Effektivität konvektiver Luftwärmung zur Vermeidung perioperativer Hypothermie. Eine vergleichende Untersuchung von drei unterschiedlichen Geräten an einem validierten Kupfermodell des Menschen

Abstract

Background: Forced-air warming with upper body, lower body and full body blankets has gained high acceptance as a measure for the prevention of intraoperative hypothermia. This study was conducted to determine the heat transfer efficacy of three warming systems with upper body, lower body and full body blankets and to gain more insight into the principles of forced-air warming. Methods: Heat transfer of forced-air warmers can be described as follows: Q˙=h · ΔT · A where Q˙= 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]. Twelve different forced-air warming systems were tested: Bair Hugger® with upper body, lower body and full body blanket (Augustine Medical Inc. Eden Prairie, MN); Thermacare® with upper body, lower body and full body blanket (Gaymar Industries, Orchard Park, NY); Thermacare® (Gaymar Industries) with reusable Optisan® upper body, lower body (full body blanket used as lowerbody blanket) and full body blanket (Willy Rüsch AG, Kernen, Germany); WarmAir® upper body, lower body and full body blanket (Cincinnati Sub-Zero Products, Cincinnati, OH); on a previously validated copper manikin of the human body. Heat flux and surface temperature were measured with 16 (11 for upper body blanket) calibrated heat flux transducers. Blanket temperature was measured using 16 (11 for upper body blanket) thermocouples. The temperature gradient between the blanket and surface (ΔT) was varied, 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 for upper body blankets, 0.54 m2 for lower body blankets and 1.21 m2 for full body blankets. 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 (12.5-19.8 W for upper body blankets, 15.6-18.2 W for lower body blankets and 11.3-25.3 W for full body blankets) than at surface temperatures of 38°C (4.9-11.2 W for upper body blankets, 7.8-8.9 W for lower body blankets and -1.5-9 W for full body blankets). The highest total heat flow for upper body blankets was delivered by the Thermacare™ system with the reusable Optisan® upper body blanket (11.2-19.8 W), for lower body blankets the highest total heat flow was delivered by the Thermacare™ system with the Thermacare™ lower body blanket (8.4-18.2 W) and for the full body blankets the highest total heat flow was delivered by the Thermacare™ system with the reusable Optisan® full body blanket (9-25.3 W). The lowest total heat flow for upper body blankets was delivered by the Bair Hugger® system with the single use upper body blanket (4.9-13 W), for lower body blankets the lowest total heat flow was delivered by the Thermacare™ system with reusable Optisan® Full body blanket used as lower body blanket (7.8-15.6 W) and for the full body blankets the lowest total heat flow was delivered by the Bair Hugger® system with the single use full body blanket (-1.5-11.3 W) . The heat exchange coefficient varied between 17.7 and 28.8 W m−2 °C−1 for upper body blankets, between 14.4 and 25.5 W m−2 °C−1 for lower body blankets and for full body blankets between 13.4 and 23.6 W m−2 °C−1 . Mean ΔT varied between 0.5 and 2.3°C for upper body blankets, between 0.6 and 2.2 °C for lower body blankets and for full body blankets between -0.06 and 1.2 °C. Conclusion: Total heat flows of 4.9-19.8 W by forced-air warming systems with upper body blankets were found, 7.8-18.2 W by forced-air warming systems with lower body blankets and -1.5-25.3 W by forced-air warming systems with full body blankets. However, the changes in heat balance by forced-air warming systems with upper body, lower body and full 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. For upper body blankets converting heat losses of approximately 37 W to heat gain, results in a 41.9-56.8 W change in heat balance. For lower body blankets converting heat losses of approximately 58 W to heat gain, results in a 65.8-76.2 W change in heat balance. For full body blankets converting heat losses of approximately 131 W to heat gain, results in a 129.5-156,3 W change in heat balance.