Breathing frequency and PaCO2: comparison between high-flow nasal cannula oxygen therapy (HFNC) and comparative therapies in 8 studies. Breathing frequency and PaCO2: comparison between high-flow nasal cannula oxygen therapy (HFNC) and comparative therapies in 8 studies. Left: Difference of breathing frequency between comparative therapy and HFNC. In all studies, breathing frequency with HFNC was lower. Right: Difference of PaCO2. Here, no statistically significant differences between the therapies are apparent. Masaji Nishimura Respir Care 2016;61:529-541 (c) 2012 by Daedalus Enterprises, Inc.