Antibiotics are indicated for primary bacterial aspiration pneumonia and secondary bacterial infection of aspiration (chemical) pneumonitis, but not for uncomplicated chemical pneumonitis.
THREE TYPES OF ‘ASPIRATION PNEUMONIA’
Aspiration pneumonia is a broad and vague term mainly used to refer to the pulmonary consequences of abnormal entry of exogenous or endogenous substances into the lower airways. It can be classified as:
- Aspiration (chemical) pneumonitis
- Primary bacterial aspiration pneumonia
- Secondary bacterial infection of chemical pneumonitis.
These three are sometimes difficult to differentiate, as their signs and symptoms can overlap.
Aspiration of stomach contents is relatively common, even in healthy people, and usually has no clinical consequences.1 However, it has also been closely related to community-acquired and nosocomial pneumonia in some studies.2,3
Chemical pneumonitis is usually a consequence of the aspiration of a large volume (≥ 4 mL/kg) of sterile acidic (pH < 2.5) gastric contents into the lower airways (Mendelson syndrome).4,5 The clinical picture varies from asymptomatic to signs of severe dyspnea, hypoxia, cough, and low-grade fever; these signs and symptoms may develop rapidly, within minutes to hours after a witnessed or suspected episode of aspiration.2,6,7 However, they represent an inflammatory reaction to the gastric acid rather than a reaction to bacterial infection.8–10
Chemical pneumonitis affects the most dependent regions of the lungs
Chest radiography shows infiltrates in the most dependent regions of the lung. If aspiration occurs while the patient is supine, the posterior segments of the upper lobes and the apical segments of the lower lobes are most affected. The basal segments of the lower lobes are usually affected if aspiration occurs while the patient is standing or upright.1,2,11,12
Clinical course varies
The clinical course varies. In almost 60% of cases, the patient’s condition improves and the lung infiltrates resolve rapidly, within 2 to 4 days. On the other hand, in about 15% of cases, the patient’s condition deteriorates quickly, within 24 to 36 hours, and progresses to hypoxic respiratory failure and acute respiratory distress syndrome.
In the other 25% of cases, the patient’s condition may improve initially but then worsen as a secondary bacterial infection sets in. The death rate in these patients is almost three times higher than the rate in patients with uncomplicated chemical pneumonitis.11,13
Treatment of uncomplicated cases is mainly supportive
The treatment of uncomplicated chemical pneumonitis involves supportive measures such as airway clearance, oxygen supplementation, and positive pressure ventilation if needed. An obstructing foreign body may need to be removed.12,14 Corticosteroids have been tried, without success.11–13,15
Empiric antibiotic treatment is controversial
Chemical pneumonitis can be difficult to differentiate from bacterial aspiration pneumonia, and whether to give antibiotics is controversial. 16 A survey of current practices among intensivists showed that antimicrobial therapy was often given empirically for noninfectious chemical pneumonitis.17 This practice raises concerns of higher treatment costs and antibiotic resistance.16,18,19 Additionally, antibiotics do not seem to alter the clinical outcome, including radiographic resolution, duration of hospitalization, or death rate, nor do they influence the subsequent development of infection.1,11,13,20
In cases of witnessed or strongly suspected aspiration of gastric contents, antibiotics are not warranted since bacterial infection is not likely to be the cause of any signs or symptoms. 2,7,16 However, to detect secondary infection early, the patient’s respiratory status should be monitored carefully and chest radiography should be repeated.
In less clear-cut cases, ie, if it is not clear whether the patient actually has chemical pneumonitis or primary bacterial aspiration pneumonia, it is prudent to start antibiotics empirically after obtaining lower-respiratory-tract secretions for stains and cultures, and then to reassess within 48 to 72 hours. The antibiotics can be discontinued if the patient has rapid clinical and radiographic improvement and negative cultures. Those whose condition does not improve or who have positive cultures should receive a full course of antibiotics.21,22