The infectious pulmonary process that occurs after abnormal entry of fluids into lower respiratory tract is termed as aspiration pneumonia. The aspirated fluid can be formed from oropharyngeal secretions or particulate matter or can also be gastric content. The term aspiration pneumonitis refers to inhalational acute lung injury that occurs after aspiration of sterile gastric contents. In an observational study, it is found that the risk of patients hospitalized for community-acquired pneumonia in developing aspiration pneumonia is found to be about 13.8%. The mortality rate from aspiration pneumonia is largely dependent on volume and content of aspirate and can range to 70%.
Failure of the natural defense mechanisms like the closure of glottis and cough reflex increases the risk of aspiration. The common risk factors for aspiration include altered mental status, Neurologic disorders, esophageal motility disorders, protracted vomiting and gastric outlet obstruction. Although the common organisms involved in etiology of community-acquired pneumonia are Streptococci, Haemophilus, and gram-negative bacilli, the etiology of aspiration pneumonia depends on the content of aspirate. A prospective study in 95 patients showed that gram-negative bacilli contributed to 49% followed by anaerobes (16%). The major anaerobes isolated were Fusobacterium, Bacteroides, and Peptostreptococcus. In hospital-acquired aspiration pneumonia, common etiology include gram-negative organisms specifically Pseudomonas auruginosa has to be considered.
Due to lack of biomarkers the epidemiological studies to find the incidence of aspiration pneumonia has been difficult. Several studies showed that aspiration pneumonia contributes to 5% to 15% of all community-acquired pneumonia. A retrospective study done on 628 patients with aspiration pneumonia by Lanspa et al. showed a 30-day mortality of 21%. The study also showed that CURB 65 a predictor of mortality in Community-acquired pneumonia is not a reliable tool for aspiration pneumonia. Aspiration pneumonia remains one of the common complication following general anesthesia and occurs in one in every 2000 to 30000 cases. A study conducted on the elderly population aged older than 65 years who underwent cardiovascular surgery the incidence of aspiration pneumonia was found to be 9.8% with 12 out of 123 patients having aspiration pneumonia post-extubation. A case-control study conducted by Marrie TJ et al. showed that the incidence of aspiration pneumonia was 18% in nursing home patients and 15% in community-acquired aspiration pneumonia. Since most cases of aspiration pneumonia are silent or unwitnessed, the true incidence rate is difficult to calculate.
In normal healthy adults, the mucociliary mechanism and alveolar macrophages act as defenses in clearing micro aspirations from the oropharyngeal secretions. The pathological process of aspiration pneumonia occurs when the normal defenses mechanisms fail in a predisposed individual. The entry of fluid into the bronchi and alveolar space triggers an anti-inflammatory reaction with the release of proinflammatory cytokines, tumor necrosis factor alpha, and Interleukins. Inoculation of organisms of common flora from the oropharynx and esophagus results in the infectious process. Mendelson first studied the pathophysiology of aspiration pneumonitis by inducing gastric contents in rabbit’s lung and comparing to 0.1N hydrochloric acid. Later studies conducted in rats using diluted hydrochloric acid demonstrated the biphasic response with initial corrosive phase by acidic pH followed by a neutrophil-mediated inflammatory response. Inoculation of normal oropharyngeal flora in the aspirate results in infectious process and results in aspiration pneumonia. If the bacterial load of aspirate is low normal host defenses will clear the secretions and prevent infection.
The common clinical features that should raise suspicion for aspiration include sudden onset dyspnea, fever, hypoxemia, radiological findings of bilateral infiltrates and crackles on lung auscultation in a hospitalized patient. The common site involved depends on the position at the time of aspiration, commonly the lower lobes are involved in an upright position, and superior lobes can be involved in the recumbent position. The radiological findings will develop within 2 hours after aspiration and bronchoscopy can reveal erythematous bronchi.
A high level of suspicion should be made in diagnosing aspiration pneumonia, especially in critically ill hospitalized patients and antibiotic treatment should be initiated immediately, and imaging studies should not delay the treatment. The commonly utilized imaging studies are chest x-ray, computed tomography of the chest which will help in localizing the site of aspiration. However, most cases are unwitnessed aspiration which makes it very hard to distinguish between aspiration pneumonitis and aspiration pneumonia. Clinical features may help to distinguish. In aspiration pneumonitis, a large volume of gastric content has to be aspirated to produce chemical pneumonitis and can quickly progress to acute lung injury with subsequent development of acute respiratory distress syndrome (ARDS). Whereas, in aspiration pneumonia, aspiration can be of smaller volumes and can be unwitnessed which with inoculation of bacteria progress to features of pneumonia and subsequent development of acute respiratory distress syndrome (ARDS).
The treatment varies between aspiration pneumonia and aspiration pneumonitis. To prevent further aspiration, patient’s position should be adjusted followed by suction of oropharyngeal contents with the placement of the nasogastric tube. In patients who are not intubated humidified oxygen is administered and the head end of the bed should be raised by 45 degrees. A close monitoring of patients oxygen saturation is important and immediate intubation with mechanical ventilation should be provided if hypoxia is noted. A flexible bronchoscopy is usually indicated for large volume aspiration to clear the secretion and also for obtaining the sample of bronchoalveolar lavage for quantitative bacteriological studies. In general practice, antibiotics are initiated immediately even though are not required in aspiration pneumonitis to prevent progression of the disease. The choice of antibiotics for community-acquired aspiration pneumonia are ampicillin-sulbactam, or a combination of metronidazole and amoxicillin can be used. In patients with penicillin allergy clindamycin is preferred. However, in hospital-acquired aspiration pneumonia antibiotics that cover resistant gram-negative bacteria and S.aureus; so the use of a combination of vancomycin and piperacillin-tazobactam is most widely used. Once the culture results are obtained the antibiotic regimen should be narrowed to organism-specific.
The disease syndrome that occurs after macro aspiration of gastric and oropharyngeal contents leading to acute pulmonary infection is Aspiration pneumonia. It is important to understand and distinguish between aspiration pneumonitis which is an inflammatory process that occurs post-aspiration and aspiration pneumonia for guiding treatment. Since the major complication of this disease syndrome results in ARDS, prompt treatment with supplemental oxygen and antibiotics and measures to prevent further aspiration has to be undertaken. A close monitoring of the patient for hypoxia is mandated and if the present patient should be intubated and mechanically ventilated. In a meta-analysis conducted by Kosaku K et al. it is found that aspiration pneumonia is associated with high-30-day and in-hospital, but outside the intensive care setting, mortality rates in patients with community-acquired pneumonia.