Physiologic effects of prone positioning on gas exchange and ventilation-perfusion matching in awake patients with AHRF

Background Prone positioning (PP) improves oxygenation in awake patients with acute hypoxemic respiratory failure (AHRF). However, the underlying mechanisms remain unclear in patients with diverse lung morphology. We aimed to determine the short-term effects of awake prone positioning (APP) in AHRF patients with focal and non-focal lung morphology. Methods This is a prospective physiological study. Twenty-four non-intubated patients with PaO2/FiO(2) <= 300 mm Hg were included. Gas exchange, ventilation and perfusion distribution, and hemodynamics variables were recorded in the supine position (SP1), 2 h after PP, and 1 h after re-supine (SP2). Lung morphology was classified as focal and non-focal patterns using computed tomography. Results Twelve of the included patients were classified to the focal group and 12 to the non-focal group. PaO2/FiO(2) improved after PP in all patients (161 [137, 227] mmHg vs. 236 [202, 275] mmHg, p < 0.001). Ventilation-perfusion (V/Q) matching increased after PP in all patients (61.9 [53.9, 66.5] vs. 77.5 [68.3, 80.0], p < 0.001). Shunt exhibited a significant decrease in patients of the non-focal group (28.6 [22.5, 30.3] vs. 11.3 [9.0, 14.5], p < 0.001), whereas no difference was found in the focal group after PP. Dead space decreased significantly in patients of the focal group (25.6 [21.5, 28.4] vs. 12.0 [10.8, 14.1], p < 0.001), whereas no difference was found in the non-focal group after PP. Conclusions APP improves V/Q matching, and large-scale, bias-free studies are needed to find more definitive differences between patients with focal and non-focal lung morphyology. Trial registrationThe study is registered in ClinicalTrials.gov (trial No. NCT04754113, date of registration: 2021-02-15).