Physiological responses and perceived comfort to high-flow nasal cannula therapy in awake adults: effects of flow magnitude and temperature

Clinical use of heated, high -flow nasal cannula (HFNC) for noninvasive respiratory support is increasing and may have a therapeutic role in stabilizing the upper airway in obstructive sleep apnea (OSA). However, physiological mechanisms by which HFNC therapy may improve upper airway function and effects of different temperature modes are unclear. Accordingly, this study aimed to determine effects of incremental flows and temperature modes (heated and nonheated) of HFNC on upper airway muscle activity (genioglossus), pharyngeal airway pressure, breathing parameters, and perceived comfort. Six participants (2 females, aged 35 +/- 14 yr) were studied during wakefulness in the supine position and received HFNC at variable flows (0-60 L/min) during heated (37 degrees C) and nonheated (21 degrees C) modes. Breathing parameters via calibrated Respitrace inductance bands (chest and abdomen), upper airway pressures via airway transducers, and genioglossus muscle activity via intramuscular bipolar fine wire electrodes were measured. Comfort levels during HFNC were quantified using a visual analog scale. Increasing HFNC flows did not increase genioglossus muscle activation despite increased negative epiglottic pressure swings (P = 0.009). HFNC provided similar to 7 cmH(2)O positive airway pressure at 60 L/min in nonheated and heated modes. In addition, increasing the magnitude of HFNC flow reduced breathing frequency (P = 0.045), increased expiratory time (P = 0.040), increased peak inspiratory flow (P = 0.002), and increased discomfort (P = 0.004). Greater discomfort occurred at higher flows in the nonheated versus the heated mode (P = 0.034). These findings provide novel insight into key physiological changes that occur with HFNC for respiratory support and indicate that the primary mechanism for improved upper airway stability is positive airway pressure, not increased pharyngeal muscle activity. NEW & NOTEWORTHY This study evaluated upper airway muscle function, breathing, and comfort across different HFNC flows and temperatures. There were no increases in genioglossus muscle activity at higher flows despite greater negative epiglottic pressure swings. Increasing negative pressure swings was associated with increasing discomfort in the nonheated mode. HFNC was associated with similar to 7 cmH(2)O increase in positive airway pressure, which may be the primary mechanism for upper airway stability with HFNC rather than increases in pharyngeal muscle activity.