HOW DO FORCED-AIR WARMING SYSTEMS PREVENT HYPOTHERMIA A PHYSICAL ANALYSIS OF DIFFERENT SYSTEMS USING A VALIDATED MANIKIN

Abstract
Forced-air warming has gained high acceptance as a measure for the prevention of intraoperative hypothermia. Heat flow of forced-air warmers can be described as follows: Q = h•ΔT•A Q = heat flow [W], h = heat exchange coefficient [W m^-2 °C^-1], ΔT = temperature gradient between blanket and surface [°C], A = covered area [m²]. We tested the six forced-air warmers with upper body and lower body blankets using a validated copper manikin of the human body. Heat flux and surface temperature were measured with calibrated heat flux transducers. Blanket temperature was measured using thermocouples. ΔT was varied and h was determined by a linear regression analysis as the slope of ΔT versus heat flux. Mean ΔT was determined for surface temperatures between 36°C and 38°C. The area covered by the blankets was estimated to be 0.35 m² for upper body blankets and 0.54 m² for lower body blankets. Heat flow was than calculated. Heat flow from the blanket to the manikin was different for surface temperatures between 36°C and 38°C. At a surface temperature of 36°C heat flow was higher (between 4.0 W and 26.6 W) than at surface temperatures of 38°C (2.6 W to 18.1 W). Forced-air warmers prevent hypothermia by two mechanisms. Firstly they transfer heat from the power unit via the blanket to the body surface. Secondly they reduce heat losses from the covered area to zero.

Key words
Hypothermia, warming devices, forced-air warming systems

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