BACKGROUND:Hemodynamic response to prone position (PP) has never been studied in a large series of patients with acute respiratory distress syndrome (ARDS). The primary aim of this study was to estimate the rate of PP sessions associated with cardiac index improvement. Secondary objective was to describe hemodynamic response to PP and during the shift from PP to supine position. METHODS:The study was a single-center retrospective observational study, performed on ARDS patients, undergoing at least one PP session under monitoring by transpulmonary thermodilution. PP sessions performed more than 10 days after ARDS onset, or with any missing cardiac index measurements before (T1), at the end (T3), and after the PP session (T4) were excluded. Changes in hemodynamic parameters during PP were tested after statistical adjustment for volume of fluid challenges, vasopressor and dobutamine dose at each time point to take into account therapeutic changes during PP sessions. RESULTS:In total, 107 patients fulfilled the inclusion criteria, totalizing 197 PP sessions. Changes in cardiac index between T1 and T2 (early response to PP) and between T1 and T3 (late response to PP) were significantly correlated (R2 = 0.42, p < 0.001) with a concordance rate amounting to 85%. Cardiac index increased significantly between T1 and T3 in 49 sessions (25% [95% confidence interval (CI95%) 18-32%]), decreased significantly in 46 (23% [CI95% 16-31%]), and remained stable in 102 (52% [CI95% 45-59%]). Global end-diastolic volume index (GEDVI) increased slightly but significantly from 719 ± 193 mL m-2 at T1 to 757 ± 209 mL m-2 at T3 and returned to baseline values at T4. Cardiac index and oxygen delivery decreased slightly but significantly from T3 to T4, without detectable increase in lactate level. Patients who increased their cardiac index during PP had significantly lower CI, GEDVI, global ejection fraction at T1, and received significantly more fluids than patients who did not. CONCLUSION:PP is associated with an increase in cardiac index in 18% to 32% of all PP sessions and a sustained increase in GEDVI reversible after return to supine position. Return from prone to supine position is associated with a slight hemodynamic impairment.