Asymmetry from an asymmetrical cannula interface and nasogastric tube during nasal high flow enhances dead-space clearance: a fluid dynamics study.

Background: During non-invasive respiratory support with nasal high flow (NHF), an asymmetrical interface (AI) featuring nasal prongs of different sizes has been shown to reduce re-breathing and improve gas exchange in hypoxemic and COPD patients. The objective of this study was to investigate the mechanisms of expired gas clearance during NHF in the presence of a nasogastric (NG) tube, which is commonly used post extubation and causes partial upper-airway occlusion. Methods: A computational fluid dynamics study was conducted using an averaged adult upper-airway model with breathing patterns representative of both healthy adults and patients with respiratory failure. Gas flow containing an end-tidal CO Results: During NHF, the AI splits the flow both inside and outside the prongs, resulting in differential pressure across the nasal cavities. At an NHF rate of 60 L/min this generated reverse flow through the choanae and increased gas clearance by more than 65%, with up to 91% of expired CO Conclusion: In the computational experiments, asymmetry in the upper-airway cross-sectional area produced by differences in nasal prong size and the presence of an NG tube during NHF alters the gas kinetics and may accelerate anatomical dead-space clearance, thereby reducing re-breathing of expired gas.