Biomedical Engineering Reference
In-Depth Information
characterized by airflow limitation that is not fully reversible. The airflow
limitation is usually progressive and associated with an abnormal inflamma-
tory response of the lungs to noxious particles and gases'' (11). For the first
time, this definition encompasses the idea that COPD is a chronic inflamma-
tory disease and much of the recent research has focused on the nature of
this inflammatory response.
COPD includes chronic obstructive bronchiolitis with fibrosis and
obstruction of small airways, and emphysema with enlargement of airspaces
and destruction of lung parenchyma, loss of lung elasticity, and closure of
small airways. Chronic bronchitis, by contrast, is defined by a productive
cough of more than three months duration for more than two successive
years; this reflects mucous hypersecretion and is not necessarily associated
with airflow limitation. Most patients with COPD have all three pathologi-
cal mechanisms (chronic obstructive bronchiolitis, emphysema, and mucus
plugging) as all are induced by smoking, but may differ in the proportion
of emphysema and obstructive bronchiolitis (4). In developed countries,
cigarette smoking is by far the commonest cause of COPD accounting for
over 95% of cases, but there are several other risk factors, including air pol-
lution (particularly indoor air pollution from burning fuels), poor diet, and
occupational exposure. COPD is characterized by acceleration in the normal
decline of lung function seen with age. The slowly progressive airflow limita-
tion leads to disability and premature death and is quite different from the
variable airway obstruction and symptoms in asthma, which rarely pro-
gresses in severity. While COPD and asthma both involve inflammation
in the respiratory tract, there are marked differences in the nature of the
inflammatory process, with differences in inflammatory cells, mediators,
response to inflammation, anatomical distribution, and response to anti-
inflammatory therapy (7,12). Some patients appear to share the characteris-
tics of COPD and asthma, however. Rather than this representing a graded
spectrum of disease, it is more likely that these patients have both of these
common diseases at the same time.
A. Differences from Asthma
Histopathological studies of COPD show a predominant involvement of
peripheral airways (bronchioles) and lung parenchyma, whereas asthma
involves inflammation in all airways but usually without involvement of
the lung parenchyma (13). There is obstruction of bronchioles, with fibrosis
and infiltration with macrophages and T-lymphocytes. There is destruction
of lung parenchyma and an increased number of macrophages and T-
lymphocytes, with a greater increase in CD8
þ
(cytotoxic) than CD4
þ
(helper) cells (14). Bronchial biopsies show similar changes with an infiltra-
tion of macrophages and CD8
þ
cells and an increased number of neutro-
phils in patients with severe COPD (15). Bronchoalveolar lavage (BAL)
