Compliance of peripheral airways deduced from morphometry

Insights into airway mechanics were sought by applying morphometric techniq ues to rabbit lungs fixed at several lung recoil pressures. Rabbits were tr eated with either nebulized carbachol followed by iv administration of carb achol or with saline solution (sham). The lungs were held at one of six val ues of positive end-expiratory pressure (PEEP; 10, 7, 4, 2, 0, and -4 cmH(2 )O) while the animal was killed and formalin was circulated through the lun gs. The lungs were removed and left in a bath of formalin for 24 h. Standar d airway morphometric measurements were made an membranous bronchiole slice s taken from representative blocks of tissue. Reductions in PEEP produced t he expected reductions in lumen area in the carbachol-treated airways but n ot in the sham-treated airways for PEEP > 2 cmH(2)O. Sham-treated airways r emained more open than expected until they collapsed into an oval shape at PEEPs between 4 and 2 cmH(2)O. The carbachol-treated airways exhibited this behavior at PEEP = -4 cmH(2)O. The smallest airways, which had relatively thicker walls, collapsed less than larger airways. We postulate that this b ehavior implies that peribronchial stress is greater than lumen pressure on collapse into the oval shape. Resistance to buckling increases with the th ickness-to-radius ratio of the airway wall, which explains why the smallest airways are the most open. The development of epithelial folds appeared to follow the theoretical prediction of a previous study (Lambert RK, Codd SL , Alley MR, and Pack RJ. J Appl Physiol 77: 1206-1216, 1994).