Respiratory Infection in the Chronically Critically Ill Patient: VENTILATOR-ASSOCIATED PNEUMONIA AND TRACHEOBRONCHITIS

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Hospital-acquired pneumonia (HAP) is a serious cause of morbidity and mortality. It is the second leading cause of nosocomial infection in the United States and the one with the highest rate of mortality.6, 26 Hospital-acquired pneumonia is defined as any pneumonia occurring after the third day of hospitalization6; that is, more than 48 to 72 hours after admission, exclusive of any pneumonia that may be incubating at the time of admission. The diagnostic criteria for nosocomial pneumonia require that the patient have an examination showing rales, dullness to percussion, or an infiltrate on chest radiograph, accompanied by at least one of the following: purulent sputum, isolation of a pathogen (from respiratory samples or blood culture), or histopathologic evidence of pneumonia. Ventilator-associated pneumonia (VAP) is a variant of HAP and is defined as bacterial pneumonia arising in any patient who has been mechanically ventilated for at least 48 hours. Criteria for definite and probable VAP have been developed (table 1), and the definition is designed to eliminate pneumonia that is incubating at the time of intubation.

In the intensive care unit (ICU), most (>85%) pneumonias are associated with mechanical ventilation6, 18, 70 and are generally VAPs. In the ICU, many patients on mechanical ventilation require tracheostomy, and many of these patients develop pneumonia, but the pathogenesis, epidemiology, and clinical significance of respiratory infection in this population may be different from those seen in the endotracheally intubated patient.

In this review, the authors focus on the problems associated with VAP, with a particular emphasis on the patient on long-term ventilation with tracheostomy who is recovering from acute (often on top of chronic) illness.

Section snippets

Incidence

A large body of literature has shown that the incidence of VAP is related directly to the duration of mechanical ventilation, suggesting that patients on long-term ventilation should have a high incidence of respiratory infection.6, 7, 8, 10, 24, 25, 28, 46 The risk for VAP is not linear over time, however, and the greatest risk for infection occurs shortly after intubation, with the daily risk actually falling over time (Table 2). In fact, in long-term ventilated patients with tracheostomy,

PATHOGENESIS OF RESPIRATORY INFECTION IN INTUBATED AND TRACHEOSTOMIZED PATIENTS

For any patient to develop pneumonia, there must be an adequate inoculum of microbes to overcome existing host defenses. In the critically ill patient, the quantity of bacteria may be high because of earlier airway colonization or exposure to large numbers of organisms from intrinsic (stomach) or extrinsic (ventilator circuits) reservoirs. In addition, many critically ill patients have impaired host defenses as a result of acute illness (shock, sepsis, and so forth), chronic illness (renal

CLINICAL FEATURES OF RESPIRATORY INFECTION IN VENTILATED AND TRACHEOSTOMIZED PATIENTS

Respiratory infection is diagnosed by most clinicians on clinical grounds, using patient symptoms, clinical findings, radiographic features, and culture results to make decisions whether to use antibiotic therapy and which organisms to target. Although the major focus for most patients is whether pneumonia is present, it is important to recognize that many patients also can develop infectious tracheobronchitis. This possibility is especially true of patients with long-term ventilation provided

MICROBIOLOGY OF RESPIRATORY INFECTION

In hospitalized patients, EGN bacteria are the most common pathogens, and among the sickest patients (those with ARDS, malnutrition, and prolonged mechanical ventilation), P. aeruginosa is predominant. In recent studies, however, the frequency of infection with S. aureus, including methicillin-resistant organisms (MRSA), has been increasing, making this the second most common organism leading to pneumonia in ICU patients.70

In addition to these organisms being common, it is important to

Empiric Therapy

When a patient meets the clinical definition of respiratory infection, antibiotic therapy should be started, using regimens that are delivered promptly and that are likely to cover the most likely causative pathogens. If initial antibiotic therapy is timely and accurate, mortality will be lower than if the initial therapy is delayed or is inappropriate.29, 43, 47 Although tracheal aspirate (or bronchoscopic) cultures should be collected in all patients, the results will not be available for

SUMMARY

The long-term ventilated patient is at high risk for developing nosocomial pneumonia or tracheobronchitis. In general, the frequency of infection increases with the duration of mechanical ventilation, but the risk appears to be greatest in the first week of intubation. Although these types of infection are common and may have morbidity and mortality impact, the daily risk is less in the long-term ventilated patient than in the acutely ill intubated patient. This reduced daily risk may reflect a

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    Address reprint requests to Michael S. Niederman, MD, Department of Medicine, Division of Pulmonary and Critical Care Medicine, 222 Station Plaza North, Suite 400, Winthrop-University Hospital, Mineola, NY 11501, e-mail: [email protected]

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