Occasional review Prevention of nosocomial bacterial pneumonia

The term “nosocomial pneumonia” broadly covers all infections occurring 48 hours or more after hospital admission excluding any infection incubating at the time of admission, and has also been called hospital acquired pneumonia. Intensive care unit (ICU) acquired pneumonia (occurring within 48 hours of admission to the ICU) and ventilator associated pneumonia (occurring within 48 hours of starting mechanical ventilation) are also included in the broader term “nosocomial pneumonia”. The development of nosocomial pneumonia remains a major problem in the ICU with most studies reporting an incidence of between 9% and 45%, depending on the groups of patients being studied, the definition of nosocomial pneumonia, and the criteria used to diagnose it. It has been shown that nosocomial pneumonia acquired in the ICU markedly increases the length of hospital stay 16 20 21 and the costs of hospital care. 22 Mortality rates may also be increased, 5 7 16 17 19 23 although it is not entirely clear whether all deaths from nosocomial pneumonia are directly related to the development of an infection. The so-called “attributable mortality”, defined as the mortality occurring as the direct result of the nosocomial pneumonia, may be especially high when Pseudomonas or Acinetobacter species are involved as pathogens. The diagnosis of nosocomial pneumonia is not straightforward, particularly in patients who are critically ill, as routine parameters do not have a high specificity for pneumonia in these patients. For example, infiltrates on chest radiographs consistent with pneumonia may be due to many other processes including oedema, atelectasis, and infarction. Positive cultures from tracheal aspirates are also non-specific as the upper respiratory tract of most critically ill patients is colonised by potential pulmonary pathogens. Alternative diagnostic techniques such as protected specimen brush biopsies and bronchoalveolar lavage have therefore been used, although a recent pilot study suggested that complicated culture sampling using these techniques has no beneficial therapeutic influence over more simple endotracheal aspirate cultures. The various methods employed in the diagnosis of nosocomial pneumonia have been reviewed elsewhere. 29 This review focuses on methods of preventing the development of bacterial nosocomial pulmonary infections. Pathogenesis of nosocomial pneumonia For a nosocomial pneumonia to occur, one or both of the following factors must be present: (1) the lower respiratory tract must be invaded by bacteria in suYcient numbers or of particular virulence and (2) pulmonary and systemic host defences must be downregulated. With the high costs of nosocomial pulmonary pneumonia and the associated increased mortality, measures to prevent the development of such infections are important and can be considered in two groups—those aimed at preventing colonisation and those aimed at increasing host defences (table 1).