The oxygenation index (OI) before and during high-frequency oscillatory ventilation (HFOV) in patients with diffuse alveolar disease (DAD) (●) and in patients with small airway disease (SAD) (■). The OI became significantly higher 6 hours prior to HFOV therapy and remained higher. The observed rise in the OI in the first hour after transition to HFOV in both groups is due to the applied higher CDP when compared with the mean airway pressure during conventional mechanical ventilation. The SAD patients had a higher, but not significant, PaCO2 before transition to HFOV than the DAD patients. PaCO2 returned to normal values after transition to HFOV. * P < 0.05.