Rate Constant for Forced Expiration Decreases with Lung Growth during Infancy

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Abstract
Airway caliber and lung volume (V L ) increase many fold between infancy and adulthood; however, these two components of the lung may not increase proportionately during lung growth and devel- opment. We evaluated in infants the rate of emptying during forced expiration from near total lung capacity to residual volume. From the flow-volume curves we calculated ( 1 ) a rate constant (k) as the change in flow divided by the change in volume between 50% and 75% of expired forced vital capac- ity (FVC), and ( 2 ) the fraction of the FVC expired in 0.5 s (FEV 0.5 /FVC). Seventeen normal healthy in- fants were evaluated twice; mean ages (ranges) at first and second tests were 30 (5 to 76) and 58 (28 to 98) wk. Analysis of cross-sectional and longitudinal data indicated that the rate of emptying dur- ing forced expiration measured by both parameters was greatest in the youngest infants and de- creased during infancy. Our findings are consistent with the concept that younger infants have large airways relative to their V L and that V L increases more rapidly than airway caliber early in life. Tepper RS, Jones M, Davis S, Kisling J, Castile R. Rate constant for forced expiration decreases with lung growth during infancy.