F IO ₂ in an Adult Model Simulating High-Flow Nasal Cannula Therapy

BACKGROUND: High-flow nasal cannula therapy (HFNC) is widely used for patients with acute respiratory failure. HFNC has a number of physiological effects. Although F IO 2 is considered to be constant, because HFNC is an open system, F IO 2 varies according to inspiratory flow, tidal volume (V T ), and HFNC gas flow. We investigated the influence of HFNC gas flow and other respiratory parameters on F IO 2 during HFNC. METHODS: We evaluated an HFNC system and, for comparison, a conventional oxygen therapy system. The HFNC apparatus was composed of an air/oxygen blender, a heated humidifier, an inspiratory limb, and small, medium, and large nasal prongs. HFNC gas flow was set at 20, 40, and 60 L/min, and F IO 2 was set at 0.3, 0.5, and 0.7. We measured F IO 2 for 1-min intervals using an oxygen analyzer and extracted data for the final 3 breaths of each interval. Spontaneous breathing was simulated using a mechanical ventilator connected to the muscle compartment of a model lung. The lung compartment passively moved with the muscle compartment, thus inspiring ambient air via a ventilator limb. With a decelerating flow waveform, simulated V T was set at 300, 500, and 700 mL, breathing frequency at 10 and 20 breaths/min, and inspiratory time at 1.0 s. RESULTS: With HFNC gas flow of 20 and 40 L/min, at all set F IO 2 values, inspiratory oxygen concentration varied with V T ( P < .001). As the set value for F IO 2 increased, the difference between set F IO 2 and measured F IO 2 increased. Neither breathing frequency nor prong size influenced F IO 2 . CONCLUSIONS: During HFNC with simulated spontaneous breathing, when HFNC gas flow was 60 L/min, measured F IO 2 was similar to set F IO 2 at 0.3 and 0.5, whereas at 0.7, as V T increased, measured F IO 2 decreased slightly. However, at 20 or 40 L/min, changes in V T related with deviation from set F IO 2 .