Growth of the Lung Parenchyma Early in Life

Rationale: Early in life, lung growth can occur by alveolarization, an increase in the number of alveoli, as well as expansion. We hypothesized that if lung growth early in life occurred primarily by alveolarization, then the ratio of pulmonary diffusion capacity of carbon monoxide (DLCO) to alveolar volume (VA) would remain constant; however, if lung growth occurred primarily by alveolar expansion, then DLCO/VA would decline with increasing age, as observed in older children and adolescents. Objectives: To evaluate the relationship between alveolar volume and pulmonary diffusion capacity early in life. Methods: In 50 sleeping infants and toddlers, with equal number of males and females between the ages of 3 and 23 months, we measured DLCO and VA using single breath-hold maneuvers at elevated lung volumes. Measurements and Main Results: DLCO and VA increased with increasing age and body length. Males had higher DLCO and VA when adjusted for age, but not when adjusted for length. DLCO increased with VA; there was no gender difference when DLCO was adjusted for VA. The ratio of DLCO/VA remained constant with age and body length. Conclusions: Our results suggest that surface area for diffusion increases proportionally with alveolar volume in the first 2 years of life. Larger DLCO and VA for males than females when adjusted for age, but not when adjusted for length, is primarily related to greater body length in boys. The constant ratio for DLCO/VA in infants and toddlers is consistent with lung growth in this age occurring primarily by the addition of alveoli rather than the expansion of alveoli.