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.

Fig. 1

Tracings: comparisons between supine (in blue, left panel) and prone position (in red, right panel) for tidal volume (solid line), flow (dotted line), esophageal pressure. In the two top rows, average tidal impedance variation (TIV), flow and esophageal pressure are displayed. Average breaths from all patients were synchronized and interpolated. The resulting mean values (thick lines) and standard variation (shading) are displayed. Figures: end-expiratory lung volume increased and was dorsally shifted in the prone position. At the bottom, comparisons between the regional distribution of dynamic strain in the supine (left panel) and prone position (right panel). Pixel with a dynamic strain > 2 are displayed in red. These values represent the average values from the whole cohort

Fig. 2

Individual patient values and medians of PaO₂/FiO₂, PaCO₂, respiratory rate, and VAS-measured patient dyspnea and discomfort during the three phases of the study

Fig. 3

Individual patient values and medians of esophageal pressure inspiratory swings (ΔP_ES), simplified pressure‒time product of the esophageal pressure per minute (PTP_ES), quasi-static transpulmonary pressure (ΔP_L), end-expiratory transpulmonary pressure, time constant and airway resistance during the three phases of the study

Fig. 4

Individual patient values and medians of tidal impedance variation, end-expiratory lung impedance (EELI), standardized minute ventilation, lung compliance, dynamic strain and Pendelluft extent during the three phases of the study

Fig. 5

Tidal volume distribution (expressed in % of global tidal volume) in supine and prone position. Results are expressed as means (standard deviation). Prone position promoted tidal volume distribution towards dorsal, dependent lung regions