Physiological effects of awake prone position in acute hypoxemic respiratory failure

Background: The effects of awake prone position on the breathing pattern of hypoxemic patients need to be better understood. We conducted a crossover trial to assess the physiological effects of awake prone position in patients with acute hypoxemic respiratory failure. Methods: Fifteen patients with acute hypoxemic respiratory failure and PaO2/FiO2 < 200mmHg underwent high-flow nasal oxygen for 1h in supine position and 2h in prone position, followed by a final 1-h supine phase. At the end of each study phase, the following parameters were measured: arterial blood gases, inspiratory effort (ΔP ES), transpulmonary driving pressure (ΔP L), respiratory rate and esophageal pressure simplified pressure–time product per minute (sPTPES) by esophageal manometry, tidal volume (V T), end-expiratory lung impedance (EELI), lung compliance, airway resistance, time constant, dynamic strain (V T/EELI) and pendelluft extent through electrical impedance tomography. Results: Compared to supine position, prone position increased PaO2/FiO2 (median [Interquartile range] 104mmHg [76–129] vs. 74 [69–93], p < 0.001), reduced respiratory rate (24 breaths/min [22–26] vs. 27 [26–30], p = 0.05) and increased ΔP ES (12 cmH2O [11–13] vs. 9 [8–12], p = 0.04) with similar sPTPES (131 [75–154] cmH2Osmin−1 vs. 105 [81–129], p > 0.99) and ΔP L (9 [7–11] cmH2O vs. 8 [5–9], p = 0.17). Airway resistance and time constant were higher in prone vs. supine position (9 cmH2Osarbitrary units−3 [4–11] vs. 6 [4–9], p = 0.05; 0.53s [0.32–61] vs. 0.40 [0.37–0.44], p = 0.03). Prone position increased EELI (3887 arbitrary units [3414–8547] vs. 1456 [959–2420], p = 0.002) and promoted V T distribution towards dorsal lung regions without affecting V T size and lung compliance: this generated lower dynamic strain (0.21 [0.16–0.24] vs. 0.38 [0.30–0.49], p = 0.004). The magnitude of pendelluft phenomenon was not different between study phases (55% [7–57] of V T in prone vs. 31% [14–55] in supine position, p > 0.99). Conclusions: Prone position improves ...