Heat and moisture exchangers (HMEs) are used to provide humidification and warming of the inspiratory gases during general anesthesia. The performance specifications provided by manufacturers of HMEs are based on in vitro measurements of moisture output using the International Standards Organization (ISO) 9360 method. We studied the in vivo performance of three different HMEs with similar ISO specifications in a randomized crossover fashion in patients under general anesthesia. The effect of each HME on temperature, convective heat loss, evaporative heat loss, total heat loss, relative humidity, and absolute humidity of inspiratory gases was determined. Although all HMEs in general improved baseline variables, we found significant differences in performance for the different HMEs. In only one type did the moisture output correspond with ISO specifications. We conclude that the in vivo performance of HMEs may not correspond with manufacturer's specifications.
Implications: There is considerable variability in the in vivo performance of heat and moisture exchangers that have similar manufacturer specifications. These specifications, based on the International Standards Organization 9360 standard, which is measured in vitro, cannot be used to predict clinical performance.