Physiological effects of awake prone position in acute hypoxemic respiratory failure

Abstract

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 < 200 mmHg underwent high-flow nasal oxygen for 1 h in supine position and 2 h 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 (sPTP_ES) 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] 104 mmHg [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 sPTP_ES (131 [75-154] cmH₂O s min^-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 cmH₂O s arbitrary units^-3 [4-11] vs. 6 [4-9], p = 0.05; 0.53 s [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 oxygenation, increases EELI and promotes V_T distribution towards dependent lung regions without affecting V_T size, ΔP_L, lung compliance and pendelluft magnitude. Prone position reduces respiratory rate and increases ΔP_ES because of positional increases in airway resistance and prolonged expiratory time. Because high ΔP_ES is the main mechanistic determinant of self-inflicted lung injury, caution may be needed in using awake prone position in patients exhibiting intense ΔP_ES. Clinical trail registeration: The study was registered on clinicaltrials.gov (NCT03095300) on March 29, 2017.

Keywords: Acute respiratory failure; Awake prone position; High-flow nasal oxygen; Inspiratory effort; Patient self-inflicted lung injury.