Investigating the Effect of Asymmetrical Therapy Delivery on CO₂ Distribution During High Flow Nasal Cannula Therapy

Computations were run to investigate the effect of asymmetrical therapy delivery on CO₂ inhalation, which should be minimized, during nasal high flow therapy. Therapy flow ranged from 5-45 L/min and was delivered using either a single-prong or dual-prong cannula geometry. Realistic breathing was mimicked using a temporally varying esophageal boundary condition. The single-prong cannula resulted in less CO₂ inhalation than the dual-prong cannula if all other factors were held constant. As therapy flow was decreased, the distinction between cannula geometries (in terms of CO₂ inhalation) grew. Pathlines, colored by residence time in the upper airway, were released from the trachea during exhalation to investigate the internal flow of CO₂-rich exhaled air. The dual-prong cannula resulted in higher residence times for expiratory air, reinforcing the notion that the single-prong cannula results in improved CO₂ clearance in difficult-to-flush scenarios. The blockage of both nares by the cannula prongs and injected therapy flow, in conjunction with the partially (80%) closed mouth, caused a more restrictive flush scenario in which there were limited pathways for expiratory flow to escape the airway unimpeded. CO₂ contours revealed the fact that the dominant mixing region between therapy flow and expiratory flow was located at different points in the airway depending on the cannula geometry used.