Airway branching morphology of mature and immature rabbit lungs

The scheme of Horsfield et al. for describing the pulmonary airway tree (J Appl Physiol 52: 21-26, 1982) catalogs each airway according to its order a nd the difference in order of its two daughters (denoted Delta). Although t his scheme captures the natural asymmetry in the airway tree, it is still d eterministic, because it assumes that all airways of a given order are the same; yet such variability is extremely important in determining the overal l behavior of the lungs. We therefore analyzed complete lung lobes from thr ee mature and two immature rabbits and determined the Horsfield order and D elta of every airway down to the terminal bronchioles. We also measured the diameter of each airway. This allowed us to determine the average structur e of the rabbit airway tree, the variation about this average, and also how the structures of mature and immature airway trees compare. We found some variation in branching asymmetry and airway diameter at a given order betwe en animals but no evidence of systematic differences in structure between m ature and immature lungs. We found evidence of a difference in the branchin g structure of the peripheral vs. the central part of the airway tree (the break point being around order 20). We also determined the nature of the va riation in Delta and diameter as a function of order, which should be valua ble for the development of computer models seeking to encapsulate the natur ally occurring regional variation in airway geometry in the normal rabbit l ung.