| Characteristic* | Pre-ARDSNet period 1996–2000(n = 74) | Post-ARDSNet period 2000–2004(n = 74) | p Value |
| Pressure-regulated ventilation (%) | 16 (22) | 29 (39) | 0.020 |
| Mean (SD) Vt† (ml/kg) | 9.1 (2.0) | 6.6 (1.5) | <0.001 |
| Mean (SD) RR† (breaths/min) | 22 (5.8) | 25 (6.2) | <0.001 |
| Median (IQR) Ve† (l/min) | 14 (11–17) | 11 (10–14) | 0.002 |
| Mean (SD) PIP† (cm H2O) | 36 (8.2) | 35 (7.5) | 0.37 |
| Median (IQR) Pplat† (cm H2O) | 29 (23–31) | 26 (23–29) | 0.082 |
| Median (IQR) Paw† (cm H2O) | 15 (12–18) | 17 (13–19) | 0.098 |
| Median (IQR) PEEP† (cm H2O) | 7.0 (5.0–9.3) | 8.3 (5.0–10) | 0.037 |
| Mean (SD) Crs† (ml/cm H2O) | 36 (11) | 28 (11) | <0.001 |
| Median (IQR) Pao2/Fio2† (mm Hg) | 144 (111–193) | 137 (103–179) | 0.49 |
| Median (IQR) worst Pao2/Fio2† (mm Hg) | 125 (84–174) | 120 (90–159) | 0.88 |
| Median (IQR) O2 index† (mm Hg/cm H2O) | 10 (6.5–16) | 12 (8.8–16) | 0.20 |
*Up to 11 observations missing, as for Vt and PIP. †Physiological variables averaged over days 1–3. ALI, acute lung injury; ARDSNet, Acute Respiratory Distress Syndrome Network study; Crs, respiratory system compliance; Fio2, fractional inspired oxygen concentration; ICU, intensive care unit; IQR, interquartile range; O2, oxygenation; Pao2, arterial oxygen tension; PIP, peak inspiratory pressure; Pplat, end-inspiratory plateau pressure; Paw, mean airway pressure; PEEP, positive end-expiratory pressure; RR, respiratory rate; SD, standard deviation; Ve, minute ventilation; Vt, tidal volume.