Rate constant for forced expiration decreases with lung growth during infancy

Airway caliber and lung volume (VL) increase many fold between infancy and adulthood; however, these two components of the lung may not increase propo rtionately during lung growth and development. 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 const ant (k) as the change in flow divided by the change in volume between 50% a nd 75% of expired forced vital capacity (FVC), and (2) the fraction of the FVC expired in 0.5 s (FEV0.5/FVC). Seventeen normal healthy infants were ev aluated twice; mean ages (ranges) at first and second tests were 30 (5 to 7 6) and 58 (28 to 98) wk. Analysis of cross-sectional and longitudinal data indicated that the rate of emptying during forced expiration measured by bo th parameters was greatest in the youngest infants and decreased during inf ancy. Our findings are consistent with the concept that younger infants hav e large airways relative to their VL and that VL increases more rapidly tha n airway caliber early in life.