Although HFV had not been evaluated either with regard to efficacy or safety and although results of fundamental studies had not provided a good understanding of how gas exchange occurred during HFV, there was considerable interest in introducing this type of ventilatory support in neonatal intensive care. HFV involves the use of small tidal volumes, delivered at respiratory frequencies ranging from 1 to 40 Hz with the aid of, for example, a piston pump or a high speed jet of gas. Compared to conventional mechanical ventilation, HFV offers several potential advantages, including reduced intrapulmonary pressure swings and fluctuation in alveolar pressures and the possibility of lowered levels of inspired oxygen. At that time, theories suggested that HFV produced a pattern of flow that enhanced gas mixing and 'homogenized' the distribution of ventilation. Experimental observations in adult animals (cats, dogs and rabbits) or healthy newborn lambs had shown HFV to be effective in promoting gas exchange without apparent adverse effects. Studies in prematurely delivered subhuman primates, that develop RDS and subsequently bronchopulmonary dysplasia indistinguishable from that of human infants, supported the notion the HFV could provide better oxygenation and lower C02 levels than conventional mechanical ventilation at similar mean airway pressure. The HIFI trial provided badly needed controlled data on the safety and efficacy of HFV in premature infants.