Effect of lung inflation and airway muscle tone on airway diameter in vivo

How normal airway dimensions change with lung volume is of great importance in determining flow limitation during the normal forced vital capacity maneuver as well as in the manifestation of obstructive lung disease. The literature presents a confusing picture, with some results suggesting that airway diameter increases linearly with the cube root of lung volume and others showing a highly nonlinear relation. The effect of smooth muscle contraction on lung-airway interdependence is even less well understood. Recent morphological work explicitly assumes that airway basement membrane is nondistensible, although the lung volume at which this maximal airway size is reached is unknown. With smooth muscle contraction, folding of the epithelium and basement membrane accounts for the changes in luminal area. In this study, we measured the effect of lung inflation on relaxed and contracted airway areas by using high-resolution computed tomography at different transpulmonary pressures, each held for 2 min. We found that fully relaxed airways are quite distensible up to a pressure of 5-7 cmH₂O (P < 0.001), where they reach a maximal size with no further distension up to an airway pressure of 30 cmH₂O (P = 0.49). Thus relaxed airways clearly do not expand isotropically with the lung. With smooth muscle tone, the airways in different animals responded differently to lung inflation, with some animals showing minimal airway dilation up to an airway pressure of 20 cmH₂O and others showing airways that were more easily dilated with lung expansion. However, maximal diameter of these moderately constricted airways was not usually achieved even up to an airway pressure of 30 cmH₂O. Thus a transient deep inspiration in vivo would be expected to have only a small effect on contracted airways.