Relationship Between Back Pressure and FIO2 on Venturi-Effect Type High-Flow Nasal Cannula

Background: Nishimura reported in his article regarding respiratory care in 2016 that high-flow nasal cannula (HFNC) was a reliable therapy device for respiratory failure patients with hypercapnia. However, we often experienced carbon dioxide narcosis in patients using Venturi-effect typed HFNC (HFNC/Venturi). PURPOSE We hypothesized that oxygen concentration was increased by back pressure in the circuit of HFNC/Venturi which was produced during breathing. Methods: FIO2 on HFNC/Ventrui was adjusted to 32%, which is the minimum concentration (MaxVenturi, Maxtec, US). Three volunteers using the Venturi-effect typed HFNC maintained stable breathing for 30 seconds. Then, the back pressure and the oxygen concentration in the circuit during their stable breathing were measured three times for each examinee. The experimental conditions used are shown as below. Total flow: 20 to 60 L/min. Breathing style: mouth and nose. Results: Back pressure in the mouth breathing when the total flow was 60 L/min reached a median of 14.2 cm H2O (interquartile range [25th to 75th percentile], 14.0 to 14.2), as compared with 1.65 cm H2O (IQR, 1.60 to 1.80) in that of the total flow 20 L/min (P