Physiologic Effects of High-Flow Nasal Cannula in Acute Hypoxemic Respiratory Failure

Rationale: High-flow nasal cannula (HFNC) improves the clinical outcomes of nonintubated patients with acute hypoxemic respiratory failure (AHRF). Objectives: To assess the effects of HFNC on gas exchange, inspiratory effort, minute ventilation, end-expiratory lung volume, dynamic compliance, and ventilation homogeneity in patients with AHRF. Methods: This was a prospective randomized crossover study in nonintubated patients with AHRF with Pa-O2/setF(IO2) less than or equal to 300mmHgadmitted to the intensive care unit. Werandomly applied HFNC set at 40 L/min compared with a standard nonocclusive facial mask at the same clinically set F-IO2 (20 min/step). Measurements and Main Results: Toward the end of each phase, we measured arterial blood gases, inspiratory effort, and work of breathing by esophageal pressure swings (Delta Pes) and pressure time product, and we estimated changes in lung volumes and ventilation homogeneity by electrical impedance tomography. We enrolled 15 patients aged 60614 years old with Pa-O2/setF(IO2) 130 +/- 35 mm Hg. Seven (47%) had bilateral lung infiltrates. Compared with the facial mask, HFNC significantly improved oxygenation (P < 0.001) and lowered respiratory rate (P < 0.01), Delta Pes (P < 0.01), and pressure time product (P < 0.001). During HFNC, minute ventilation was reduced (P < 0.001) at constant arterial CO2 tension and pH (P = 0.27 and P = 0.23, respectively); end-expiratory lung volume increased (P < 0.001), and tidal volume did not change (P = 0.44); the ratio of tidal volume to Delta Pes (an estimate of dynamic lung compliance) increased (P < 0.05); finally, ventilation distribution was more homogeneous (P < 0.01). Conclusions: In patients with AHRF, HFNC exerts multiple physiologic effects including less inspiratory effort and improved lung volume and compliance. These benefits might underlie the clinical efficacy of HFNC.