Physiological effects of awake prone position in acute hypoxemic respiratory failure

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.

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.

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).

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.