BACKGROUND:

High-flow nasal cannula (HFNC) therapy is used for patients with respiratory failure. Recently, HFNC therapy with very high gas flows (ie, gas flows of 60–100 L/min) was reported to generate higher positive airway pressure and an associated decrease in breathing frequency. However, the humidification of HFNC therapy with very high gas flow remains to be clarified.

METHODS:

We evaluated 3 heated humidifier systems: a single MR850, the Hummax2, and parallel MR850s. The MR850 is a pass-over humidifier system, and the Hummax2 works with a porous hollow polyethylene fiber membrane. The parallel MR850 system included 2 MR850s connected in parallel to the lung with a 22 mm Y-piece. Gas flow was set at 40–90 L/min in increments of 10 L/min, and F_IO₂ was set at 0.21. Heated humidifiers in the MR850 systems were set in invasive mode (40°C/−3), and with the Hummax2 the vapor temperature was set at 39°C. The simulated external nares were connected to a test lung via a standard ventilator circuit. One-way valves prevented mixing of inspired and expired gases. Compliance of the test lung was 0.05 L/cm H₂O and resistance 5 cm H₂O/L/s. Simulated tidal volumes (V_T) were 300, 500, and 700 mL, with a breathing frequency of 10 or 20 breaths/min and an inspiratory time of 1.0 s. Temperature, relative humidity, and absolute humidity (AH) of inspired gas downstream of the external nares were measured using a hygrometer for 1 min, and results for the last 3 breaths were extracted.

RESULTS:

With the single MR850, when gas flow was > 80 L/min, AH decreased as gas flow increased (P < .001). With the Hummax2, as gas flow increased, AH decreased (P < .001). With the parallel MR850s, regardless of gas flow, AH was constant. As breathing frequency increased, AH increased in all systems.

CONCLUSIONS:

During HFNC therapy with very high gas flows in this bench study, conventional heated humidifiers did not provide adequate humidification. Caution is advised when using HFNC therapy with very high gas flows with conventional heated humidifiers.

Keisuke Morinishi

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