Why Passive Humidifier Testing Methods and Clinical Use Matter for Patient Safety and Need to Be Updated

Insufficient inspiratory gas humidification in intubated patients is associated with several complications, including life-threatening endotracheal tube occlusion (ETO). Passive humidifiers, or heat-and-moisture exchangers (HMEs), are widely used in clinical practice, and their performance is commonly assessed using the ISO 9360 standard, implemented more than 30 years ago. Recent evidence suggests that this standard is insufficient to detect poorly performing HMEs. During the COVID-19 pandemic, unexpectedly high rates of ETO were reported in patients ventilated with HMEs that complied with ISO requirements, prompting renewed interest in reassessing humidification performance evaluation. We conducted a narrative and critical review of current methods used to evaluate HME and propose changes to improve device assessment. The ISO method relies on gravimetric measurements, a technique with several limitations, and assumes an expiratory absolute humidity (AH) of 44 mg H 2 O/L, which is substantially higher than the AH actually entering the HMEs. As a result, bench performance overestimates the humidity delivered. In addition, the ISO standard uses derived metrics rather than directly reporting absolute humidity, the parameter most closely associated with clinical outcomes. This review highlights the limitations of the current ISO approach and proposes several key revisions: adoption of validated hygrometric measurements; use of physiologic expiratory humidity (∼35 mg H 2 O/L); and definition of humidity thresholds adapted to clinical settings (≥28 mg H 2 O/L for prolonged ventilation, 20–25 mg H 2 O/L for the operating room, and intermediate values for short-term ventilation). Revising the ISO standard appears warranted to further improve purchasing decisions for the safe use of HMEs.