Improved oxygenation in prone positioning of mechanically ventilated patients with COVID-19 acute respiratory distress syndrome is associated with decreased pulmonary shunt fraction: a prospective multicenter study

Background Prone position is used in acute respiratory distress syndrome and in coronavirus disease 2019 (Covid-19) acute respiratory distress syndrome (ARDS). However, physiological mechanisms remain unclear. The aim of this study was to determine whether improved oxygenation was related to pulmonary shunt fraction (Q’s/Q’t), alveolar dead space (Vd/Vtalv) and ventilation/perfusion mismatch (V’ A /Q’). Methods This was an international, prospective, observational, multicenter, cohort study, including six intensive care units in Sweden and Poland and 71 mechanically ventilated adult patients. Results Prone position increased PaO 2 :FiO 2 after 30 min, by 78% (83–148 mm Hg). The effect persisted 120 min after return to supine ( p < 0.001). The oxygenation index decreased 30 min after prone positioning by 43% (21–12 units). Q’s/Q’t decreased already after 30 min in the prone position by 17% (0.41–0.34). The effect persisted 120 min after return to supine ( p < 0.005). Q’s/Q’t and PaO 2 :FiO 2 were correlated both in prone (Beta -137) ( p < 0.001) and in the supine position (Beta -270) ( p < 0.001). V’ A /Q’ was unaffected and did not correlate to PaO 2 :FiO 2 ( p = 0.8). Vd/Vtalv increased at 120 min by 11% (0.55–0.61) ( p < 0.05) and did not correlate to PaO 2 :FiO 2 ( p = 0.3). The ventilatory ratio increased after 30 min in the prone position by 58% (1.9–3.0) ( p < 0.001). PaO 2 :FiO 2 at baseline predicted PaO 2 :FiO 2 at 30 min after proning (Beta 1.3) ( p < 0.001). Conclusions Improved oxygenation by prone positioning in COVID-19 ARDS patients was primarily associated with a decrease in pulmonary shunt fraction. Dead space remained high and the global V’ A /Q’ measure could not explain the differences in gas exchange.