Background <p>Prone positioning (PP) and inhaled nitric oxide (iNO) are widely utilized rescue therapies for severe hypoxemia during invasive mechanical ventilation. However, their combined effects on regional ventilation–perfusion (V/Q) matching remain incompletely characterized. This study utilized single-plane saline-contrast electrical impedance tomography (EIT) to achieve real-time bedside visualization and evaluate the relative spatial mechanisms of these interventions in patients with COVID-19-related ARDS.</p> Methods <p>We conducted a prospective sequential physiological study in mechanically ventilated patients with moderate-to-severe COVID-19-related acute respiratory distress syndrome (ARDS). Relative regional ventilation, perfusion, and V/Q matching distributions were monitored across four predefined, fixed sequential conditions: supine position ventilation (SPV), SPV with iNO (20&#xa0;ppm for 1&#xa0;h; SPV + iNO), prone position ventilation after stabilization (PPV), and PPV with an additional 1&#xa0;h of iNO (PPV + iNO). Primary analyses focused on prespecified within-patient contrasts, supplemented by exploratory difference-in-differences (DiD) analyses to evaluate position-dependent vascular responses.</p> Results <p>Twenty-eight patients completed all study phases. Hemodynamics and conventional respiratory mechanics remained remarkably stable across all four conditions. Systemic oxygenation improved progressively; both SPV + iNO and PPV significantly increased PaO₂ and PaO₂/FiO₂ compared to SPV alone, while the addition of iNO during PPV (PPV + iNO) yielded further oxygenation improvements and reduced FiO₂ requirements. Within the EIT-assessed thoracic slice, SPV + iNO produced the most widespread V/Q improvements, significantly reducing the global non-perfused fraction (−7.28%), wasted ventilation (−8.14%), wasted perfusion (−5.89%), and the global inhomogeneity of perfusion (GIper). Transitioning to PPV similarly reduced the global non-perfused fraction (−8.76%) and GIper, while ventral-to-dorsal ventilation distribution remained stable. Notably, while PPV + iNO improved systemic oxygenation, it did not produce additional significant reductions in global EIT V/Q metrics, and locally increased the dorsal low V/Q compartment (+ 4.01%). DiD analyses revealed that while body position did not significantly modify the acute systemic oxygenation response to iNO, it significantly altered EIT-derived V/Q responses, indicating an attenuated capacity for iNO to improve regional V/Q matching in the imaged slice when administered in the prone position.</p> Conclusions <p>Both prone positioning and iNO improved oxygenation in moderate-to-severe COVID-19-related ARDS. Within the EIT-assessed thoracic slice, these improvements were accompanied by better regional V/Q matching, which was mainly perfusion-related rather than ventilation-related and suggests limited recruitability of the imaged region; the effect of iNO on regional V/Q matching was attenuated in the prone position. As EIT samples a single thoracic cross-section, these are localized findings and should not be generalized to the whole lung.</p> <p><i>Trial registration</i> ClinicalTrials.gov, NCT05715762. Registered 08 February 2023.</p>