An Experimental and Numerical Investigation of CO2 Distribution in the Upper Airways During Nasal High Flow Therapy

Nasal high flow (NHF) therapy is used to treat a variety of respiratory disorders to improve patient oxygenation. A CO 2 washout mechanism is believed to be responsible for the observed increase in oxygenation. In this study, experimentally validated Computational Fluid Dynamics simulations of the CO 2 concentration within the upper airway during unassisted and NHF assisted breathing were undertaken with the aim of exploring the existence of this washout mechanism. An anatomically accurate nasal cavity model was generated from a CT scan and breathing was reproduced using a Fourier decomposition of a physiologically measured breath waveform. Time dependent CO 2 profiles were obtained at the entrance of the trachea in the experimental model, and were used as simulation boundary conditions. Flow recirculation features were observed in the anterior portion of the nasal cavity upon application of the therapy. This causes the CO 2 rich gas to vent from the nostrils reducing the CO 2 concentration in the dead space and lowering the inspired CO 2 volume. Increasing therapy flow rate increases the penetration depth within the nasal cavity of the low CO 2 concentration gas. A 65% decrease in inspired CO 2 was observed for therapy flow rates ranging from 0 to 60 L min −1 supporting the washout mechanism theory.