RESUMO
Sequential lung function was measured in 12 very low-birth weight infants (less than or equal to 1,250 g) within 14 hours of birth, and at daily intervals thereafter for the first week of life, using an esophageal balloon and pneumotachograph system. All infants were clinically free of respiratory distress syndrome and radiographically showed no evidence of atelectasis or pulmonary edema. The alveolar-arterial oxygen tension gradient was high at birth and remained elevated over the period during which arterial blood gases were monitored. Increases of lung compliance and tidal volume between the first day and the end of the first week of life were not significant. Day-to-day determinations of lung compliance revealed an individual and group variability without a definite pattern. Lung resistance measurements indicated no clear trend for the group as a whole, but inspiratory resistance was generally lower than expiratory resistance. Possible causes, in addition to technical factors, that may account for the variability in the pulmonary mechanics of these small infants include an instability of lung volume and uneven distribution of pleural pressure due to chest wall distortion, differences in sleep-state, and alteration in the distribution of body fluids, resulting in a change in lung water. Any or all of these mechanisms may result in an unstable lung, even in an apparently clinically stable very low-birth weight infant.