It is postulated that many of the effects of environmental irritants on respiratory epithelium are mediated by local release of cytokines such as IL-13 from T cells and innate lymphoid cells. The transcription of the mucin gene in bronchial epithelium and the production of neutrophil elastase are increased as a consequence of exposure to tobacco smoke. Microbial infection often is present but has a secondary role, chiefly by maintaining inflammation and exacerbating symptoms.
MORPHOLOGYAsseeningrossspecimens,themucosalliningofthelargerairwaysusuallyishyperemic and swollen byedemafluidandiscoveredbyalayerofmucinousormucopurulentsecretions. Thesmallerbronchiandbronchiolesalsomaybefilledwithsecretions.Thediagnosticfeatureofchronicbronchitisinthetracheaandlargerbronchiisenlargement of the mucus-secreting glands (
Fig.13.9 ).Themagnitudeoftheincreaseinsizeisassessedbytheratioofthethicknessofthesubmucosalglandlayertothatofthebronchialwall(theReidindex—normally0.4).Variablenumbersofinflammatorycells,largelylymphocytesandmacrophagesbutsometimesalsoadmixedneutrophils,arefrequentlyseeninthebronchialmucosa.Chronic bronchiolitis (smallairwaydisease),characterizedbygobletcellmetaplasia,mucousplugging,inflammation,andfibrosis,alsoisseen.Inseverecases,theremaybecompleteobliterationofthelumenasaconsequenceoffibrosis(bronchiolitis obliterans). Itisthesubmucosalfibrosisthatleadstoluminalnarrowingandairwayobstruction.Emphysematouschangesoftencoexist.
Fig. 13.9 Chronicbronchitis.Thelumenofthebronchusisabove.Notethemarkedthickeningofthemucousglandlayer(approximatelytwice-normal)andsquamousmetaplasiaoflungepithelium.(From the Teaching Collection of the Department of Pathology, University of Texas, Southwestern Medical School, Dallas,Texas.)
The course of chronic bronchitis is quite variable. In some patients, cough and sputum production persist indefinitely without ventilatory dysfunction, while others develop COPD with significant outflow obstruction marked by hypercapnia, hypoxemia, and cyanosis. Patients with chronic bronchitis and COPD have frequent exacerbations, more rapid disease progression, and poorer outcomes than those with emphysema alone. Progressive disease is marked by the development of pulmonary hypertension, sometimes leading to cardiac failure (Chapter 11); recurrent infections; and ultimately respiratory failure.
Chronicbronchitisisdefinedaspersistentproductivecoughforatleast3consecutivemonthsinatleast2consecutiveyears.
Cigarettesmokingisthemostimportantunderlyingriskfactor;airpollutantsalsocontribute.
Chronicairwayobstructionlargelyresultsfromsmallairwaydisease(chronicbronchiolitis)andcoexistentemphysema.
Histologicexaminationdemonstratesenlargementofmucus-secretingglands,gobletcellmetaplasia,andbronchiolarwallfibrosis.
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