Evolution of PaO₂/fraction of inspired oxygen (FIO₂; A), tidal volume (B), and positive end-expiratory pressure (PEEP; C) in the first week after acute respiratory distress syndrome (ARDS) onset according to the evolution group. (A) Evolution of PaO₂/FIO₂ and comparison between the improving (green), persisting (orange), and worsening (red) categories. On day 1, PaO₂/FIO₂ was significantly lower in the worsening group as compared to the improving group (P = 0.009) and the persisting group (P = 0.031). Each following day, PaO₂/Fio₂ was significantly different between each of the three groups with a gradual increase from the worsening to the persisting and to the improving group. As compared to day 1, PaO₂/FIO₂ on day 2 significantly decreased in the worsening group and then remained stable, whereas it increased in the improving and persisting groups. (B) Evolution of tidal volume and comparison between the improving (green), persisting (orange), and worsening (red) categories. Tidal volume was similar between groups at day 1 and remained similar each day of data collection. Within each group, tidal volume also remained similar to the initial one at each day of follow-up. (C) Evolution of PEEP and comparison between the improving (green), persisting (orange), and worsening (red) categories. On days 1, 2, and 3, PEEP was significantly higher in the worsening group as compared to the improving groups. Every day of data collection except on day 1, PEEP was higher in the worsening group as compared to the persisting groups. In the persisting groups, PEEP decreased from day 1 to day 2 and remained stable the following days. Detailed P values are shown in tables 2–4 in the Supplemental Digital Content (https://links.lww.com/ALN/B814). *P < 0.05 for overall comparison; †P < 0.05 for two-by-two comparison; ‡P < 0.05 versus day 1 for the same group; §P < 0.05 versus day 2 for the same group; ||P < 0.05 versus day 3 for the same group; #P < 0.05 versus day 5 for the same group. PBW, predicted body weight.