First The effects of lung volume recruitment manouvres on pulmonary blood flow (PBF) during high-frequency oscillatory ventilation (HFOV) in preterm neonates are unknown. Since increased airway pressure adversely affects PBF, we compared the effects of two HFOV recruitment strategies on PBF and oxygenation index (OI). Preterm lambs (128 +/- 1 day gestation; term similar to 150 days) were anesthetized and ventilated using HFOV (10 Hz, 33% t(I)) with a mean airway pressure (Pao) of 15 cmH(2)O. Lung volume was recruited by either increasing Pao to 25 cmH(2)O for 1 min, repeated five times at 5-min intervals incrementing up to 30 cmH(2)O then decrementing back to 15 cmH(2)O ( Ramp group; n = 6). Controls (n = 5) received constant HFOV at 15 cmH2O. PBF progressively decreased (by 45 +/- 4%) and OI increased (by 15 +/- 6%, indicating reduced oxygenation) in controls during HFOV, which was similar to the changes observed in the Sigh group of lambs. In the Ramp group, PBF fell (by 54 +/- 10%) as airway pressure increased (r(2) = 0.99), although the PBF did not increase again as the Pao was subsequently reduced. The OI decreased (by 47 +/- 9%), reflecting improved oxygenation at high Pao levels during HFOV in the Ramp group. However, high Pao restored retrograde PBF during diastole in four of six lambs, indicating the restoration of right-to-left shunting through the ductus arteriosus. Thus the choice of volume recruitment maneuvre influences the magnitude of change in OI and PBF that occurs during HFOV. Despite significantly improving OI, the ramp recruitment approach causes sustained changes in PBF.