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1 Department of Respirology and Allergology, Fujita Health University, Toyoake, Japan 470-1192; 2 UBC Pulmonary Research Laboratory, St Paul's Hospital, Vancouver, Canada V6Z 1Y6; and 3 Institute of Fundamental Sciences-Physics, Massey University, Palmerston North, New Zealand 5331
Insights into airway mechanics were sought by applying
morphometric techniques to rabbit lungs fixed at several lung recoil pressures. Rabbits were treated with either nebulized carbachol followed by iv administration of carbachol or with saline solution (sham). The lungs were held at one of six values of positive
end-expiratory pressure (PEEP; 10, 7, 4, 2, 0, and
4
cmH2O) while the animal was killed and formalin was
circulated through the lungs. The lungs were removed and left in a bath
of formalin for 24 h. Standard airway morphometric measurements
were made on membranous bronchiole slices taken from representative
blocks of tissue. Reductions in PEEP produced the expected reductions
in lumen area in the carbachol-treated airways but not in the
sham-treated airways for PEEP > 2 cmH2O. Sham-treated
airways remained more open than expected until they collapsed into an
oval shape at PEEPs between 4 and 2 cmH2O. The
carbachol-treated airways exhibited this behavior at PEEP =
4
cmH2O. The smallest airways, which had relatively thicker
walls, collapsed less than larger airways. We postulate that this
behavior implies that peribronchial stress is greater than lumen
pressure on collapse into the oval shape. Resistance to buckling
increases with the thickness-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).
rabbit; airway mechanics; carbachol; elastic buckling
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