Changes of time constants during infancy and early childhood

We used respiratory inductance plethysmography to record tidal respiration in 27 healthy unsedated infants and children 1 mo to 8 yr of age during sleep. Rib cage and abdominal outputs were present at approximately equal gains and summed to obtain an estimate of volume. Flow-volume curves were generated from the uncalibrated volume signal and its flow derivative. Expiratory time constants (τ) were obtained by visually drawing a line through the linear portion of the expiratory flow-volume relationship. τ increased significantly during the first 10 mo of life. After 10 mo, the estimated rate of increase of τ for older children was <5% of the estimated initial rate and not significantly different from zero. Prolongation of τ was paralleled by an increase in expiratory time (TE), and no changes in TE/τ were observed in the first 2 yr of life. These changes in τ likely reflect the increase in lung compliance induced by rapid alveolar growth during infancy. After the first year, expiratory time constants appear to remain relatively constant and may be consistent with balanced changes in compliance and resistance beyond infancy.