Nosocomial pneumonia in the intensive care unit: mechanisms and significance.

The true incidence and importance of nosocomial pneumonia in the intensive care unit are difficult to assess. The commonly quoted incidence of 21-26%'-3 may be an overestimate resulting from the lack of specificity of clinical diagnostic criteria in the critically ill.45 Conventionally, fever, leucocytosis, purulent sputum, and the appearance of new and persistent infiltrates on the chest radiograph are diagnostic of pneumonia.67 Although these criteria are probably satisfactory for patients in general wards, in the intensive care unit such features as fever and leucocytosis may result from various infectious or even non-infectious disease processes. The presence of an endotracheal tube may cause local inflammation and the production of purulent sputum. Radiographic infiltrates consistent with pneumonia may represent other conditions, such as mucus retention, atelectasis, or pulmonary infarction. The diagnosis of nosocomial pneumonia is particularly difficult in patients with widespread infiltrates due to the adult respiratory distress syndrome. Even positive blood cultures cannot be accepted as definitive proof of lung infection as the organisms may arise from a non-pulmonary source. Microbiological analysis of sputum or -tracheal aspirates obtained via the endotracheal tube adds little to diagnostic accuracy and indeed may be misleading. Colonisation of the endotracheal tube and tracheobronchial secretions with potential pathogens occurs within hours of intubation, and distinguishing between tracheobronchial colonisation and pulmonary infection is difficult even if quantitative cultures are used.28 A clinical pulmonary infection score has recently been proposed that combines individually weighted clinical indices and tracheal aspirate results.9 Its predictive value for the diagnosis of pneumonia approaches that of bronchoscopic criteria. Only relatively recently have studies of nosocomial pneumonia used techniques for obtaining microbiological specimens that minimise contamination by upper airway secretions.4 ""16 Quantitative culture of bronchoscopic protected brush specimens, telescoping plugged catheter specimens, and bronchoalveolar lavage fluid has enabled bacteriological criteria to be more accurately defined for the diagnosis of pneumonia in terms of the number of organisms or colony forming units (cfu) per millilitre. Studies in ventilated patients and in the ventilated baboon model indicate that counts of more than 1iO cfu/ml from protected brush specimens or telescoping plugged catheter specimens 01516 and of more than IO' or IO' cfu/ml from lavage fluid9 1217 signify lung infection. The sensitivity and specificity of the techniques using protected brush specimens, telescoping plugged catheter specimens, and lavage fluid range from 70% to 100% (table 1). The gold standard for the diagnosis of nosocomial pneumonia is probably the histological examination and quantitative culture of lung biopsy specimens. The reliability of the techniques using lavage fluid and protected brush specimens has been shown by culture and histopathological examination of postmortem lung biopsy material obtained immediately after the bronchoscopic procedures.0 12 In the intensive care unit transthoracic, transbronchial or open lung biopsies are generally reserved for immunocompromised patients and for those with suspected interstitial or vasculitic lung disease. In these patients the potential benefit outweighs the morbidity and mortality associated with such invasive procedures. The use of cultures of bronchoscopy specimens reduces the estimated incidence of pneumonia from 26% to 9%. 14 In Oxford our use of a combination of a weighted clinical criteria score and alternate day bronchiolar lavage via a non-directed suction catheter (non-directed bronchiolar lavage) has yielded an incidence of 1 %. We believe that the decision to initiate, withhold, or discontinue antibiotic treatment should be based on these more stringent diagnostic criteria.