Comparison of forced-air warming systems with upper body blankets using a copper manikin of the human body

Abstract

Background: Forced-air warming with upper body blankets has gained high acceptance as a measure for the prevention of intraoperative hypothermia. However, data on heat transfer with upper body blankets are not yet available. This study was conducted to determine the heat transfer efficacy of eight complete upper body warming systems and to gain more insight into the principles of forced-air warming. Methods: Heat transfer of forced-air warmers can be described as follows: Q̇=h·ΔT·A, where Q̇ = heat flux [W], h=heat exchange coefficient [W m-2°C-1], ΔT=temperature gradient between the blanket and surface [°C], and A = covered area [m2]. We tested eight different forced-air warming systems: (1) Bair Hugger® and upper body blanket (Augustine Medical Inc. Eden Prairie, MN); (2) Thermacare® and upper body blanket (Gaymar Industries, Orchard Park, NY); (3) Thermacare® (Gaymar Industries) with reusable Optisan® upper body blanket (Willy Rüsch AG, Kernen, Germany); (4) WarmAir® and upper body blanket (Cincinnati Sub-Zero Products, Cincinnati, OH); (5) Warm-Gard® and single use upper body blanket (Luis Gibeck AB, Upplands Väsby, Sweden); (6) Warm-Gard® and reusable upper body blanket (Luis Gibeck AB); (7) WarmTouch® and CareDrape® upper body blanket (Mallinckrodt Medical Inc., St. Luis, MO); and (8) WarmTouch® and reusable MultiCover™ upper body blanket (Mallinckrodt Medical Inc.) on a previously validated copper manikin of the human body. Heat flux and surface temperature were measured with 11 calibrated heat flux transducers. Blanket temperature was measured using 11 thermocouples. The temperature gradient between the blanket and surface (ΔT) was varied between -8 and +8°C, and h was determined by linear regression analysis as the slope of ΔT vs. heat flux. Mean ΔT was determined for surface temperatures between 36 and 38°C, as similar mean skin surface temperatures have been found in volunteers. The covered area was estimated to be 0.35 m2. Results: Total heat flow from the blanket to the manikin was different for surface temperatures between 36 and 38°C. At a surface temperature of 36°C the heat flows were higher (4-26.6 W) than at surface temperatures of 38°C (2.6-18.1W). The highest total heat flow was delivered by the WarmTouch™ system with the CareDrape™ upper body blanket (18.1-26.6W). The lowest total heat flow was delivered by the Warm-Gard® system with the single use upper body blanket (2.6-4W). The heat exchange coefficient varied between 15.1 and 36.2W m-2°C-1, and mean ΔT varied between 0.5 and 3.3°C. Conclusion: We found total heat flows of 2.6-26.6 W by forced-air warming systems with upper body blankets. However, the changes in heat balance by forced-air warming systems with upper body blankets are larger, as these systems are not only transferring heat to the body but are also reducing heat losses from the covered area to zero. Converting heat losses of approximately 37.8 W to heat gain, results in a 40.4-64.4 W change in heat balance. The differences between the systems result from different heat exchange coefficients and different mean temperature gradients. However, the combination of a high heat exchange coefficient with a high mean temperature gradient is rare. This fact offers some possibility to improve these systems. © Acta Anaesthesiologica Scandinavica 46 (2002).