INTRODUCTION

Aspiration is the introduction of oropharyngeal or gastric contents into the respiratory tract. Three major syndromes may develop as a consequence of aspiration: chemical pneumonitis, bronchial obstruction secondary to aspiration of particulate matter, and bacterial aspiration pneumonia. Less commonly, interstitial lung disease occurs in persons with chronic aspiration. Which of these consequences emerges is determined by the amount and nature of the aspirated material as well as by the integrity of host defense mechanisms. Aspiration is the main means of bacterial contamination of the lower airways.

The term aspiration pneumonia is used to refer to the infectious consequences of introduction of relatively large volumes of material (macroaspiration). Although healthy persons frequently aspirate small volumes of pharyngeal secretions during sleep, the development of pneumonia after such microaspiration is normally prevented by mechanical (eg, cough and mucociliary transport) and immunological responses. Pneumonia arises when these host defenses are not able to limit bacterial proliferation either because of microaspiration of highly virulent pathogens to which the host lacks specific immunity (eg, Streptococcus pneumoniae or enteric gram-negative bacteria) or because of macroaspiration of lower or higher virulence organisms.

Aspiration may be clinically obvious, as when acute pulmonary complications follow inhalation of vomited gastric contents. Such acute chemical pneumonitis is often referred to as Mendelson syndrome. On the other extreme, so-called silent aspiration, as occurs in persons with neurological impairment who lack cough responses, is often followed by the indolent onset of infectious pneumonia consequent to the inhalation of the low virulence mixtures of aerobic and anaerobic microorganisms normally resident in the oropharynx.