Laboratory Evaluation of Moisture Output of Seven Airway Heat and Moisture Exchangers

Abstract

Humidification of gases inspired during mechanical ventilation is accepted as important, and such humidification is often provided by devices called heat and moisture exchangers (HME), most of which are also known as hygroscopic condenser humidifiers (HCH); another general term is "artificial noses." In a laboratory simulation of mechanical ventilation, we studied the moisture output of seven commercially available HME, six of which were HCH. Method: We used a lung model to simulate patient use, determining each HME's peak moisture output at tidal volumes of 0.5 L, 0.75 L, and 1.0 L and at each tidal volume—when FIO₂ was 0.21 and 1.0. Results: All seven devices provided at least 21 mg H₂O/L (range, 21-28 mg H₂O/L) at FIO₂ 0.21. As tidal volume increased (constant inspiratory flow), moisture output fell slightly in all HME. At the higher FIO₂ (decreased water vapor content of 'inspired' gas), output also was less than at the lower FIO₂. The highest outputs were provided by the Engstrom and Siemens 150 humidifiers. The one non-hygroscopic device we tested was not efficient enough for us to recommend its use during mechanical ventilation. Conclusion: Hygroscopic condenser humidifiers represent an improvement over earlier devices, but their employment during mechanical ventilation should be limited to short-term use in patients who are adequately hydrated, normothermic, and who do not require therapeutic humidity.