Modelling of Dead Space Clearance and Rebreathing During Asymmetrical Nasal High Flow

Nasal high flow (NHF) is an established form of non-invasive respiratory support. Asymmetrical nasal cannula interface (AI) leads to flow asymmetry in the upper airways that accelerates clearance of expired gas. The purpose of the study was to model clearance of expired gas and rebreathing during NHF with AI and symmetrical cannula interface (SI). Gas flow in the upper airway with end-tidal CO2 concentration 5% was modelled using Ansys Fluent software (Ansys, USA). Modelling was done using a breathing waveform at 15min⁻¹ with commercially available interfaces (Optiflow+ Duet and Optiflow+ Fisher & Paykel Healthcare, New Zealand). During the breathing cycle, AI was more effective at clearing the expired CO2, resulting in a reduction in rebreathing as the inspired CO2 per breath was decreased from 1.30, 1.05, 0.86 mL to 0.31, 0.28, and 0.27 mL/breath at 20, 40, and 60 L/min NHF rates, respectively. This effect was caused by asymmetrical flow in the nasal cavity, where over 90% of the CO2 was purged via the less occluded nare. NHF with AI resulted in the reverse flow in the nasal cavity, leading to faster and more effective clearance of CO2 and, hence, a decrease in the rebreathing. <fig><object-id>erj;64/suppl_68/OA959/F1</object-id><object-id>F1</object-id><object-id>F1</object-id><graphic></graphic></fig> Figure 1: (a) Volume of CO2 in the airway per breath during NHF 20, 40 and 60 L/min. (c) CO2 flow through nares during NHF with AI. Top-down view of CO2 mass fraction in the airway for AI (b) and SI (d) NHF interfaces.