Abstract

--- - "High-flow nasal cannula (HFNC) is extensively used for acute respiratory failure. However, questions remain regarding its physiological effects. We explored 1) whether HFNC produced similar effects to continuous positive airway pressure (CPAP); 2) possible explanations of respiratory rate changes; 3) the effects of mouth opening. Two studies were conducted: a bench study using a manikin's head with lungs connected to a breathing simulator while delivering HFNC flow rates from 0 to 60 L/min; a physiological crossover study in 10 healthy volunteers receiving HFNC (20-60 L/min) with the mouth open or closed and CPAP 4 cmH(2)O delivered through face mask. Nasopharyngeal and esophageal pressures were measured; tidal volume and flow were estimated using calibrated electrical impedance tomography. In the bench study, nasopharyngeal pressure at end-expiration reached 4 cmH(2)O with HFNC at 60 L/min, whereas tidal volume decreased with increasing flow. In volunteers with HFNC at 60 L/min, nasopharyngeal pressure reached 6.8 cmH(2)O with mouth closed and 0.8 cmH(2)O with mouth open; P < 0.001. When increasing HFNC flow, respiratory rate decreased by lengthening expiratory time, tidal volume did not change, and effort decreased (pressure-time product of the respiratory muscles per minute); effort was equivalent between CPAP and HFNC 40 L/min and became lower at 60 L/min (P = 0.045). During HFNC with mouth closed, and not during CPAP, resistance to breathing was increased, mostly during expiration. In conclusion, mouth closure during HFNC induces a positive nasopharyngeal pressure proportional to flow rate and an increase in expiratory resistance that might explain the prolonged expiration and reduction in respiratory rate and effort, and contribute to physiological benefits." - NEW & NOTEWORTHY High-flow nasal cannula generates positive airway pressure and reduces respiratory rate. Our findings confirm these physiological effects when the mouth is kept closed. An increased resistance to breathing induces a longer expiratory phase, which leads to decreased respiratory rate and minute ventilation, and therefore to a decreased respiratory workload, counterbalancing the increased pressure-time product per liter. These effects are less apparent with the mouth open and are not observed with continuous positive airway pressure.