What determines the efficacy of forced-air warming systems? A manikin evaluation with upper body blankets.

BACKGROUND: Forced-air warming has gained acceptance as an effective means to prevent perioperative hypothermia. However, little is known about the influence of air flow and air temperature at the nozzle and the influence of heat distribution in the blankets on the efficacy of these systems. METHODS: We conducted a manikin study with heat flux transducers using five forced-air warming systems to determine the factors that are responsible for heat transfer from the blanket to the manikin. RESULTS: There was no relation between air temperature at the nozzle of the power unit and the resulting heat transfer. There was also no relation between the air flow at the nozzle of the power unit and the resulting heat transfer. However, all blankets performed best at high air flows above 19 L/s. The heat exchange coefficient, the mean temperature gradient between the blanket and the manikin correlated positively with the resulting heat transfer and the difference between the minimal and maximal blanket temperature correlated negatively with the resulting heat transfer. CONCLUSIONS: The efficacy of forced-air warming systems is primarily determined by the blanket. Modern power units provide sufficient heat energy to maximize the ability of the blanket to warm the patient. Optimizing blanket design by optimizing the mean temperature gradient between the blanket and the manikin (or any other surface) with a very homogeneous temperature distribution in the blanket will enable the manufacturers to develop better forced-air warming systems.

Conductive heat exchange with a gel-coated circulating water mattress.

The use of forced-air warming is associated with costs for the disposable blankets. As an alternative method, we studied heat transfer with a reusable gel-coated circulating water mattress placed under the back in eight healthy volunteers. Heat flux was measured with six calibrated heat flux transducers. Additionally, mattress temperature, skin temperature, and core temperature were measured. Water temperature was set to 25 degrees C, 30 degrees C, 35 degrees C, and 41 degrees C. Heat transfer was calculated by multiplying heat flux by contact area. Mattress temperature, skin temperature, and heat flux were used to determine the heat exchange coefficient for conduction. Heat flux and water temperature were related by the following equation: heat flux = 10.3 x water temperature - 374 (r(2) = 0.98). The heat exchange coefficient for conduction was 121 W . m(-2) . degrees C(-1). The maximal heat transfer with the gel-coated circulating water mattress was 18.4 +/- 3.3 W. Because of the small effect on the heat balance of the body, a gel-coated circulating water mattress placed only on the back cannot replace a forced-air warming system.