Airway geometry models of children's lungs for use in dosimetry modeling

Single-path whole-lung and lobar models of the lungs of 11 children between 3 mo and 21 yr of age were developed based on a combination of cast data and published information on distal airway dimensions. The cast data used to generate these models were taken from one of the largest databases of actual measurements in children. The methods used to develop the children's models were based on techniques that have been used to develop adult single-path airway geometry models. Model dimensions for the conducting airways, as well as the estimated dead space, for all children fell within the range of the limited published information. Thus, the method for estimating airway dimensions in adults may be successfully applied to develop estimates of airway dimensions in children. The predicted total lung capacity (TLC) for the older children (aged 8 to 21 yr) fell within or near the range arising from published scaling equations. The assumptions used to generate the gas exchange region for children 8 yr and older produced results that were reasonably consistent with available physiological data. However, these assumptions do not result in a physiologically consistent gas exchange region for children 3 yr of age and younger; also, to maintain physiologically reasonable relationships between dead space and alveolar volume, the models for children 3 yr of age and younger resulted in predicted TLCs well below those predicted using published scaling equations. These discrepancies may be reflective of dysanaptic growth, in which the alveolar region is growing more rapidly than the airways. The results for children 3 yr of age and under suggest the need for a greater understanding of lung development during this critical period. This is particularly important considering the increasing evidence that exposure to pollutants and other toxicants and allergens during the first 2 yr of life may have long-term consequences on respiratory disease outcomes. Our results suggest that the geometry model airway dimensions for all ages are appropriate for use with dosimetry models, but dosimetry modelers need to assess carefully the reasonableness of TLC and functional residual capacity volumes to which airway dimensions are scaled for children 3 yr of age and under.