Evaluation of the Hygrometric Properties and Resistances of Several Passive Humidifiers After Prolonged Use

Adequate humidification of inspired gases during invasive mechanical ventilation is essential to prevent complications such as endotracheal tube occlusion, sub-occlusions, and airway mucosal dysfunction. It is uncertain whether humidification performance remains stable over time when these devices are used for several consecutive days. The objective of the study was to assess the hygrometric performance and flow resistance of 5 heat-and-moisture exchangers (HMEs) with similar dead space (∼50 mL) but different materials, over 96 h of simulated ventilation using physiological expiratory humidity.

Five devices were tested during 96 h on a bench reproducing physiologic ventilation with expiratory humidity set at 35 mg H₂O/L Sedaconda ACD-S (carbon based), Hygrobac S (paper based), HumidStar 55Plus (foam based), Filtraflux (foam based), and Bact-HME Midi (fiber based). Inspiratory humidity, efficiency, water loss, and pressure-flow relationships were measured at multiple time points (90 min to 96 h). Ventilator settings reflected current protective ventilation practices (tidal volume = 500 mL, breathing frequency = 20 breaths/min).

Only Sedaconda ACD-S (mean = 29.0 mg H₂O/L) and Hygrobac S (mean = 28.2 mg H₂O/L) met the recommended threshold. Others remained <28 mg H₂O/L. Hygrobac S remained stable over time, while Sedaconda ACD-S showed a gradual decline (30.2 to 27.5 mg H₂O/L). Tested foam-based devices (HumidStar 55 Plus, Filtraflux) and fiber-based Bact-HME Midi performed poorly throughout. Resistances varied widely among tested devices (0.63-5.7 cm H₂O/L/s at 1 L/s). Foam-based HMEs had the lowest resistance; carbon-based Sedaconda had the highest. Changes in resistance after 96 h were minimal.

Not all HMEs delivered sufficient humidity. Among the tested devices, only Sedaconda ACD-S and Hygrobac S met the recommended humidification criteria for prolonged humidification. Variability in performance over time and differences in resistance highlight the need for updated standards and informed device selection to ensure patient safety. Assessing the stability of humidification performance should be required when manufacturers claim safe prolonged use.