Specific airway conductance in relation to postconceptional age during infancy.

Thoracic gas volume (TGV) and airway conductance (Gaw) were measured in 69 healthy infants during the 1st yr of life, using an adaptation of the whole body plethysmographic technique in which a heated rebreathing system was used to eliminate temperature differences in respired gas. There was highly significant correlations between TGV and body wt (r=0.99), and between Gaw and TGV (r=0.97) in all the infants. However, the relationship between Gaw and TGV (i.e., specific conductance (SGaw)) was dependent on postconceptional age (PCA) in preterm infants, falling from 0.50 s-1-cmH2O-1 at 31 wk to 0.32 s-1-cmH2O-1 at 40 wk PCA. Gestational age and postnatal age per se did not affect this relationship, showing it to be a maturational event unaffected by the time of birth. After 40 wk, the decrease in SGaw was far more gradual throughout the 1st yr of life. Black and Chinese infants were found to have significantly higher values for SGaw than their White counterparts, which may be due to anatomical differences in nasal structure.