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Table 3 Invasive procedures in nosocomial pneumonia

Bronchoscopic distal airway sampling

Because of its minimally invasive character, bronchoscopy has been considered the ideal method for studying lung infections invasively, providing rapid access to lower airways under direct visual control. However, the tracheobronchial tree is usually contaminated by upper respiratory secretions during the procedure. This problem has been solved by designing various protected devices. In addition, quantitative culture of recovered material has been introduced in order to differentiate between colonizing and pathogenic bacteria.

Two bronchoscopic procedures, protected specimen brush and bronchoalveolar lavage, have been widely used in the etiological diagnosis of ventilator-associated pneumonia. Recovered bacteria are considered to be pathogenic, thus confirming the presence of pneumonia, when cultured in concentrations equal to or higher than 103 colony-forming units per milliliter in the case of protected brush or 10 4 to 105 colony-forming units per milliliter in samples obtained by bronchoalveolar lavage. However, because antibiotics rapidly affect quantitative cultures, the validity of these thresholds in patients already on antibiotics has never been well established.

Bronchoscopic sampling procedures are easily carried out in ventilated patients and, although they are not innocuous, complications are infrequent. This is not the case in patients who are not carrying an orotracheal or a tracheostomy tube, where these techniques are poorly tolerated and could precipitate the need of ventilation. Further, bronchoscopy in this setting requires considerable experience.

Distal bronchial sampling by these bronchoscopic methods has proved to be very sensitive and specific compared with lung tissue cultures. It has also provided essential epidemiological data about the etiology of ventilator-associated pneumonia. Nevertheless, the role of quantitative bronchoscopic techniques in daily medical practice is a matter of controversy, mainly because no adequate cost-benefit or outcome studies have been performed.

Non-bronchoscopic distal airways sampling

Distal bronchial sampling can also be carried out by different non-bronchoscopic 'blind' protected devices, thus avoiding the need of bronchoscopy. The evident risk of sampling a non-consolidated area does not seem to be very relevant owing to the fact that ventilator-associated pneumonia is a multifocal infection, and causative organisms are usually present even in non-consolidated areas. Although the sensitivity of these 'blind' techniques is somewhat lower than that of bronchoscopic procedures, they are much simpler and, therefore, available at any time. So far, however, the precise indications of these 'blind' have not been established.

Transthoracic needle aspiration

Because of its simplicity and low cost, transthoracic needle aspiration could be a useful procedure for diagnosing pneumonia in non-ventilated nosocomial pneumonia. This procedure has proven to be sensitive and its specificity is excellent. Because the sample is seldom contaminated, its sensitivity could be enhanced by the use of rapid immunological methods and, eventually, molecular biology techniques. Although considered hazardous, transthoracic puncture using the 25G ultrathin needle appears to be a safe procedure, although mechanical ventilation is considered to be an absolute contraindication. Transthoracic needle aspiration has proved to have a significant therapeutical relevance in non-ventilated nosocomial pneumonia. The procedure could be indicated in the case of large or easily located consolidations.

Transtracheal aspiration

Transtracheal aspiration could also be indicated in non-ventilated nosocomial pneumonia. Although it is very sensitive, the specificity in hospital in-patients could be lower owing to the increased rate of tracheobronchial colonization by nosocomial flora.

Lung biopsy

Lung biopsy by means of thoracoscopy or thoracotomy is seldom indicated in the diagnosis of nosocomial pneumonia except for immunosuppressed patients. Diagnostic approach

A variety of diagnostic strategies, such as the exclusive use of classical non-invasive procedures, the quantitative culture of endotracheal aspirates, and the use of quantitative invasive diagnostic 'blind' or bronchoscopic techniques, can be applied to nosocomial pneumonia affecting ventilated patients ( T.able.,4). At present, the cost-benefit analysis of these different approaches and their potential influence on the outcome of pneumonia are a matter of controversy. Simpler methods such as a quantitative culture of endotracheal aspirates or distal samples obtained by 'blind' procedures, are generally preferable. Ideally, these procedures should be carried out before antibiotic treatment. More sophisticated bronchoscopic procedures, particularly protected specimen brush, could eventually be used for non-responding cases.

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Table 4 Diagnostic approach to nosocomial pneumonia

The diagnostic efficacy of non-invasive procedures in non-ventilated nosocomial pneumonia is very poor. Bronchoscopic procedures are usually not well tolerated by these patients and could precipitate the need of mechanical ventilation. Transthoracic and transtracheal aspiration could be a reasonable alternative for establishing a reliable etiological diagnosis in this setting.

Bibliography

Campbell, G.D., et al. (1995). Hospital-acquired pneumonia in adults: diagnosis, assessement of severity, initial antimicrobial therapy and preventive strategies. American Journal of Respiratory and Critical Care Medicine, 153, 1711-25.

Meduri, G.U. and Johanson, W.G. (ed.) (1992). International Consensus Conference on Clinical Investigation of Ventilator-associated Pneumonia. Chest, 102 (Supplement 1).

Niederman, M.S., et al. (1993). Guidelines for the initial management of adults with community-acquired pneumonia: diagnosis, assessment of severity and initial antimicrobial treatment. American

Review of Respiratory Disease, 148, 1418-26.

Torres, A. (1996). Severe pulmonary infections, Vols I and II. W.B. Saunders, Philadelphia, PA.

Torres, A. and Woodhead, M. (1996). Pneumonia. European Respiratory Monograph 3, Vol. 2. European Respiratory Society, Sheffield.

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