Hospital-acquired pneumonia

Hospital-acquired pneumonia develops at least 48 hours after admission to hospital. The most common pathogens are gram-negative bacilli and Staphylococcus aureus; drug-resistant organisms are a significant problem. The causes are the same as for community-acquired pneumonia, but in patients on mechanical ventilation, pneumonia may also present with decreased oxygenation and increased tracheal secretions. The diagnosis is suspected on the basis of clinical manifestations and chest radiography and is confirmed by bacteriologic examination of blood or lower respiratory tract samples taken during bronchoscopy. Treatment is with antibiotics. Hospital-acquired pneumonia has a poor prognosis, partly due to comorbidities.

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Causes hospital-acquired pneumonia

The most common cause of hospital-acquired pneumonia is microaspiration of bacteria that have colonized the oropharynx and upper respiratory tract in seriously ill patients.

Causal organisms and their antibiotic resistance patterns vary among institutions and may change within a single institution over short periods (eg, monthly). In general, the most important pathogen is Pseudomonas aeruginosa, which is the most common causative agent in intensive care–acquired pneumonia and in patients with cystic fibrosis, neutropenia, early AIDS, and bronchiectasis. Other important organisms include enteric gram-negative bacteria (Enterobacter, Klebsiella pneumoniae, Escherichia coli, Serratia marcescens, Proteus Acinetobacter ) and methicillin-sensitive and -resistant Staphylococcus aureus.

Staphylococcus aureus, pneumococcus, and Haemophilus influenzae are more common when pneumonia develops within 4 to 7 days of hospitalization, and enteric gram-negative organisms are more common with increasing duration of intubation.

Previous antibiotic therapy greatly increases the likelihood of polymicrobial infection, infection with resistant organisms, especially methicillin-resistant Staphylococcus aureus, and Pseudomonas infection. Infection with resistant organisms significantly increases mortality and complicates the course of the disease.

Glucocorticoids in high doses increase the risk of Legionella and Pseudomonas infection.

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Risk factors

Endotracheal intubation with mechanical ventilation poses the greatest overall risk; ventilator-associated pneumonia accounts for more than 85% of all cases, and pneumonia occurs in 17% to 23% of ventilated patients. Endotracheal intubation compromises airway defenses, impairs cough and mucociliary clearance, and facilitates microaspiration of bacterial-laden secretions that accumulate above the inflated endotracheal tube cuff. In addition, bacteria form a biofilm on and in the endotracheal tube that protects them from antibiotics and host immunity.

In nonintubated patients, risk factors include prior antibiotic therapy, high gastric pH (due to prophylactic treatment of stress ulcers), and underlying cardiac, pulmonary, hepatic, and renal insufficiency. The main risk factors for postoperative pneumonia are age over 70 years, abdominal or thoracic surgery, and dependent functional status.

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Symptoms hospital-acquired pneumonia

In general, the symptoms of hospital-acquired pneumonia in nonintubated patients are the same as those of community-acquired pneumonia. Hospital-acquired pneumonia in critically ill, mechanically ventilated patients more often causes fever and an increase in respiratory rate and/or heart rate or changes in respiratory parameters such as increased purulent secretions or worsening hypoxemia. Noninfectious causes of deteriorating pulmonary function, such as acute respiratory distress syndrome (ARDS), pneumothorax, and pulmonary edema, should be excluded.

Forms

Hospital-acquired pneumonia includes mechanical ventilation-associated pneumonia, postoperative pneumonia, and pneumonia that develops in patients without mechanical ventilation but hospitalized in moderate or severe condition.

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Complications and consequences

The mortality rate associated with hospital-acquired pneumonia due to gram-negative infections is approximately 25% to 50%, despite the availability of effective antibiotics. It is unclear whether death results from the underlying disease or from the pneumonia itself. Women have a higher risk of death. Mortality from Staphylococcus aureus pneumonia ranges from 10% to 40%, partly because of the severity of comorbid conditions (eg, need for mechanical ventilation, older age, chemotherapy for malignancy, chronic lung disease).

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Diagnostics hospital-acquired pneumonia

Diagnosis is imperfect. In practice, nosocomial pneumonia is often suspected on the basis of a new infiltrate on chest radiograph or leukocytosis. However, no nosocomial pneumonia symptom, sign, or radiographic finding is sensitive or specific for the diagnosis, since all symptoms may be caused by atelectasis, pulmonary embolism, or pulmonary edema and may be part of the clinical picture of ARDS. The usefulness of Gram stain, sputum examination, and culture of endotracheal aspirates is questionable because specimens are often contaminated with bacteria that are either colonizing or pathogenic, so that a positive culture does not always indicate the etiologic role of the isolated organism. Bronchoscopic collection of lower respiratory tract secretions probably provides more reliable specimens, but the effectiveness of this approach is controversial. Studies of inflammatory mediators in bronchoalveolar lavage fluid may have a role in diagnosis in the future; For example, a concentration of soluble myeloid cell-expressed trigger receptor (a protein expressed by immune cells during infection) greater than 5 pg/mL may help distinguish bacterial and fungal pneumonia from noninfectious causes of clinical and radiographic changes in mechanically ventilated patients. However, this approach requires further study, and the only finding that reliably identifies both pneumonia and the causative organism is a culture of a respiratory pathogen isolated from blood or pleural fluid.

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Treatment hospital-acquired pneumonia

Some patients may have a low risk index for pneumonia that requires an alternative diagnosis. However, hospital-acquired pneumonia is treated with antibiotics that are chosen empirically based on the patient's perception of certain risk factors and the setting.

Uncontrolled use of antibiotics is the main cause of antimicrobial resistance. Therefore, treatment may begin with broad-spectrum drugs, which are replaced by the most specific drug effective against the organisms identified in culture. Alternative strategies to limit resistance, which have not been shown to be effective, include stopping antibiotics after 72 hours in patients whose pulmonary infection scores have decreased to less than 6, and regularly rotating empirically prescribed antibiotics (e.g., every 3 to 6 months).

Initial antibiotics

There are many regimens, but all should include antibiotics that cover resistant gram-negative and gram-positive organisms. Choices include carbapenems (imipenem-cilastatin 500 mg IV every 6 hours or meropenem 1-2 g IV every 8 hours), monobactams (aztreonam 1-2 g IV every 8 hours), or antipseudomonal beta-lactams (ticarcillin 3 g IV with or without clavulanic acid every 4 hours, piperacillin 3 g IV with or without tazobactam every 4-6 hours, ceftazidime 2 g IV every 8 hours, or cefepime 1-2 g every 12 hours) given alone or in combination with an aminoglycoside (gentamicin or tobramycin 1.7 mg/kg IV every 8 hours or 5-6 mg/kg once daily or amikacin 15 mg/kg every 24 hours) and/or vancomycin 1 g every 12 hours. Linezolid may be used for some lung infections, including methicillin-resistant Staphylococcus aureus (MRSA), especially in patients who cannot be treated with vancomycin. Daptomycin should not be used to treat lung infections.

Prevention

Noninvasive ventilation using continuous positive airway pressure (CPAP) or bilevel positive airway pressure (BiPAP) prevents the disruption of airway protection that occurs with endotracheal intubation and eliminates the need for intubation in some patients. The semi-upright or upright position reduces the risk of aspiration and pneumonia compared with the prone position.

Continuous aspiration of sublingual secretions through a special endotracheal tube connected to a suction device probably reduces the risk of aspiration.

Selective decontamination of the oropharynx (using topical gentamicin, colistin, and vancomycin cream) or the entire GI tract (using polymyxin, aminoglycosides, or quinolones and/or nystatin or amphotericin) also appears to be effective, although it may increase the risk of colonization with resistant organisms.

Hospital-acquired pneumonia is prevented by culture monitoring and routinely changing ventilator circuits or endotracheal tubes.

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