Biomedical Engineering Reference
In-Depth Information
The major risk factor for a rapid decline in FEA
1
that leads to COPD
is the chronic inhalation of toxic particles and gases over a person's lifetime
(12). The tobacco smoking habit (13) is the dominant factor contributing to
this exposure but atmospheric pollution produced by the exhaust from the
internal combustion engines, and many other types of exposures that are
specific to the home and work place contribute to a lifetime exposure.
Although childhood infections of the lower respiratory tract increase the
risk for COPD later in life (13) the role of adult infection in the pathogenesis
of COPD is a controversial issue. The British hypothesis introduced in the
1950s was based on the concept that there was a progressive change from
the chronic cough and sputum production found in smokers to infective
bronchitis indicated by the appearance of purulent sputum and finally
chronic airways obstruction. However, the study by Fletcher and associates
(see Fig. 3) showed that the presence of chronic bronchitis did not predict
the rapid decline in lung function that led to severe (GOLD-3) and very
severe (GOLD-4) COPD. It is now quite well established that smoking
interferes with the clearance of microbes that gain entry into the lower
respiratory tract and that the lung becomes colonized by a number of
micro-organisms in severe (GOLD-3) and very severe (GOLD-4) COPD.
It has also been established that lower respiratory tract infections account
for about a third of acute exacerbations of COPD and that these infections
are initiated by the appearance of new strains of organisms that chronically
colonize the lungs. Although these acute exacerbations are troublesome
and expensive, especially if they result in hospitalization they do not appear
to influence the rate of decline in COPD except in those that continue to
smoke (14).
II. CHRONIC BRONCHITIS AND SMALL AIRWAY
OBSTRUCTION
It is a common but incorrect practice to use the terms bronchitis and airways
obstruction interchangeably in patients with COPD because the site of
obstruction is in the small conducting airways (6-8). The diagnosis of
chronic bronchitis is based on the symptom of persistent cough with exces-
sive sputum production that must be present on most days for at least 3
months per year for two consecutive years before the diagnosis can be estab-
lished (15). Reid (16,17), showed that the presence of these symptoms corre-
lated with increased size of the bronchial mucus glands and she proposed the
ratio of the width of the gland to the distance between the reticular base-
ment membrane and cartilage (now referred to the Reid index) as a diagnos-
tic yardstick for the pathologic diagnosis of chronic bronchitis. Mullen
et al. (18) and Saetta et al. (19) subsequently showed that the symptoms
of chronic bronchitis were associated with an inflammatory process invol-
