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Evolution and variability of pulmonary mechanics during postnatal transition in term infants.

This study was designed to assess whether progressive changes related to the transition to extrauterine life explain in part the interassay variability of pulmonary mechanics in the immediate postnatal period. We performed sequential pulmonary function tests in 21 normal term infants with the use of a pneumotachometer and an esophageal balloon. The average percent increase in values from the first period (2 to 10 hours) to the last period of study (66 to 78 hours) was 27% for tidal volume, 24% for minute ventilation, and 42% for dynamic compliance. The maximum difference between two pulmonary function measurements obtained within 30 minutes of each other ranged between 10% for dynamic compliance and 27% for minute ventilation. The coefficient of variation for dynamic compliance over a 2-hour period (12%) was less than half the 3-day coefficient of variation (31%). The reliability coefficient ranged between 0.72 for tidal volume and 0.92 for dynamic compliance. We conclude that dynamic compliance in term infants increases progressively during the first 3 days of life, whereas tidal volume and minute ventilation increase mostly during the first day. The variability of pulmonary mechanics reflects in part those progressive physiologic changes during postnatal transition.