one third of the trunk and the extremities does not take part in heat exchange by forced-air warming, because this surface is in direct contact with the bed. Therefore the covered area was calculated as two thirds of the circumference times the length of the trunk, arms and legs. This resulted in the following covered areas:
Trunk: 2/3·0.84 m·0.74 m = 0.41 m²
Arm: 2/3·0.33 m·0.705 m = 0.16 m²
Leg: 2/3·0.485 m·0.75 m = 0.24 m²
Trunk and extremities: 1.21 m²
Calculation of heat exchange at the manikin
Heat exchange at the manikin was calculated for surface temperatures of 32°C and 38°C according to equation 1 for each system.
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Results |
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Heat flow delivered to the blanket
The nozzle temperatures ranged between 41.5°C and 47.6°C and air flow ranged between 9.4 L·sec–1 and 26.2 L·sec–1, resulting in heat flows ranging from 249 W to 623 W (Table I).
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Ts at the lower surface temperatures (Table II). The heat exchange coefficients varied by a factor of 2 among the systems and ranged between 13.4 and 32.2 W·m–2·°C–1. Figure 2 shows a typical example for the determination of h for a single system.
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T varied between 1.24 and 3.18°C for surface temperatures of 32°C and between 0.26 and 1.59°C for surface temperatures of 38°C (Table II
). Figure 3 shows a typical example of how mean T was derived for the defined surface temperatures.
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Discussion |
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Forced-air warming is a common method for rewarming after cardiac surgery. In 1994 Giesbrecht et al.17 described significant differences between forced-air warming devices in combination with full body blankets. However, the clinical relevance of these results from a study of normothermic volunteers is uncertain, as the skin temperature of normothermic volunteers is generally higher than the mean skin temperatures of hypothermic postoperative cardiac surgical patients. Therefore, we investigated the heat transferring properties of 11 forced air warming systems with full body blankets using a validated copper manikin at temperatures reflective of the early postoperative setting. This manikin has been used previously for comparison of forced-air warming systems with upper18 or lower body blankets.19
We found relevant differences in heat transfer between the different forced-air warming systems tested. Heat transfer ranged between 30.6 to 77.3 W for surface temperatures of 32°C, and between –8.8. to 29.6 W for surface temperatures of 38°C. This divergence in heat transfer at different surface temperatures is caused by the higher mean temperature gradient between the blanket and the manikin surface. This effect can be observed with every system, and limits heat transfer to an already warm surface temperature.
Three forced-air warming systems failed to maintain a positive temperature gradient between the blanket and the manikin surface at a surface temperature of 38°C and as a result, cooled the manikin. However, our results are at variance with those of Giesbrecht et al.17 who found a heat transfer of 40 to 95 W. There are three possible explanations for the differences between their study and ours: 1) mean skin temperatures may not have been comparable; 2) our manikin may be limited in its ability to simulate realistic conditions for forced-air warming devices; and 3) forced-air warming systems have evolved during the 13-yr interval between these studies.
In the study of Giesbrecht et al. mean skin temperature under the forced-air warming system ranged between 36.5°C and 37.5°C.17 In post-cardiac surgery patients5,10 the mean skin temperature under a forced-air warming blanket rises slowly from 32°C to almost 38°C, and we have calculated heat transfer values to reflect the response at these surface temperatures. Therefore, the testing conditions of the studies were not comparable. However, as the temperature gradient between the blanket and the surface was higher at lower surface temperatures, we would have expected higher heat transfers in our study. When comparing the heat transfer values from our study at surface temperatures of 36°C to 38°C to the heat transfers observed by Giesbrecht et al.,17 the difference becomes even greater. Here we can only see heat transfers from –8.8 to 44 W (Table II), which is much less than 40 to 95 W observed by Giesbrecht et al.17 Accordingly, the different testing conditions cannot alone account for the different findings of these two studies.
The heat exchanging properties of this manikin model have been carefully validated.20 The combined heat exchange coefficient for radiation and convection of the manikin is 11 W·m–2·°C–1. This corresponds well with the combined heat exchange coefficient for radiation and convection of 10.8 W·m–2·°C–1 that was measured previously in human volunteers using the same methodology.20 The emissivity of the manikin is 0.96, whereas the emissivity of human skin is 0.98.21 In a previous study in volunteers, we tested four different forced-air warming systems with upper body blankets22 and demonstrated that we could confidently predict the heat transfer of these forced-air warmers with a previous investigation in manikins.18 The heat transfer of three forced-air warming systems could be predicted exactly, whereas a fourth system was underestimated by 1.1 W, a value which is of minimal clinical importance. Therefore, the manikin is able to accurately simulate heat transfer of forced-air warming systems.
Discrepancies between studies may also reflect, to a certain degree, advances in the technology of forced-air warming systems which have taken place over the past decade. In the early 90’s forced-air warming devices were used primarily for postoperative rewarming of conscious hypothermic patients. This application allowed for higher nozzle temperatures of the power units than today. In an unpublished series in 1994 we found that the Bair Hugger® 200 had a nozzle temperature of 51.3°C, the WarmAir® 133 power unit used a nozzle temperature of 45.0°C and the WarmTouchTM had a nozzle temperature of 48.8°C. The air flows of these earlier devices were also much higher than in the series of warming devices from the current investigation, where flow rates ranged between 17.4 L·sec–1 to 31.5 L·sec–1.
The increasing intraoperative use of forced-air warming systems has led to a reduction of nozzle temperatures, because there are reports of burns associated with the use of forced-air warming systems.23 Another factor to consider is that most forced-air warming systems operate with a lower air flow in Europe compared to North America, because the AC power source in Europe uses 50 Hz, compared to 60 Hz in North America. The motors of most forced-air warming systems operate at reduced speed at 50 Hz, which decreases the air flow of the blower by approximately 20%. Only the motor of the Bair Hugger® Model 750 operates at the same speed at either 50 Hz or 60 Hz.
Finally, the higher nozzle temperatures and air flows from warming devices of the 1990s may explain why the heat transfer values in the investigation of Giesbrecht et al.17 were higher than in our investigation, although Giesbrecht et al. did not report these data.
In conclusion, the evaluation of commercially available forced-air warming devices using a validated manikin model demonstrates clinically relevant differences between systems.
Several systems were unable to provide adequate heat transfer to the manikin at a surface temperature of 38°C.
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Acknowledgments |
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Footnotes |
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for each company whose product was evaluated. The companies were explicitly restricted from having any influence on study design, data analysis or data interpretation. The laboratory has been funded, in part, by Augustine Medical, Mallinckrodt, Rüsch and several other companies which manufacture and distribute body warming devices. However, no author has a financial interest in the products which were evaluated. Accepted for publication June 23, 2006. Revision accepted October 19, 2006.
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References |
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2 Joachimsson PO, Nystrom SO, Tyden H. Postoperative ventilatory and circulatory effects of extended rewarming during cardiopulmonary bypass. Can J Anaesth 1989; 36: 9–19.
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23 Azzam FJ, Krock JL. Thermal burns in two infants associated with a forced air warming system. Anesth Analg 1995; 81: 661.[Medline]
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