Abstract
In autopsy lungs from 28 patients with bronchial asthma, the grade and the distribution of airway smooth muscles in the bronchial tree were studied by morphometry using microscopic lung sections and were compared with 19 normal control cases and 14 cases with other types of COPD. In each case, cross sections of airways were prepared to cover the entire range of airways from large segmental bronchi to terminal bronchioles. The varying constricted state of an airway was standardized by measuring the perimeter length L of the epithelial basement membrane (BM) and the sectional area S of muscles with a digital image analyzer, reducing the airway into a circular state with completely stretched BM, where the anatomical airway radius R and the mean thickness D of muscular layer were calculated from L and S. A close correlation of D with R was shown to exist on bilogarithmic coordinates in every case in both controls and asthmatics. The values of D in the asthmatics were higher than in controls in the larger airways, where the asthmatic constriction of bronchi was mainly considered to occur. However, in asthmatics, there appeared two types of distribution of muscular hypertrophy: In what we designated as type 1, muscular hypertrophy was found mainly in larger airways, while in type 2, it involved the entire range of airways including the peripheral bronchioles as well as the large segmental level. In 14 cases of COPD (emphysema or bronchiolitis), only mild hypertrophy was found in large airways. The ratio D/R, the relative thickness of muscles, was calculated in all bronchi sampled; this value proved to increase toward the periphery in the controls and type 2 asthmatic groups, suggesting that in these patients the peripheral airways are more hyperactive than the central bronchi. Normally, the small airways are likely to be the major site of gas-flow control.
