Epidemiology

HAP is defined as an acute infection of the pulmonary parenchyma occurring in a patient who has been in hospital for at least 48 h that was not present at the time of admission. In 1995 the American Thoracic Society (ATS) classified HAP on the basis of risk factors, severity (mild to moderate or severe), and the time of the pneumonia relative to the patient’s admission (early or late). The applicability of these classifications remains controversial and, as such, is beyond the scope of this article. In 2005 the ATS and the IDSA revised the guidelines to include healthcare-associated pneumonia (HCAP), as well as simplifying the classification of HAP to early and late regardless of disease severity. Broadly, HCAP includes pneumonia in any patient who was in an acute care hospital for 2 or more days within 90 days preceding the infection; who resided in a nursing home or long-term care facility; who received IV antibiotic therapy, chemotherapy, or wound care within 30 days preceding the current infection; or who attended a hospital or hemodialysis clinic. VAP is defined as pneumonia occurring 2 to 3 days after endotracheal intubation.

HAP accounts for 15% of all nosocomial infections and 27% of hospital-acquired infections in the ICU. The incidence of HAP ranges from 5 to 10 cases per 1000 hospital admissions. The National Nosocomial Infections Surveillance (NNIS) system has reported that the median rate of VAP per 1000 ventilator days among participating hospitals ranged from 2.9 to 15.1 in pediatric and trauma ICUs, respectively. The mortality rate attributed to HAP ranged from 20% to 33%, while VAP, the more serious form of HAP, may account for 60% of deaths due to a nosocomial infection. HAP may prolong patient stay in the ICU by 4.3 to 6.1 days and the total stay in hospital by 4 to 9 days. The cost of the increased length of hospital stay in the United States was estimated at $41 000 per patient, and direct annual costs of US$2 billion have been estimated.
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Risk Factors

The risk factors for HAP and VAP have been grouped into 4 categories: factors that enhance colonization of the oropharynx or stomach, conditions favouring aspiration or reflux from the gastrointestinal tract, conditions requiring mechanical ventilation and exposure to contamination through personnel or respiratory devices, and host factors; the latter include advanced age, malnutrition, and severe underlying conditions such as immunosuppression (see also Table 6).

Table 6. Risk Factors in Hospital-Acquired Pneumonia and Ventilator-Associated Pneumonia




Factors Predisposing




Factors Predisposing to



to
Aspiration




Colonization


Witnessed aspiration


Chronic obstructive pulmonary disease


Supine positioning


Antacids or histamine type-2 antagonists


Coma


Tracheostomy


Enteral nutrition


Acute respiratory distress syndrome


Nasogastric tube


Prior exposure to antibiotics


Reintubation


Age > 60 years


Tracheostomy


Patient transport


Acute respiratory distress


syndrome


Head trauma


Monitoring of intracranial


pressure

If colonized by pathogenic organisms, the oropharynx and stomach may serve as reservoirs for those organisms. In HAP and VAP, the oropharynx appears to be the predominant reservoir. Factors facilitating bacterial colonization include antibiotic exposure, admission to the ICU, or underlying chronic pulmonary disease. In healthy individuals, the corrosive pH of the stomach (below 2) eradicates most bacteria entering the gastric space, which results in an essentially sterile environment. Alterations in stomach flora, resulting from an increase in gastric pH, have been observed in patients receiving an H2-antagonist or a proton pump inhibitor. Among patients receiving omeprazole, organisms found in the gastric contents were similar to those in the patients’ oral cavity. In a study of ICU patients, Donowitz and others noted a predominance of hospital-acquired gram-negative bacteria in the gastric contents of patients receiving cimetidine and antacids (both of which result in a gastric pH above 4). Other factors that may increase gastric pH include advanced age, achlorhydria, ileus or upper gastrointestinal disease, and enteral feeding.
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Conditions favouring aspiration or gastrointestinal reflux include endotracheal intubation, insertion of a nasogastric tube, supine position, coma, surgery of the head, neck, or upper torso, and immobilization because of trauma or illness. Prolonged mechanical ventilation increases the probability of exposure to contamination through respiratory devices and the probability of nosocomial spread of pathogens by the health care team (e.g., contaminated or colonized hands). Factors such as more than one intubation, mechanical ventilation for longer than 72 h, prior aspiration of gastric contents, and COPD are associated with an increased risk of VAP in mechanically ventilated patients. In addition to the endotracheal tube providing a direct passage for oropharyngeal organisms into the lungs, bacteria may collect and form a glycocalyx matrix (biofilm) on the tube, which serves as a reservoir for infection. The lung may be inoculated if the biofilm is dislodged as a result of mechanical air flow or manipulation of the endotracheal tube (movement or suctioning).