Twenty infants, mechanically ventilated in the neonatal period for respiratory distress syndrome, were compared with 15 healthy controls, matched for birthweight(less than 1501 g) but greater in mean gestational age. Clinical features and lung mechanics (by whole body plethysmography) were recorded at 6-monthly intervals until about one year. THe neonatal course of the mechanically ventilated infants was commonly complicated by tracheobronchial hypersecretion and the later course by a fairly high incidence of lower respiratory tract illness. In this group, thoracic gas volume, dynamic compliance, pulmonary and airways conductance were all abnormal during the middle 4 months of the first year and reverted towards normal towards the end of the first year. The control group had normal lung mechanics. Early lung function tests were of limited value in predicting later lower respiratory tract illness, which was more common in boys, after neonatal mechanical ventilation for longer than 24 hours or raised ambient oxygen for longer than 5 days. There were few predictive physical signs. In this group of very low birthweight infants, respiratory distress syndrome of sufficient severity to require mechanical ventilation led to significant physiological and clinical disturbances of lung function which lasted into the second 6 months of life and which were particularly severe in those who had recurrent lower respiratory tract illness.
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