Fig.13.3 ).Suchmembranesconsistoffibrin-richedemafluidadmixedwithremnantsofnecroticepithelialcells.Overall,thepictureisremarkablysimilartothatseeninrespiratorydistresssyndromeofthenewborn(Chapter6).Intheorganizing stage, typeIIpneumocytesproliferatevigorouslyinanattempttoregeneratethealveolarlining.Resolutionisunusual;morecommonly,thefibrin-richexudatesorganizeintointraalveolarfibrosis.Markedthickeningofthealveolarseptaensuesduetoproliferationofinterstitialcellsanddepositionofcollagen.
The clinical syndrome of acute lung injury or ARDS affects approximately 190,000 patients per year in the United States. In 85% of cases, it develops within 72 hours of the initial insult. The overall hospital mortality rate is 38.5% (27%, 32%, and 45% for mild, moderate, and severe ARDS, respectively). Predictors of poor prognosis include advanced age, bacteremia (sepsis), and the development of multiorgan failure. Most patients who survive the acute insult recover normal respiratory function within 6 to 12 months, but the rest develop diffuse interstitial fibrosis leading to chronic respiratory insufficiency.
ARDSisaclinicalsyndromeofprogressiverespiratoryinsufficiencycausedbydiffusealveolardamageinthesettingofsepsis,severetrauma,ordiffusepulmonaryinfection.
NeutrophilsandtheirproductshaveacrucialroleinthepathogenesisofARDSbycausingendothelialandepithelialinjury.
Thecharacteristichistologicpictureisthatofalveolaredema,epithelialnecrosis,accumulationofneutrophils,andpresenceofhyalinemembranesliningthealveolarwallandducts.
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Fig. 13.3 Acutelunginjuryandacuterespiratorydistresssyndrome.
(A)Diffusealveolardamageintheacutephase.Somealveoliarecollapsed,whileothersaredistended;manyarelinedbybrightpinkhyalinemembranes(arrow). (B)Thehealingstageismarkedbyresorptionofhyalinemembranesandthickeningofalveolarseptabyinflammatorycells,fibroblasts,andcollagen.NumerousreactivetypeIIpneumocytesalsoareseenatthisstage(arrows), associatedwithregenerationandrepair.
Diffuse pulmonary diseases can be classified into two categories: (1) obstructive (airway) disease, characterized by an increase in resistance to air flow caused by partial or complete obstruction at any level; and (2) restrictive disease, characterized by reduced expansion of lung parenchyma and decreased total lung capacity.
The major diffuse obstructive disorders are emphysema, chronic bronchitis, bronchiectasis, and asthma. In patients with these diseases, forced vital capacity (FVC) is either normal or slightly decreased, while the expiratory flow rate, usually measured as the forced expiratory volume at 1 second (FEV1), is significantly decreased. Thus, the ratio of FEV to FVC is characteristically decreased. Expiratory obstruction may result from anatomic airway narrowing, classically observed in asthma, or from loss of elastic recoil, characteristic of emphysema.