Page 847 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

578 PART 5: Infectious Disorders

episode and could therefore be technically called aspiration pneumonia. Airway Defense: Airway defenses are critical once an infectious bolus
However, this term often is associated with a clinical entity of anaerobic is aspirated into the trachea or proximal airways and for inhaled
pleuropneumonia (Fig. 65-1) resulting from a past episode of aspira- pathogens and particulate matter. The critical importance of muco-
tion associated with loss of consciousness, usually associated with acute ciliary clearance is abundantly illustrated in the genetic defects in
alcohol intoxication or a seizure disorder. The key differentiating factor cystic fibrosis and primary cilial dysfunction. A variety of mucins
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for this diagnosis is that the aspiration episode occurred days to weeks, are induced by pathogens but may actually dampen the inflammatory
even months, prior to presentation to the hospital. While this disorder is response in some cases. The effect of cigarette smoking and anteced-
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one of the aspiration syndromes, its frequency has markedly decreased ent viral infection are well-known risk factors for development of
in the last few decades. pneumonia. However, the airway epithelium cells are also immuno-
In contrast, viral CAP and some forms of bacterial pneumonia result logically active. A number of antimicrobial peptides are secreted by
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from droplet inhalation. Legionella pneumophila HAP from contami- these cells, including defensins, lactoferrin, lysozyme, and cathelici-
nated water sources is a good example of an inhalational bacterial pneu- dins. Immunoglobulin (Ig)-A and complement components are also
monia. However, many of these infections actually represent contiguous secreted into the airway during inflammation.
extension from the infected oropharynx, rather than true inhalation
pneumonia. Therefore, microaspiration may still play a significant role Alveolar Defense: The resident alveolar macrophage is the key compo-
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even in these infections. nent of host defense at the alveolar level. They are able to clear most
inhaled infectious and noninfectious challenges, as well as smaller
■ HOST DEFENSE degrees of aspiration. More importantly, macrophages are the source
of a host of cytokines and chemokines, which recruit neutrophils and
Because the lower respiratory tract is discontinuously exposed to infec- initiate a variety of other components of innate immunity. In addi-
tious microorganisms, a wide variety of redundant host defense mecha- tion, macrophages and dendritic cells are key antigen-presenting cells,
nisms are available to deal with this infectious challenge. important in the initiation of humoral immunity.

A

B

R L R L

FIGURE 65-1. Anaerobic pleuropneumonia. Note intrathoracic air fluid level on the plain chest x-ray (A). Chest CT (B) confirms rim enhancement of the pleural space along
intrapleural air, and necrotic pneumonia.

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