Abstract
Background Delivering heated and humidified medical gas at 20 – 60 L/min, high flow nasal cannula (HFNC) creates low level of PEEP, and ameliorates respiratory mechanics. It has become a common therapy for patients with respiratory failure. However, independent measurement of heat and humidity during HFNC and comparison of HFNC devices are lacking.
Method We evaluated 2 HFNC (AIRVO™ 2 and Optiflow™ system) devices. Each HFNC was connected to simulated external nares using the manufacturer's standard circuit. The AIRVO™ 2 outlet-chamber temperature was set at 37°C. The Optiflow™ system incorporated an O2/air blender and a heated humidifier, which was set in 40°C /–3. For both systems, HFNC flow was tested at 20, 40 and 50 L/min. Simulating spontaneous breathing using a mechanical ventilator and TTL test lung, we tested tidal volumes (VT) of 300, 500 and 700 mL, and breathing frequencies of 10 and 20 breath/min. The TTL was connected to the simulated external nares with a standard ventilator circuit. To prevent condensation, the circuit was placed in an incubator maintained at 37°C. Small, medium and large nasal prongs were tested. Absolute humidity (AH) of inspired gas was measured at the simulated external nares.
Results At 20, 40, and 50 L/min of flow, respective AH values for the AIRVO™ 2 were 35.3±2.0, 37.1±2.2 and 37.6±2.1 mg/L, and for the Optiflow™ system, 33.1±1.5, 35.9±1.7 and 36.2±1.8 mg/L. AH was lower at 20 L/min of HFNC flow than at 40 and 50 L/min (P<0.01). While AH remained constant at 40 and 50 L/min, at 20 L/min of HFNC flow, AH decreased as VT increased for both devices.
Conclusions During bench use of HFNC, AH increased with increasing HFNC flow. When the inspiratory flow of spontaneous breathing exceeded the HFNC flow, AH was influenced by VT. At all experimental settings, AH remained above 30 mg/L.
Footnotes
- Address correspondence, Masaji Nishimura, MD, Emergency and Critical Care Medicine, The University of Tokushima Graduate School, 3-18-15 Kuramoto, Tokushima, Japan 770-8503, Tel +81-88-633-9445, Fax +81-88-633-9425, E-mail: nmasaji{at}tokushima-u.ac.jp
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