Background <p>Prone positioning (PP) has been shown to improve oxygenation in patients with acute respiratory distress syndrome (ARDS); with a focus on its early physiological effects. However, the time-dependent effects of PP on ventilation-perfusion (V/Q) matching have not been fully investigated. In this study we aimed to investigate the longitudinal effects of PP on regional V/Q matching and the distribution of ventilation and perfusion in patients with coronavirus disease 2019 (COVID-19)-associated ARDS.</p> Methods <p>This study analyzed patients with COVID-19 ARDS who were mechanically ventilated and underwent their first PP treatment. V/Q mismatching was assessed using electrical impedance tomography (EIT). At five intervals during the initial PP session PaO₂/FiO₂ measurements and EIT evaluations were performed including: before the initiation of PP while in the supine position (SP), 1&#xa0;h after PP (PP₁), 3&#xa0;h after PP (PP₃), 16&#xa0;h after PP (PP_end), and 3&#xa0;h after reverting to the supine position (RE-SP₃).</p> Results <p>In this study eighteen COVID-19 ARDS patients were enrolled. In comparison with SP, PP led to significant improvements in oxygenation, with PaO₂/FiO₂ consistently increasing at each PP time point and peaking at PP_end. Dorsal ventilation significantly increased at PP₁ (<i>P</i> = .047), and steadily rose during PP, with a higher increase at PP_end than PP₁ (<i>P</i> &lt; .001). Dorsal perfusion remained unchanged during the first three hours of PP; however, significantly increased by PP_end. Ventilation and perfusion returned to their baseline levels at RE-SP₃. PP increased normal V/Q (%), and decreased non-perfused (%), low V/Q (%), particularly in the dorsal lung regions, compared with SP. At RE-SP₃, there was a marked increase in the non-ventilated (%), low V/Q (%), and non-perfused (%) compared with PP. The global inhomogeneity (GI)-V/Q ratio was noted to have decreased during PP and correlated with an increase in PaO₂/FiO₂.</p> Conclusions <p>In COVID-19-induced ARDS patients, prone positioning initially improves oxygenation and V/Q matching by enhancing ventilation distribution and decreasing low V/Q (%). Over time, perfusion changes further improve V/Q matching, but these benefits diminish once the patient returns to the supine position, leading to increased V/Q mismatch.</p> <p><i>Trial registration</i> Clinical Trials.gov, NCT04725227. Registered 25 January 2021, <a href="https://clinicaltrials.gov/study/NCT04725227?cond=NCT04725227&amp;rank=1">https://clinicaltrials.gov/study/NCT04725227?cond=NCT04725227&amp;rank=1</a></p>