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Aim: Inhaled nitric oxide (iNO) is used to reduce right-to-left extrapulmonary shunting by decreasing pulmonary vascular resistance in term or near-term infants. The objectives of this study were to determine, first, the pulmonary blood flow status of very preterm infants with hypoxaemic respiratory failure, then the response of oxygenation to iNO therapy according to pulmonary blood flow (PBF) and, finally, to verify the lack of adverse side effects of iNO on the ductus arteriosus. Methods: Infants below 32 wk gestational age (GA) with hypoxic respiratory failure and aAO 2 < 0.22 were randomized as the control or iNO group. PBF was evaluated by pulsed Doppler measurement of mean pulmonary blood flow velocity (MPBFV) in the left pulmonary artery. Low PBF (LPBF) was defined as MPBFV <0.2 m/s. Results: Seventy infants of 23 to 31 wk GA with hypoxic respiratory failure were randomized either to receive or not to receive 5 ppm iNO in addition to optimal care. Twenty-eight infants were diagnosed with LPBF (11/35 in iNO vs 17/35 in the control groups). Thirty minutes after receiving iNO the number of LPBF infants dropped to 8/35. In the iNO group, aAO 2 increased significantly from 0.14 ± 0.05 to 0.24 ± 0.08 after iNO, but only in the LPBF infants (mean ± SD; p = 0.027). Conclusion: In infants below 32 wk GA with hypoxic respiratory failure, Doppler echocardiographic assessment of LPBF seems to be able to determine which patients are likely to benefit from iNO therapy on systemic oxygenation. [ABSTRACT FROM AUTHOR]