Hospital-Acquired Pneumonia
Introduction
Pneumonia is one of the commonest of the ‘nosocomial infections’, that is, infections which are acquired by patients as a result of their stay in hospital. The incidence rises with age with rates of up to 15 per 1,000 hospitalized patients >65 years of age. It is associated with significant mortality, particularly when it affects patients in intensive care units (10-50% in ventilated patients).
Several factors contribute to the increased risk of pneumonia in hospitalized patients: the proximity of other patients harbouring potential respiratory pathogens; immunocompromisation due to existing disease or treatment; impaired airways clearance due to anaesthesia, depressed consciousness, or post-surgical pain; disturbance of the normal commensal flora of the upper respiratory tract due to treatment with broad-spectrum antibiotics and intubation to facilitate mechanical ventilation.
The pathogenic organisms which invade the lung in hospitalized patients are often derived from the population of organisms which have colonized the patient’s own upper respiratory tract. The intensity of this colonization, the species of organisms involved, and their resistance to antibiotics are adversely influenced by treatment with broad-spectrum antibiotics and access for mechanical ventilation. Established hospital strains of antibioticresistant organisms may become predominant in this colonization. Poor infection control practice, particularly by clinical staff who come in close contact with the patients, contributes to cross-infection with such hospital strains.
Accurate clinical diagnosis of hospital-acquired lung infection is often difficult because a number of disease processes apart from infection show similar clinical and radiological manifestations. Microbiological diagnosis is also often uncertain because of the difficulty in distinguishing the organisms which are invasive from those which are merely colonizers.
Aetiology
The aetiological agents involved in hospital-acquired pneumonia are related to the origin of the infection. Crossinfection, from other patients or health care workers, involves viruses such as influenza A or B, adenoviruses, respiratory syncytial virus and, more recently, SARS coronavirus. Diagnosis is rapid by immunofluorescence tests or specific molecular probes and polymerase chain reaction (PCR). Bacterial pathogens such as methicillin-resistant Staphylococcus aureus (MRSA) and resistant Gram-negative bacilli (Pseudomonas aeruginosa, Acinetobacter spp., antibioticresistant ‘coliforms’) may also be acquired by cross-infection and may cause pneumonia in vulnerable patients, e.g. ventilated patients or neutropenic patients. Infections can also arise from the hospital environment. Legionella pneumophila commonly originates from faulty hospital water and ventilation systems, Aspergillus fumigatus from airborne contamination, and resistant Gram-negative organisms from contaminated equipment.
Patients on ventilators are particlarly at risk. Infection usually derives from hospital-acquired colonization of the patient’s upper respiratory tract with potential pathogens such as MRSA, resistant Gram-negative bacilli, or yeasts. These are often strains which are selected for resistance because of antibiotic therapy. Less commonly, infection arises from contamination of airways, equipment, or humidifiers with resistant Gram-negative bacilli.
Presentation
Patients with hospital-acquired pneumonia may present with the following features:
New onset pyrexia.
Change of sputum or tracheal aspirate characteristics such as increasing volume, increased viscosity, and/or increasing purulence.
Development of leucocytosis on the full blood count.
New lung infiltrate on the chest radiograph unexplained by other causes.
Investigations
Isolation of specific aetiological agents and antibiotic susceptibility tests should always be attempted, since infection is often due to resistant organisms. Samples of respiratory secretions for microscopy and culture may include:
Expectorated sputum.
Aspirates of nasopharyngeal secretions (for viruses).
Endotracheal secretions.
Bronchoalveolar lavage.
Pleural fluid.
Transbronchial biopsy.
Open lung biopsy.
Antibiotics agents
Methicillin-resistant strains of Staphylococcus aureus are always resistant to all penicillin and cephalosporin antibiotics. They are also often resistant to other groups of antibiotics such as quinolones (e.g. ciprofloxacin, ofloxacin) and macrolides (e.g. erythromycin, clarithromycin), and some strains are resistant to aminoglycosides (e.g. gentamicin, tobramycin). They are almost always sensitive to glycopeptide antibiotics such as vancomycin or teicoplanin, although there have been rare reports of partial resistance.
Antibiotics for oral administration which are effective against many strains of MRSA include rifampicin, fucidin, tetracycline, and cotrimoxazole. Effective new antibiotics such as linezolid have been recently introduced into clinical care.
Illustrative cases
Figures 2.1,2.2,2.3,2.4 show Gram-staining and culture of sputum samples from patients with possible hospital-acquired infection. A Gram stain of a sample from a neutropenic patient with severe hospital-acquired pneumonia and sepsis is shown (2.1), demonstrating long, slender, Gram-negative bacilli. Culture yielded nonlactose fermenting colonies of Pseudomonas aeruginosa which are oxidase positive (2.2). Nonmucoid and mucoid strains are identified.
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