Hygrometric Performance of 113 Passive Humidifiers: ISO Standard Method Versus Psychrometry

Background: Passive humidifiers, also known as heat-and-moisture exchangers (HMEs) are used to warm and humidify inspired gases delivered during mechanical ventilation. Recent data demonstrate that the evaluation and selection of these devices is not optimal, leading to severe complications, such as endotracheal tube occlusions. The aim of the study was to assess the humidification performances of a large number of commercially available devices, with the psychrometric method, and to compare the results with manufacturer's data. Methods: We assessed 113 devices using a bench test that simulated physiological ventilation conditions. Seventy-one devices were described by the manufacturers as HME without or with filtration properties (HMEF) and 42 as antimicrobial filters. Among the 71 HME/HMEF, 60% were foam-based, 32% were paper-based, 6% were fiber-based, and 1 had carbon media (HMEF with inhaled sedation function). Three hygrometric measurements using the psychrometric method for each device were performed after reaching a steady state and compared with manufacturers' data (based on the International Organization for Standardization [ISO] method). Results: Among the 71 HME/HMEF tested, only 24 (34%) delivered absolute humidity above 28 mg H₂O/L. Nineteen percent of the foam-based and 57% of the paper-based HME/HMEF delivered desired levels of humidity. Humidification data provided by manufacturers were available for 59 HME/HMEFs. In all but one device, humidity output was overestimated using the ISO method. The median (interquartile) difference between our measurements and the manufacturers' data were 6.0 (3.8-8.7) mg H₂O/L for devices described as HMEs (P < .001). Poor performing devices were detected only with the psychrometric method. Conclusions: Several HME/HMEFs performed poorly and should not be used for prolonged mechanical ventilation. The values determined by independent assessments were lower than values reported by manufacturers. Evaluation of a passive humidifier using the current ISO method does not guarantee provision of adequate humidification.