Optimum support by high-flow nasal cannula in acute hypoxemic respiratory failure: effects of increasing flow rates

Purpose: Limited data exist on the correlation between higher flow rates of high-flow nasal cannula (HFNC) and its physiologic effects in patients with acute hypoxemic respiratory failure (AHRF). We assessed the effects of HFNC delivered at increasing flow rate on inspiratory effort, work of breathing, minute ventilation, lung volumes, dynamic compliance and oxygenation in AHRF patients. Methods: A prospective randomized cross-over study was performed in non-intubated patients with patients AHRF and a PaO₂/FiO₂ (arterial partial pressure of oxygen/fraction of inspired oxygen) ratio of ≤300 mmHg. A standard non-occlusive facial mask and HFNC at different flow rates (30, 45 and 60 l/min) were randomly applied, while maintaining constant FiO₂ (20 min/step). At the end of each phase, we measured arterial blood gases, inspiratory effort, based on swings in esophageal pressure (ΔPes) and on the esophageal pressure–time product (PTP_Pes), and lung volume, by electrical impedance tomography. Results: Seventeen patients with AHRF were enrolled in the study. At increasing flow rate, HFNC reduced ΔPes (p < 0.001) and PTP_Pes (p < 0.001), while end-expiratory lung volume (ΔEELV), tidal volume to ΔPes ratio (V_T/ΔPes, which corresponds to dynamic lung compliance) and oxygenation improved (p < 0.01 for all factors). Higher HFNC flow rate also progressively reduced minute ventilation (p < 0.05) without any change in arterial CO₂ tension (p = 0.909). The decrease in ΔPes, PTP_Pes and minute ventilation at increasing flow rates was better described by exponential fitting, while ΔEELV, V_T/ΔPes and oxygenation improved linearly. Conclusions: In this cohort of patients with AHRF, an increasing HFNC flow rate progressively decreased inspiratory effort and improved lung aeration, dynamic compliance and oxygenation. Most of the effect on inspiratory workload and CO₂ clearance was already obtained at the lowest flow rate.