Braunlich J, et al. - Here, the authors assessed the mechanism of nasal highflow (NHF) mediated PCO₂ reduction in patients with chronic obstructive pulmonary disease (COPD). With NHF therapy, effective PCO₂ reduction was noted in stable hypercapnic COPD patients. This effect did not correlate with an increase in mean airway pressure but with increased leakage and airflow. Thereby indicating airway wash out and reduction of functional dead space as important mechanisms of NHF therapy. When considering NHF treatment in hypercapnic COPD patients, these outcomes could be useful.