Transit-time analysis of the forced spirogram in healthy children and adults.

Mean transit time (MTT), coefficient of variation (COV), and index of skewness (IOS) of transit times were measured, with the more conventional derivatives of the forced expiratory spirogram, in 178 healthy people aged 6-71 yr breathing air, and in 18 healthy adults breathing 20% oxygen in helium (O2He) in addition. Breathing O2He very significantly altered MTT and all derivatives with dimensions of flow or time, but altered COV and IOS very little. Between 6 and 30 yr mean MTT, after adjustment for expired volume, remained constant and then increased to 71 yr. Mean COV and IOS increased from 6 to 71 yr, though with an apparent change in rate during the early teens. The results suggest that whereas the resistance of large airways falls in proportion to growth of the lungs, small-airway resistance becomes less uniform during both childhood and adult life.