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

Background: Forced-air warming has gained high acceptance as a measure for the prevention of intraoperative hypothermia. However, data on heat transfer with lower body blankets are not yet available. This study was conducted to determine the heat transfer efficacy of six complete lower body warming systems. Methods: Heat transfer of forced-air warmers can be described as follows: Q.=h.DeltaT.A where Q. = heat transfer [W], h = heat exchange coefficient [W m(-2) degreesC(-1) ], DeltaT = temperature gradient between blanket and surface [degreesC], A = covered area [m(2) ]. We tested the following forced-air warmers in a previously validated copper manikin of the human body: (1) Bair Hugger((R)) and lower body blanket (Augustine Medical Inc., Eden Prairie, MN); (2) Thermacare((R)) and lower body blanket (Gaymar Industries, Orchard Park, NY); (3) WarmAir((R)) and lower body blanket (Cincinnati Sub-Zero roducts, Cincinnati, OH); (4) Warm-Gard((R)) and lower body blanket (Luis Gibeck AB, Upplands Vasby, Sweden); (5) Warm-Gard((R)) and reusable lower body blanket (Luis Gibeck AB); and (6) WarmTouch((R)) and lower body blanket (Mallinckrodt Medical Inc., St. Luis, MO). Heat flux and surface temperature were measured with 16 calibrated heat flux transducers. Blanket temperature was measured using 16 thermocouples. DeltaT was varied between -10 and +10 degreesC and h was determined by a linear regression analysis as the slope of DeltaT vs. heat flux. Mean DeltaT was determined for surface temperatures between 36 and 38 degreesC, because similar mean skin temperatures have been found in volunteers. The area covered by the blankets was estimated to be 0.54 m(2) . Results: Heat transfer from the blanket to the manikin was different for surface temperatures between 36 degreesC and 38 degreesC. At a surface temperature of 36 degreesC the heat transfer was higher (between 13.4 W to 18.3 W) than at surface temperatures of 38 degreesC (8-11.5 W). The highest heat transfer was delivered by the Thermacare((R)) system (8.3-18.3 W), the lowest heat transfer was delivered by the Warm-Gard((R)) system with the single use blanket (8-13.4 W). The heat exchange coefficient varied between 12.5 W m(-2) degreesC(-1) and 30.8 W m(-2) degreesC(-1) , mean DeltaT varied between 1.04 degreesC and 2.48 degreesC for surface temperatures of 36 degreesC and between 0.50 degreesC and 1.63 degreesC for surface temperatures of 38 degreesC. Conclusion: No relevant differences in heat transfer of lower body blankets were found between the different forced-air warming systems tested. Heat transfer was lower than heat transfer by upper body blankets tested in a previous study. However, forced-air warming systems with lower body blankets are still more effective than forced-air warming systems with upper body blankets in the prevention of perioperative hypothermia, because they cover a larger area of the body surface.