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  • Pneumonia in the critically ill hospitalized patient: part 6

critically ill

Gram-negative bacilli, such as Escherichia colt, Klebsiella pneumoniae, and Pseudomonas aeruginosa are the most common types of bacteria causing noso­comial pneumonia. Staphylococcus aureus, which can be methicillin-sensitive or resistant, accounts for a large portion of hospital-acquired pneumonia in some hospitals. Nosocomial pneumonia due to Le­gionella pneumophila has been reported in certain geographic areas such as Pittsburgh, Burlington (Ver­mont), and Los Angeles. These infections are usually associated with cooling-tower reservoirs or hospital water which is heavily colonized with the organism. Anaerobes have been cultured from approximately 30 percent of infected patients, but appear to be less important than aerobic pathogens. Most nosocomial pneumonias are caused by more than one pathogen.

Fatality rates for patients with nosocomial pneu­monia remain high in many series. Nosocomial pneumonia contributed to 60 percent of the fatal nosocomial infections in a study of 200 consecutive hospital deaths by Gross et al. Stevens et al reported fatality rates of 50 percent for intensive care unit patients with hospital-acquired pneumonia compared to 3.5 percent of patients without pneumonia and rates were higher for patients infected with Pseudo­monas aeruginosa. In our study of 233 mechanically ventilated patients, there was a 55 percent fatality rate for patients with pneumonia compared to a rate of 25 percent for patients without pneumonia. These data underscore the need for earlier recognition, treat­ment, and prevention.
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Table 2—Factors That May Increase the Risk of Nosocomial Pneumonia by Altering Colonization, Increasing the Risk of Aspiration, or Impairing Host Defenses

Host factors: Age
Obesity Coma
Underlying disease-(chronic lung disease, congestive heart failure, diabetes mellitus, AIDS, systemic lupus, cancer, central nervous system diseases, seizures, head trauma, uremia, malnutrition Drugs:
Heroin, cocaine, alcohol, sedatives, antibiotics, antacids, histamine type-2 blockers, steroids, cytotoxic drugs Invasive Devices:
Intubation, tracheostomy, mechanical ventilation, nasogastric tube Surgery:
Head and neck, chest, abdomen

Dr. Niederman: What are some of the risk factors for nosocomial pneumonia?

Dr. Craven: Aspiration of bacteria from the oropharynx is the primary route of entry into the lung, and a number of factors affect the type and number of bacteria that colonize the oropharynx. Most of us aspirate bacteria into our lower airways daily, but pneumonia does not develop. The development of pneumonia is not only related to the numbers and types of bacteria that are aspirated, but also ability of the lungs mechanical, humoral, and cellular defenses to contain and prevent infection.

Risk factors that may increase the risk of nosocomial pneumonia are summarized in Table 2. Host factors such as age and underlying disease may either allow more bacteria to enter the lung or impair removal of bacteria by various pulmonary host defense mecha­nisms. Drugs or medications may either alter con­sciousness or promote infection by impairing removal of pathogens from the lung. Invasive devices and surgery increase the risk of aspiration or removal of bacteria from the lung.

Table 3—Effect of Intubation on Oropharyngeal Colonization and the Pathogenesis of Pneumonia

Endotracheal Tube:
—bypasses nasopharynx and mechanical trapping of particles —impairs normal temperature and humidity of air —acts as a foreign body and produces local trauma —impairs ciliary clearance and natural removal of secretions —impairs swallowing —changes mouth flora
—cuff leaks contaminated secretions from the oropharynx —nasotracheal tube may cause sinusitis —allows organisms to directly reach the lung Nasogastric Tube:
—foreign body that impairs swallowing —causes stagnation of oropharyngeal secretions —increases reflux through lower esophageal sphincter —acts as a conduit for bacterial migration —may increase risk of sinusitis

As mentioned, the use of an endotracheal tube may increase the risk of nosocomial pneumonia at least 6- to 21-fold compared to nonintubated patients. The presence of the endotracheal tube impairs re­moval of bacteria, allows leakage of pathogens around the cuff, and causes local trauma and inflammation (Table 3). The upper part of the tracheobronchial tree contains heavily ciliated epithelium and mucus that can trap and clear bacteria from the lung. The cilia beat hundreds of times per minute, in unison, to move mucus and bacteria out of the trachea. Colonization of the tracheobronchial tree may decrease or alter this activity, and increase the number of pathogens to be cleared by alveolar macrophages, polymorpho- nuclear leukocytes, humoral antibodies (IgM, IgG, IgA), and complement. In addition, the endotracheal tube can impair swallowing, and alter the hosts diet and thus his intestinal flora. The use of a nasogastric tube is also a widely unappreciated risk factor for pneumo­nia, that may increase the risk of nosocomial pneu­monia by several different mechanisms (Table 3). Dr. Niederman: Dr Craven, could you comment on the role of respiratory therapy equipment in the pathogen­esis of pneumonia?
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