Definitions

Nosocomial pneumonia usually affects mechanically ventilated patients, hence the term "ventilator associated pneumonia (VAP)" is used synonymously. However, nosocomial pneumonia may occur in non-ventilated patients, creating a distinct entity (table 4.1). Notably, all concepts of nosocomial pneumonia refer to the non-immunosuppressed host, with absence of "immunosuppression" defined as absence of risk for infection with opportunistic pathogens.

Dividing patients with VAP into groups with early and late onset has been shown to be of paramount importance.11 Early onset pneumonia commonly results from aspiration of endogenous community acquired pathogens such as Staphylococcus aureus, Streptococcus pneumoniae, and Haemophilus influenzae, with endotracheal intubation and impaired consciousness being the main risk factors.12-15 Conversely, late onset pneumonia follows aspiration of oropharyngeal or gastric secretions containing potentially drug resistant nosocomial pathogens. Only late onset VAP is associated with an attributable excess mortality.9

The definitions of early and late onset VAP have not been standardised. Firstly, the starting point for early onset pneumonia has varied considerably, including time of hospital admission, of admission to the ICU, or of endotracheal intubation. If the time of admission to the ICU is chosen as the starting point, patients may already have been colonised in hospital and consequently differences between early and late onset pneumonia will no longer be evident.1416 In accordance with the American Thoracic Society (ATS) guidelines, we advocate using the time of hospital admission. Secondly, the cut off time separating early and late onset VAP has not been standardised. The ATS suggested using the fifth day after hospital admission.11 We have shown that colonisation of patients after head injury markedly changed between the third and fourth day in favour of nosocomial pathogens.13 Whereas the oropharynx, nose, tracheobronchial tree were initially colonized with endogenous community acquired pathogens, this pattern was subsequently changed by an increasing number of typical nosocomial pathogens. Trouillet et al have shown that isolation of drug resistant microorganisms can be predicted by the duration of intubation and antimicrobial treatment17; the cut off between early and late onset VAP used was 7 days.

Traditionally, nosocomial pneumonia is defined as occurring in patients admitted to hospital (or intubated) for at least 48 hours.18 However, this definition is no longer adequate at least for VAP because a significant number of cases occur within 48 hours of hospital admission as a consequence of intubation, particularly emergency intubation. In these patients cardiopulmonary resuscitation and continuous sedation were independent risk factors for the development of VAP while antimicrobial treatment was protective.19

Key features of the current definitions of nosocomial pneumonia are summarized in box 4.1.

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