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 2 concentration of 5% was numerically simulated using Ansys Fluent (Ansys Inc., USA) during NHF at rates of 20 L/min and 60 L/min. The computational domain included the nasopharynx and the distal portions of commercially available nasal cannula interfaces. 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 2 expelled through the less occluded naris, compared with a symmetrical cannula interface with similarly-sized nasal prongs. The position of the NG tube on the side of either the large or small prong had only a marginal effect on the almost complete clearance (100% vs. 98.6%) observed at an NHF rate of 60 L/min. However, at a lower NHF rate of 20 L/min, clearance increased from 29.6% to 53.1% when the NG tube was positioned on the side of the large prong. 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.