Answer:

Aspiration of gastric and oropharyngeal contents causes three overlapping syndromes. First, aspiration of large particulate matter may obstruct the airways and lead to atelectasis. Acute aspiration pneumonitis is more common and is a result of chemical burning of the tracheobronchial tree and pulmonary parenchyma by gastric and oral fluids. Finally, aspiration of bacteria from the oropharynx or a superinfection of the chemical pneumonitis may result in aspiration pneumonia.

Mendelson originally described aspiration pneumonitis in 66 obstetrical patients who aspirated gastric contents during anesthesia. Respiratory distress and cyanosis developed within 2 hours after the aspiration event. All of the patients (except two who had airway obstruction from solid food particles) recovered within 24 to 36 hours without antibiotics and had radiographic resolution. Signs and symptoms of aspiration pneumonitis include coughing, wheezing, dyspnea, hypoxia, fever, tachypnea, and crackles on lung auscultation. Radiography usually demonstrates diffuse bilateral infiltrates.

Aspiration pneumonitis may be mild and elude clinical detection. Rarely, aspiration is fulminant and rapidly fatal. Most patients rapidly improve after aspiration and have clearing of radiographic lung infiltrates. Occasionally, patients initially improve but then develop progressive lung infiltrates on chest radiograph. These infiltrates probably represent a secondary bacterial infection or ARDS. The volume and pH of the aspirated fluid are important factors affecting the degree of lung injury. In adults, approximately 25 mL of gastric acid with a pH <2.5 is considered a clinically relevant threat. Smaller volumes may produce a milder syndrome. Bile may elicit a potent inflammatory response and has been identified in patients’ endotracheal tubes. Atelectasis, peribronchial hemorrhage, pulmonary edema, and degeneration of bronchial epithelial cells all develop within minutes of aspiration. By 4 hours, the alveoli fill with polymorphonuclear leukocytes and fibrin, and hyaline membranes form. The lung becomes grossly edematous and hemorrhagic with alveolar consolidation.

Aspiration pneumonia may result from a superimposed infection of the chemical injury or inhalation of microorganisms from the oropharynx. Poor dentition is a risk factor for aspiration pneumonia. Common bacteria include Haemophilus influenzae, streptococci, and other anaerobes. The oral flora changes in chronically ill patients and those in health care settings for greater than 48 to 72 hours. In these patients, gram-negative bacteria and resistant Staphylococcus may colonize the oropharynx and cause pneumonia.

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