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Heat and moisture exchangers (HMEs) have been increasingly used for humidification during mechanical ventilation in pediatric patients . However, efficacy of HMEs in the pediatric population has not yet been fully evaluated and there is limited information in the literature regarding the effects of ventilator settings and endotracheal tube (ETT) leakage on the humidification performance of HMEs.
Chikata et al.  tested ten pediatric HMEs in a model lung, measuring performance under different respiratory rates, tidal volumes TVs (achieved with different pressure control levels), and leakage conditions. They found that eight of the ten HMEs maintained absolute humidity (AH) at more than 30mg/L, the minimum level recommended by the American Association for Respiratory Care (AARC). With a small leak, AH decreased below 30mg/L (26.6 to 29.5mg/L), decreasing further (19.7 to 27.3mg/L) with a larger leak. Pressure control (tidal volume) level did not statistically significantly affect AH values, although there was a trend to decreased AH at higher tidal volumes. In three of the HMEs, increased respiratory rate resulted in a significantly higher AH. In six of the other seven HMEs, there was a nonstatistically significant trend to higher AH with increased respiratory rate, with one HME showing a nonsignificant trend to lower AH.
We think this is a well-designed and conducted study and provides valuable information regarding the use of HMEs in pediatric patients under different conditions. The effect of ETT leakage on AH is nicely demonstrated and is consistent with previously published data on mechanically ventilated adults.
However, we had some questions regarding the study's other findings and their possible explanation as follows