Aspiration is a common event even in healthy individuals and usually resolves without detectable sequelae. Markers placed in the stomach can often be detected in the lungs of healthy persons using scintigraphic methods . Sensitive tests show that at least one-half of healthy adults aspirate during sleep . Pulmonary sequelae depend upon the volume and contents of the inoculum and host defense mechanisms. Aspiration pneumonia refers to the pulmonary consequences resulting from this abnormal entry of fluid, particulate exogenous substances, or endogenous secretions into the lower airways. There are usually two requirements to produce aspiration pneumonia:
●Compromise in the usual defenses that protect the lower airways including glottic closure, cough reflex, and other clearing mechanisms
●An inoculum deleterious to the lower airways by a direct toxic effect (such as gastric acid), stimulation of an inflammatory process from bacterial infection, or obstruction due to uncleared fluid or particulate matter
Most pneumonia arises following the aspiration of microorganisms from the oral cavity or nasopharynx. The term aspiration pneumonia should be reserved for pneumonitis resulting from the altered clearance defenses noted above. The pathogens that commonly produce pneumonia, such as Streptococcus pneumoniae, Haemophilus influenzae, gram-negative bacilli, and Staphylococcus aureus, are relatively virulent bacteria so that only a small inoculum is required and the aspiration is usually subtle. A true aspiration pneumonia, by convention, usually refers to an infection caused by less virulent bacteria, primarily anaerobes, which are common constituents of the normal flora in a susceptible host prone to aspiration. In a review of 1348 patients hospitalized with community-acquired pneumonia, 13.8 percent were considered to be at risk for aspiration; risk factors included neurologic disorders, reduced consciousness, esophageal disorders, vomiting, or witnessed aspiration .
The predisposing conditions, clinical syndromes, diagnosis, and treatment of aspiration pneumonia will be reviewed here. Community-acquired pneumonia, hospital-acquired pneumonia, pneumonia due to specific pathogens, empyema, and lung abscess are discussed separately. (See related topic reviews.)