Effect of lung inflation and airway muscle tone on airway diameter
in vivo.
Brown, Robert H., Wayne Mitzner.
Department of Anesthesiology and Critical Care Medicine, Department
of Environmental Health Sciences/Division of Physiology, Department
of Radiology
APStracts 3:0033A, 1996.
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
the lung volume dependence 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 account for the
changes in lumenal area. In this study we measured the effect of lung
inflation on relaxed and contracted airways area using high
-resolution CT at four different transpulmonary pressures, each held
for 2 min. We found that fully relaxed airways are quite distensible
up to a pressure of 5 - 7 cm H2O (p<0.001), where they reach a
maximal size with no further distension up to airway pressures of 30
cm H2O (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 airway pressures of 20
cm H2O, 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 airway
pressures of 30 cm H2O. Thus a transient deep inspiration in vivo
would be expected to have only a small effect on contracted airways.
Received 26 June 1995; accepted in final form 20 December 1995.
APS Manuscript Number A689-5.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 25 January 96