The effect of convective heating on evaporative heat loss in anesthetized children.

Cassey, John; Salter, Jo; Colyvas, Kim; Burstal, Richard; Stanger, Rohan

BACKGROUND Convective warming is effective in maintaining core temperature under anesthesia. It may increase evaporative water loss (EWL). If significant, further investigation of warming modifications to minimize this impact would be warranted. OBJECTIVES To quantify EWL in two groups of children (warmed and nonwarmed) having surgical procedures under anesthesia. METHODS We performed an observational study of well children having general anesthesia for elective surgical procedures lasting ≥60 min. They were recruited sequentially to each of three age groups: 1-12 months, 13 months-5 years, and 5-12 years--with each age group divided into convectively warmed (43°C) and nonwarmed (21°C) subgroups. Evaporative heat loss (EHL) was calculated from accurate measurement of net EWL during the surgical period. RESULTS Sixty children were studied. As a percentage of body mass, mean EWLs were 0.29 (warmed) and 0.09 (nonwarmed). Using an ancova model, only procedure duration had a significant impact and explained why the extended procedural time in some convectively warmed children led to higher mean EWLs for that group. For the nonwarmed group, the mean Tcore drop was 1.27°C with a contribution from EWL of 0.6°C over ~70 min. CONCLUSIONS Within the age range 1 month-12 years, EHL is not significantly influenced by convective heating under anesthesia. There is no thermal advantage in exploring technique modifications such as humidifying the warming air. Previous estimates of the contribution of EHL to total heat loss in anesthetized children may require revision.