BACKGROUND: High-flow nasal cannula therapy (HFNC) is widely used for patients with acute respiratory failure. HFNC has a number of physiological effects. Although FIO2 is considered to be constant, because HFNC is an open system, FIO2 varies according to inspiratory flow, tidal volume (VT), and HFNC gas flow. We investigated the influence of HFNC gas flow and other respiratory parameters on FIO2 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 FIO2 was set at 0.3, 0.5, and 0.7. We measured FIO2 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 VT 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 FIO2 values, inspiratory oxygen concentration varied with VT (P < .001). As the set value for FIO2 increased, the difference between set FIO2 and measured FIO2 increased. Neither breathing frequency nor prong size influenced FIO2.
CONCLUSIONS: During HFNC with simulated spontaneous breathing, when HFNC gas flow was 60 L/min, measured FIO2 was similar to set FIO2 at 0.3 and 0.5, whereas at 0.7, as VT increased, measured FIO2 decreased slightly. However, at 20 or 40 L/min, changes in VT related with deviation from set FIO2.
- Correspondence: Masaji Nishimura MD PhD, Critical Care and Emergency Medicine, Tokushima University Graduate School, 3-18-15 Kuramoto, Tokushima 770-8503, Japan. Tel.: 81-88-633-9445; Fax: 81-88-633-9425; E-mail: .
The authors have disclosed no conflicts of interest.
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