Skin-surface warming: Heat flux and central temperature

Abstract

The authors determined the efficacy of four postoperative warming devices by measuring cutaneous and tympanic membrane temperatures, and heat loss/gain using 11 thermocouples and ten thermal flux transducers in five healthy, unanesthetized volunteers. Overall thermal comfort was evaluated at 5–10 min intervals using a 10-cm visual analog scale. The warming devices were: 1) a pair of 250-W infrared heating lamps mounted 71 cm above the abdomen; 2) the Thermal Ceiling MTC XI UL (500 W) set on “high” and mounted 56 cm above the volunteer; 3) a 54-by-145 cm circulating-water blanket set to 40°C placed over the volunteer; and 4) the Bair-Hugger forced air warmer with an adult-sized cover set on “low” (≈33° C), “medium” (≈38° C), and “high” (≈43° C). Following a 10-min control period, each device was placed over the volunteer and activated for a 30-min period. All devices were started “cold” and warmed up during the study period. The Bair Hugger set on “medium” decreased heat loss more than each radiant warming device and as much as the circulating-water blanket. All methods reached maximum efficacy within 20 min. Set on “high,” the Bair Hugger increased skin-surface temperature more than the circulating-water blanket. The Bair Hugger (all settings) and the water blanket raised skin temperature more than the radiant heaters. The circulating-water blanket was the most effective device for heating an optimally placed transducer on the chest (directly under and parallel to the radiant heat sources, and touching the water and Bair Hugger blankets). However, when the entire skin surface was considered, the Bair Hugger set on “high” transferred the most heat, enough to increase mean body temperature ≈1.5 degrees C/h in a postoperative patient without thermoregulatory responses. Central temperature decreased slightly (the expected thermoregulatory response) during skin-surface warming, the decrease being roughly in proportion to the efficacy of the warming devices. Cutaneous heat flux correlated well with the skin-surface temperature, but not with thermal comfort. There was no correlation between forehead and tympanic membrane temperatures.