Studies indicate that small airways are the major
site of obstruction in patients with COPD.
A study was carried out to study the relationship
between the number and dimensions of small airways and emphysematous changes in
the lungs. The number of small airways with a diameter of between 2.0 and 2.5mm
were calculated using mutidetector CT scan in a total of 96 lungs, out of which
78 were from patients with varying degree of COPD, 4 from deceased patients and
14 from patients undergoing lung transplantation for Stage 4 COPD.
MicroCT was used to measure the average alveolar
dimension, the number of terminal bronchioles per millimeter of lung volume,
and the diameter and cross-sectional area of the terminal bronchioles.
(Terminal bronchioles are the last part of the conducting airways, which
finally divide into the respiratory bronchioles).
Histological sections of some lung tissues were also
showed that the number of airways measuring between 2 and 2.5 mm in diameter
per lung pair was progressively reduced with worsening stages of COPD
. It was however unclear whether the decrease in diameter was due
to reduction in the number of small airways or simply due to narrowing of the
COPD showed an 83 to 99.7% decrease in the terminal bronchiolar cross-sectional
area per lung and a 72 to 89% reduction in the number of terminal bronchioles
per lung depending on the type of emphysema. The narrowing and loss of terminal
bronchioles seemed to precede the development of emphysema.
The study thus
concludes that increase in airway resistance in patients with COPD is likely
due to increase in resistance of small airways, which in turn is due to
narrowing or loss of the airways. These changes are observed before the
development of emphysema.
1. McDonough JE et al. Small-Airway Obstruction and Emphysema in Chronic
Obstructive Pulmonary Disease. N Engl J Med 2011; 365:1567-1575