Correlation of high flow nasal cannula outlet area with gas clearance and pressure in adult upper airway replicas.

Background: Primary benefits of high flow nasal cannula therapy include washout of carbon dioxide rich exhaled gas and increased airway pressures during tidal breathing. This work reports on the influence of high flow nasal cannula outlet area on upper airways gas clearance and tracheal pressures using measurements in five realistic adult nose-throat airway replicas.

Methods: Two commercial high flow nasal cannulas and one generic nasal cannula of varying size were compared. 100% oxygen was supplied via cannulas at flow rates ranging from 30 to 90l/min to replicas originally filled with air, and oxygen concentrations at the larynx and trachea were compared over time. Additionally, and separately, replicas were connected to a mechanical lung simulator to simulate tidal breathing while undergoing high flow nasal cannula therapy, with tracheal pressure-time waveforms recorded.

Findings: Faster gas clearance corresponded with higher flow rates (P<0.001), and with smaller cannula outlet area (P<0.001). Observed pressures were in approximate agreement with limited available in-vivo data in the literature. Between 0 and 60L/min cannula flow rates, tracheal positive end expiratory pressures increase was greater with the smallest cannula (∆P=785SD(185) Pa) compared to the largest cannula (∆P=380SD(120)Pa). Regression analysis indicates that positive end expiratory pressure is proportional to the square of flow velocities exiting the cannula and nares (R=0.906).

Interpretation: Since increased pressure and clearance rate have been associated with improved clinical outcomes in previous studies, our results suggest that smaller cannula outlet area may be preferable.