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- Pneumonia due to the misdirection of large volume of gastric bacteria into the lung parenchyma
- Contrasts with typical pneumonia, which occurs by direct microinhalation of infectious particles from air
- Typically occurs in patients with impaired airway protection. Impairment can be secondary to neurologic, mechanical, or immunologic factors.
- Can occur with any impairment of a mechanism that protects the lower airways. This may be a mechanical impairment or a defect in humoral and cellular immunity.
- System(s) affected: pulmonary
- Risk of aspiration pneumonia is highest among nursing home patients.
- Risk of aspiration pneumonia is 6 times higher if ≥75 years of age compared to those <60 years of age.
- Typically presents with fever, cough, purulent sputum. However, symptoms may be minimal in elderly, particularly those with neurologic disorders/reduced consciousness causing swallow dysfunction.
- Prevalence data is difficult to assess given variations in the definition of aspiration pneumonia (radiographic, clinical, or bacteriologic classifications have all been used).
- Although small volume aspiration is common, even universal, aspiration pneumonia usually requires a degree of physiologic compromise.
Etiology and Pathophysiology
- Aspiration pneumonia occurs when organisms dwelling in the oropharynx or stomach are introduced into the lungs in sufficient volume to cause clinical disease.
- Mechanical factors
- Mechanical ventilation: Endotracheal tube is a direct path to lower respiratory tract, which also prevents clearance of bacteria and secretions from exiting lower airways.
- Nasogastric feeding tubes prevent normal function of esophageal sphincters and increase risk of aspiration events. They are further associated with the appearance of gram-negative bacteria in pharyngeal secretions (1).
- Reduced gag reflex due to sedation, stroke, or normal aging reduces spontaneous coughing and clearance of bacteria.
- Hospitalization has been associated with a change in microflora of stomach and oropharynx with consequences for both the severity and bacteriology of aspiration pneumonia.
- Increased colonization of bacteria in conditions such as malnutrition, alcoholism, diabetes, and proton pump inhibitor use increases likelihood of infection with each aspiration event (2).
- Gravity affects the distribution of aspiration events. If aspiration occurs while the patient is recumbent, infection is likely in the posterior segments of the upper lobes and the apical segments of the lower lobes. If aspiration occurs while upright or semirecumbent, the basal segments of the lower lobes are most likely to become infected.
- Aspiration pneumonia patients have longer hospital stays and higher in-hospital mortality than patients with pneumonia who did not aspirate (2).
- Both community-acquired pneumonia (CAP) and hospital-acquired pneumonia have a worse prognosis if aspiration (defined by both clinical and radiologic features) is likely to be the inciting event (3).
- If untreated, patients appear to have a higher incidence of cavitation and lung abscess formation than in those with nonaspiration pneumonia.
- Important contrast with chemical pneumonitis, which is due to aspiration of contents toxic to lung, independent of bacterial involvement (e.g., gastric acid), which presents with an abrupt onset of symptoms and prominent dyspnea. Chest x-ray (CXR) changes are seen within 2 hours.
- Pathogens vary according to setting:
- CAP: gram positives and some gram negatives (e.g., Streptococcus pneumonia, Staphylococcus aureus, Haemophilus influenzae, and enterobacteria)
- Health care–associated pneumonia (HCAP): mostly polymicrobial, including gram-negative bacilli such as Pseudomonas aeruginosa and anaerobes such as Bacteroides fragilis and less commonly, gram positives, including S. aureus
- Ventilator-associated pneumonia (VAP): common nosocomial bacteria, especially P. aeruginosa, Acinetobacter baumannii, methicillin-resistant S. aureus (MRSA)
- Specific bacteria are rarely identified.
- Most cases associated with predisposing factors (see “Risk Factors”)
- Reduced consciousness, alcoholism, seizures, general anesthesia, dementia/cognitive impairment, age >60 years, poor nutritional status, poor oral hygiene
- Choking incident, prior aspiration events
- Pneumonia, likely secondary to aspiration, has been observed in up to 65% of out-of-hospital cardiac arrest survivors.
- Mechanical ventilation, bronchoscopy, upper endoscopy
- Pulmonary diseases: chronic obstructive pulmonary disease (COPD)
- Dysphagia: due to stroke, neuromuscular diseases, radiation to the neck or oropharynx
- GI diseases: gastroesophageal reflux disease, esophageal disease
- Enteral feeding tubes, nasogastric tube feeding
- Immunosuppressed patients: solid organ transplantation, steroid use >20 mg/day for >2 weeks, HIV
- Treatment can reduce aspiration events, but it is important to recognize that they are not completely preventable.
- Aspiration prevention protocols, including bedside speech and swallow evaluations, progressive oral intake, sedation vacations, and ventilator-weaning protocols significantly reduce risk of aspiration pneumonia in the critically ill population.
- A soft diet and nectar-thickened liquids are better than a pureed diet for preventing pneumonia.
- Feeding tubes, often inserted in the malnourished elderly with swallowing dysfunction and dementia, do not prolong life; the American Geriatrics Society recommends against tube feeding in advanced dementia (4).
- A recent Cochrane review found no difference in aspiration pneumonia between nasogastric feeding and gastric tube feeding populations (5).
- Most cases are not preventable and are likely to be recurrent.
- A “goals of care” discussion in a patient who has recurrent aspiration events may be appropriate.
- Although typically considered a risk-factor, a recent study of >11,000 acute stroke patients found no change in rate of pneumonia in those supine versus head of bed elevated (6).
Commonly Associated Conditions
See “Risk Factors.”