The study compared the outcome of infants received iNO at OI >15 and <25, with a control group that received a simulated early procedure with iNO actually given based on the standard recommendation. iNO was delivered at 20 ppm during the initial dose-response evaluation. Infants in either group who showed subsequent deterioration with OI >25 on two consecutive measurements at least one hour apart, or a rapid deterioration with OI >30 on two consecutive measurements 15 minutes apart, received iNO therapy as part of standard medical management. Specific guidelines were followed for the use of high frequency ventilation and surfactant during study gas administration to prevent them from confounding the results of the study.