Chapter 26 ICU Infections

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SECTION II • Medical and Surgical Crises

Certain clinical findings are especially notewor- thy. Legionella, Strongyloides, and Cryptosporidium may cause diarrhea and pulmonary infiltration. Concurrent infiltration of the lungs and skin may result from Pseudomonas, Aspergillus, Candida, and varicella zoster. Hepatic and pulmonary dis- eases tend to coexist during infections with CMV, Nocardia, mycobacteria, and necrotizing bacteria ( Pseudomonas, Staphylococcus ). Evaluation and Therapeutic Approach to Pulmonary Infiltrates Unfortunately, these problems often defy easy diagnosis and a tissue biopsy frequently is needed. The pace of the disease may be very rapid, so that the objective is to cover broadly while attempting to establish a specific etiologic diagnosis expedi- ently and safely. Two important questions must be answered to deal effectively with a life-threatening pulmonary process in a compromised host. First, considering that the process seems to be infectious and that the course cannot be determined easily, does a precise diagnosis need to be established or is empirical therapy sufficient? Second, if a precise diagnosis is required, what is the most efficacious technique for a fragile, critically ill patient? The answers to these questions are not straightforward and remain the subject of intense controversy. In general, the approach should vary with the severity of illness, the pace of advancement, the coagula- tion and ventilation status, the strength of ancillary information, and the experience of available per- sonnel with specific invasive procedures. If dif- fuse infiltrates cannot be distinguished confidently from pulmonary edema despite CT imaging, a brief trial of diuresis may be prudent before proceeding to invasive diagnostic measures. Even when pneu- monitis is certain, the astute clinician considers the potential contributions of hypoproteinemia and hydrostatic forces to the density of the infiltrates. Ancillary Data Both the characteristics of the chest radiograph at any single point in time and its rate of progres- sion can provide helpful diagnostic clues. Localized infiltrates, either consolidated or nodular, are most consistent with bacterial or fungal infection, hem- orrhage, or thromboembolic disease. Bilateral “interstitial” infiltrates, on the other hand, suggest volume overload, Pneumocystis, mycobacteria, or virus. However, serious lung infections may develop without impressive pulmonary infiltrates, particu-

larly in neutropenic patients. A fulminant evolution suggests a bacterial process or a noninfectious etiol- ogy (fluid overload, embolism, ARDS). Conversely, a process requiring 1 to 2 weeks for full expres- sion calls to mind mycobacterial, parasitic, or sys- temic fungal diseases. The severity of hypoxemia is another key observation. Explosive life-threatening depressions of blood oxygen tension are typical for bacterial, viral, and Pneumocystis infections but are less common with more indolent fungal and myco- bacterial processes. Though exceptions may occa- sionally be encountered, bacterial pneumonia and sepsis reliably give rise to procalcitonin elevations. Body fluids from extrapulmonary sources can sug- gest a presumptive diagnosis for the chest infiltrate. Spinal fluid may demonstrate Cryptococcus but does not prove that the radiographic infiltrates are related. Nonetheless, in the appropriate setting, pleural and joint fluids should be tapped, examined, and cultured, and a stool specimen should be sent for parasite detection. Although blood cultures are unquestionably important, serologic testing rarely provides definitive information in an appropriate time frame. Pulmonary Secretions and Tissue Sputum is produced less frequently by the com- promised host than by immunocompetent patients, especiallywhenneutropenia is present.Nonetheless, when sputum can be obtained, its careful examina- tion may reveal the responsible pathogen. In addi- tion to the routine Gram stain, a direct fluorescent antibody test for Legionella, a phase contrast or cyto- logic preparation for Blastomycosis , an acid-fast stain for mycobacteria and Nocardia, and a silver stain for Pneumocystis and fungal elements are highly worthwhile. Concentrated sputum specimens may reveal Strongyloides . In patients with AIDS, such a profusion of Pneumocystis organisms is harbored that expectorated specimens often reveal them. Unfortunately, cultures of many pathogens require days to weeks for growth, and the nearly universal practice of early, multiple broad-spectrum antibiotic use routinely obscures the diagnosis. T he N eed for B iopsy If a specific diagnosis is not in hand after review of clinical data and laboratory results, the next step should be guided by the strength of clinical suspi- cion and the urgency of making the correct diagno- sis. In most instances, bronchoscopy should be the first invasive procedure. Although coagulopathy and