Structure-function of airway generations 0 to 4 in the preterm lamb

Five generations of airways from 15 preterm lambs of 130-137 d (90% term) gestation were studied to investigate the effect of generation on structure-function of preterm airways. Airway rings were measured to determine the internal radius (r), and wall thickness (t). The ratio r/t was then calculated as a morphometric index used in the determination of wall stress. Airway rings from each generation were placed in tissue baths to compare passive, active, and total force development. Contraction via membrane depolarization (KCl) and muscarinic receptor stimulation (acetylcholine) were evaluated. As r and t decreased, r/t declined by a factor of 3.48 down the generations. At the optimal length for active force development, the passive, active, and total stresses decreased significantly as a function of generation. The receptor-mediated response to acetylcholine was significantly less in generations 0, 1, and 2 than in generations 3 and 4. No differences were found among the various generations in contractility as measured by the response to KCl. These data suggest that based on the interrelationship between airway morphometry and force development the trachea is exposed to greater wall stress than the lower airways during continuous positive airway pressure. Taken together, these data may help to explain the structural changes, such as tracheomegaly, as well as the physiologic changes in airway reactivity seen in the premature infant after mechanical ventilation.