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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 PaO 2 /FiO 2  < 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 PaO 2 /FiO 2 (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 cmH 2 O [11-13] vs. 9 [8-12], p = 0.04) with similar sPTP ES (131 [75-154] cmH 2 O s min -1 vs. 105 [81-129], p > 0.99) and ΔP L (9 [7-11] cmH 2 O vs. 8 [5-9], p = 0.17). Airway resistance and time constant were higher in prone vs. supine position (9 cmH 2 O 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., (© 2023. BioMed Central Ltd., part of Springer Nature.)

Background: Whether respiratory physiology of COVID-19-induced respiratory failure is different from acute respiratory distress syndrome (ARDS) of other etiologies is unclear. We conducted a single-center study to describe respiratory mechanics and response to positive end-expiratory pressure (PEEP) in COVID-19 ARDS and to compare COVID-19 patients to matched-control subjects with ARDS from other causes., Methods: Thirty consecutive COVID-19 patients admitted to an intensive care unit in Rome, Italy, and fulfilling moderate-to-severe ARDS criteria were enrolled within 24 h from endotracheal intubation. Gas exchange, respiratory mechanics, and ventilatory ratio were measured at PEEP of 15 and 5 cmH 2 O. A single-breath derecruitment maneuver was performed to assess recruitability. After 1:1 matching based on PaO 2 /FiO 2 , FiO 2 , PEEP, and tidal volume, COVID-19 patients were compared to subjects affected by ARDS of other etiologies who underwent the same procedures in a previous study., Results: Thirty COVID-19 patients were successfully matched with 30 ARDS from other etiologies. At low PEEP, median [25th-75th percentiles] PaO 2 /FiO 2 in the two groups was 119 mmHg [101-142] and 116 mmHg [87-154]. Average compliance (41 ml/cmH 2 O [32-52] vs. 36 ml/cmH 2 O [27-42], p = 0.045) and ventilatory ratio (2.1 [1.7-2.3] vs. 1.6 [1.4-2.1], p = 0.032) were slightly higher in COVID-19 patients. Inter-individual variability (ratio of standard deviation to mean) of compliance was 36% in COVID-19 patients and 31% in other ARDS. In COVID-19 patients, PaO 2 /FiO 2 was linearly correlated with respiratory system compliance (r = 0.52 p = 0.003). High PEEP improved PaO 2 /FiO 2 in both cohorts, but more remarkably in COVID-19 patients (p = 0.005). Recruitability was not different between cohorts (p = 0.39) and was highly inter-individually variable (72% in COVID-19 patients and 64% in ARDS from other causes). In COVID-19 patients, recruitability was independent from oxygenation and respiratory mechanics changes due to PEEP., Conclusions: Early after establishment of mechanical ventilation, COVID-19 patients follow ARDS physiology, with compliance reduction related to the degree of hypoxemia, and inter-individually variable respiratory mechanics and recruitability. Physiological differences between ARDS from COVID-19 and other causes appear small.