Relationship Between Flow and Absolute Humidity During Active Humidification for High-Flow Therapy: A Bench Study

Active humidification of medical gases is a fundamental component of high-flow oxygen therapy (HFOT). Given the high gas flow rates employed, it remains unclear whether the theoretically expected relationship between flow rate, temperature, and absolute humidity is maintained under clinical conditions. This bench-top study aimed to characterize the relationship between absolute humidity (AH) and gas flow rate and to identify optimal AH targets across different HFOT settings. This relationship was assessed by measuring water consumption by the heated humidifier at different temperatures and across a range of high-flow settings. Statistical analysis showed that AH increased by approximately 3.7 mg/L for each 1 °C increase in temperature and decreased by approximately 0.4 mg/L for each additional 1 L/min of delivered flow. Utilizing the alveolar absolute humidity value of 44 mg/L at 37 °C as the reference, the derived slope was used to estimate the corresponding AH targets for each high-flow therapy setting: 10.4 mg/L for Continuous Positive Airway Pressure via Helmet (H-CPAP), 25.3 mg/L for High Flow Nasal Cannula (HFNC), and 36.5 mg/L for HFOT-tracheostomy. Given the decrease in absolute humidity with increasing gas flow rate, the study also proposes a temperature-compensation model that may be used to achieve target absolute humidity values across different high-flow therapy settings.