Influence of Prone Position on Regional Ventilation/Perfusion Matching in Patients With ARDS Over Time

<b>Background:</b> We sought to investigate the short- and long-term effects of prone positioning (PP) on ventilation/perfusion matching in patients with ARDS using contrast-enhanced electrical impedance tomography (EIT). <b>Methods:</b> EIT measurements were performed in 18 mechanically ventilated subjects with ARDS before PP (supine position [SP]), 1 h after turning subjects to PP (PP₁), 3 h after PP (PP₃), 9 h after (PP₉), 16 h after PP (PP₁₆; the end of PP), and 3 h after returning to the supine position (Re-SP₃). <b>Results:</b> The <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mrow><mml:msub><mml:mi>P</mml:mi><mml:mrow><mml:mi>a</mml:mi><mml:mrow><mml:msub><mml:mi>O</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:mrow></mml:msub></mml:mrow></mml:mrow></mml:math>/<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mrow><mml:msub><mml:mi>F</mml:mi><mml:mrow><mml:mi>I</mml:mi><mml:mrow><mml:msub><mml:mi>O</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:mrow></mml:msub></mml:mrow></mml:mrow></mml:math> increased gradually during the PP period (110.68 vs 158.44 vs 210.15 vs 215.22 vs 236.04 vs 163.77 mm Hg, mean values at SP, PP₁, PP₃, PP₉, PP₁₆, and Re-SP₃, respectively<i>, P</i> < .001). Global ventilation/perfusion matched percent significantly increased within PP duration (54.13% vs 63.15% vs 63.02% vs 63.75% vs 66.63% vs 57.42, <i>P</i> < .005). Compared with SP, dorsal ventilation significantly increased at PP₁ (<i>P</i> < .001) and increased gradually during PP. However, the dorsal flow commenced to improve at PP₉ and persisted in enhancement until PP₁₆ (40.61% vs 48.78% vs 50.56%, mean values at PP₃, PP₉, and PP₁₆, respectively<i>, P</i> < .05). There was a significant reduction in global Shunt-EIT percentage within PP duration, primarily localized in the dorsal area. Dead Space-EIT percentage remained unchanged during PP. <b>Conclusions:</b> Oxygenation remained improved or maintained throughout the 16-h duration of PP. Ventilation is susceptible to immediate gravitational effects; however, changes in blood flow may occur later after 9 h, which supports prolonged PP treatment. The shunt continuously decreases, but no significant changes were observed for dead space. <b>Trial registration:</b> ClinicalTrials.gov, NCT04725227. Registered on January 25, 2021.