By washing CO2  out of the upper airways, high-flow nasal cannulas (HFNC) might improve CO2  elimination. For a study, researchers sought to see how HFNC affected minute ventilation and ventilatory ratio (VR), a dead space surrogate, in patients hospitalized for acute hypercapnic COPD exacerbation. A physiological study compared HFNC at 40 L/min to low flow oxygen. Chest plethysmography was used to estimate tidal volume (VT) and minute ventilation differences between the two treatments. The arterial blood gases and respiratory rate (RR) were measured. VR variations were computed. Wilcoxon tests were used to compare data. About 10 patients had their recordings taken. With HFNC, minute ventilation was reduced by -16.2 [-30.9–0.4]% (P=0.049). VT was unchanged, but RR was lower during HFNC. When HFNC was used instead of standard oxygen, PaCO2 was lower: 48.7 [46.4–58.1] vs. 50.7 [48.4–57.5] mmHg (P=0.020). With HFNC, VR decreased by -18.0 [-34.7 – 4.0]% (P=0.020). HFNC was used in patients recovering from an acute COPD exacerbation; it reduced RR, minute ventilation, PaCO2, and VR compared to standard oxygen. These changes were consistent with HFNC causing a decrease in physiologic dead space.