Differential effect of recruitment maneuvres on pulmonary blood flow and oxygenation during HFOV in preterm lambs

The effects of lung volume recruitment manoeuvres on pulmonary blood flow (PBF) during high-frequency oscillatory ventilation (HFOV) in preterm neonates are unknown. As increased airway pressure adversely affects PBF, we compared the effects of two HFOV recruitment strategies on PBF and oxygenation index (OI). Preterm lambs (128+/-1d gestation; term 150d) were anesthetized and ventilated using HFOV (10Hz, 33 tI) with a mean airway pressure (Pao) of 15cmH2O. Lung volume was recruited by either increasing Pao to 25cmH2O for 1 min, repeated 5 times at 5 min intervals (Sigh group; n=5) or stepwise (5cmH2O) changes in Pao at 5 min intervals incrementing up to 30cmH2O then decrementing back to 15 cmH2O (Ramp group; n=6). Controls (n=5) received constant HFOV at 15cmH2O. 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 4 of 6 lambs, indicating the restoration of right-to-left shunting through the ductus arteriosus. Thus, the choice of volume recruitment manoeuvre 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. Key words: Preterm, lung volume recruitment, waveform analysis, pulmonary blood flow.