The effect of nasal patency and anatomy during asymmetrical nasal high flow.

Nasal high flow (NHF) with the asymmetrical cannula interface (AI) is increasingly used as respiratory support across a broad range of patients. By incorporating one unilaterally oversized prong, the AI enhances dead-space clearance and increases airway pressure. This study investigated the effects of nasal patency and anatomical variability of the upper airways during NHF delivered with the AI. Twenty healthy adults underwent three 3-h study periods, whereby heated, humidified air was delivered at 30 L/min via either an AI, a symmetrical interface (SI), or a control condition without NHF. Nasal patency was assessed every 30 min using acoustic rhinometry, rhinomanometry, and a self-assessment questionnaire. Measurements were quantified using a laterality index, and ventilation was monitored by respiratory inductance plethysmography. Nasal anatomy was evaluated using magnetic resonance imaging, and airway replicas reconstructed from these images, including original and mirrored models, were tested in bench-top experiments. All measures of nasal patency were significantly positively correlated (P < 0.001). There was no association between the AI laterality index and respiratory rate or tidal volume. In the three-dimensional models, both airway pressure and dead-space clearance increased when the leak area was reduced by changing the SI to the AI. Compared with the originals, mirroring the nasal passages showed no difference except for the model with severe nasal septum deviation. NHF with SI or AI did not alter the dominant side of nasal patency, which did not affect ventilation. Nares morphology, rather than physiological alternation in nasal patency, may have a greater influence on pressure generation during NHF.NEW & NOTEWORTHY The asymmetrical cannula interface used during nasal high flow (NHF) causes unequal occlusion of the nares. This study demonstrates that physiological changes of nasal patency at the internal nasal valve, known as the nasal cycle, do not affect ventilation during NHF. In contrast, anatomical variability, such as differences in the cross-sectional area of the nares or severe nasal septum deviation, may substantially influence the generated pressure and could produce physiologically relevant effects during therapy.