A55-05 Clearance of Anatomical Dead Space by Asymmetrical Nasal High Flow in Hypercapnic COPD Patients: A Computational Study

Abstract Rationale Patients with COPD experience airflow obstruction and air trapping, resulting in an increased alveolar dead space. This produces an expired CO2 capnogram with a steep alveolar “plateau”, commonly referred to as a “shark fin.” Nasal high flow (NHF) clears anatomical dead space and reduces rebreathing from the upper airways. The asymmetrical cannula interface (AI) has been shown to accelerate the clearance of expired gas by creating asymmetrical flow and differential pressures within the nasal cavities. This study investigated the clearance effects of NHF using AI and a symmetrical interface (SI) during simulated healthy and hypercapnic COPD breathing. Method Ansys Fluent™ was used to simulate nasal breathing with NHF applied through an averaged patient airway truncated at the nasopharynx. Healthy and COPD breathing patterns were specified at the nasopharynx using flow rate boundary conditions. Representative capnograms were generated based on anatomical and clinical data: the healthy capnogram was represented by a step function, and the COPD capnogram by a sawtooth function. The COPD breathing pattern represented a patient with expiratory flow limitation while respiratory rate in both conditions was 18 min-1. To simulate hypercapnia, the end-tidal CO2 concentration was set to twice that of the healthy case, ensuring equivalent expired tidal (500 mL) and CO2 volumes (17.5 mL at the nasopharynx). NHF at 30 L/min was applied through the AI and SI, modelled after existing clinical products (Optiflow+, Optiflow Duet; Fisher NHF with the SI reduced it to 2.45 mL, and with the AI to 0.31 mL. Within the estimated dead space volume, NHF with AI increased dead space clearance by 13.68 mL (45.6%) in healthy conditions, and by 20.79 mL (69.3%) in COPD, per breath. Conclusions A steep alveolar plateau in the expired CO2 waveform, commonly observed in severe hypercapnic COPD patients, increases the proportion of high-CO2 gas cleared by NHF relative to the total expired volume. The AI may further enhance dead space clearance efficiency during NHF by reducing rebreathing of CO2-rich gas through improved purging of the more distal regions of the upper airways. This abstract is funded by: Fisher and Paykel Healthcare