Chapter 26 ICU Infections

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CHAPTER 26 • ICU Infections

Pulmonary Infiltrates The problem of diagnosing pulmonary infiltrates in the immunocompromised patient is complex, and only a few salient features can be covered here. For febrile neutropenic patients, infection must always be the leading diagnostic consideration; however, progression of the primary neoplastic process, hem- orrhage, pulmonary edema, graft versus host disease, radiation, and drug reaction are frequent causes of pulmonary infiltrates. Although virtually any organ- ism can cause pulmonary infiltration in the com- promised host, the clinician often can integrate knowledge of the immune defect, epidemiology, and clinical and laboratory data to narrow the spectrum of likely possibilities and formulate a logical approach. As a first consideration, the underlying disease may give some clue to the nature of the pathogen. For example, AIDS, a problem predominantly of helper T lymphocytes, predisposes a patient to Pneumocystis, mycobacteria, fungal, and cytomegalovirus (CMV) infections that a presumptive diagnosis often is suggested by the radiographic and clinical pictures alone. Nonetheless, the spectrum of possibilities remains wide until the cause is confirmed by biopsy or fluid examination. Epidemiologic factors also are important to consider. The duration of hospitaliza- tion before the development of pneumonitis influ- ences the microbiology. For example, Pseudomonas, Candida, and Aspergillus infections are most likely to develop after many days in the hospital, whereas the likelihood of routine (community prevalent) pathogens wanes after the first few days of hospital confinement. Renal transplant recipients are unusu- ally prone to CMV, herpes simplex, Cryptococcus, Aspergillus, and P. carinii infections during the period of maximal T-cell suppression, 1 to 6 months after operation. Neutropenic patients are highly suscepti- ble to Gram-negative bacteria and fungal infections. ( Aspergillus and Mucor become common infecting organisms if neutropenia is sustained longer than 3 weeks.) Concurrent infection with two or more organisms occurs commonly in patients with AIDS and in those undergoing renal or marrow transplan- tation. CMV, Cryptococcus, and Nocardia frequently are recovered in conjunction with other pathogens. (CMV and Pneumocystis are commonly associated.) Superinfections also occur frequently in immuno- suppressed patients, particularly during sustained neutropenia and prolonged high-dose immunosup- pressive therapy.

Lethal infections that produce injurious toxins may also arise from the gastrointestinal tract, where some offending organisms are predisposed to prolif- erate in ischemic tissues. Patients having severe coli- tis in association with otherwise unexplained organ system compromise require urgent surgical consul- tation. This life-threatening syndrome is often but not invariably due to C. difficile . Patients experienc- ing colitis with progressive failure of remote organs despite appropriate antibiotics and supportive treat- ment may require urgent total colectomy to control the systemic toxicity of this disease. Infection in the Immunocompromised Host General Considerations Few clinical problems present a greater diagnostic challenge than fever in the immunocompromised host. Because such patients often have impaired function of multiple organ systems and undergo treatment with toxic chemotherapeutic agents, pos- sible etiologies span a wide range of noninfectious and infectious agents. Multiple causes frequently coexist. Patients in this category have primary deficits of T-lymphocyte (cell-mediated), B-cell (antibody), or granulocyte (phagocytic) function. Knowledge of the type of immune deficit can help narrow the dif- ferential diagnosis. For example, T-cell disorders pre- dispose patients to viruses and fungi, whereas B-cell disorders and granulocytopenia predispose patients to bacterial pathogens. Although loss of humoral immunity and T-cell functions predispose patients to infection, profound neutropenia (<1,000 granulo- cytes/mm 3 ) is the deficit that poses the greatest risk to life. Infections constitute a true medical emergency because in this setting, the speed with which appro- priate therapy is begun largely determines outcome. Unfortunately, establishing a specific diagnosis often proves difficult. Such patients frequently fail to pro- duce suppuration or other localizing signs of inflam- mation. Regardless of the type of immune defect or site of inflammation, the etiologic organism usually is one that normally resides as a commensal in the host. Although any site may be the target of infection, a few problems are characteristic in the neutrope- nic patient. These include mucosal infections (e.g., mucositis, gingivitis), “primary bacteremia,” soft tis- sue phlegmons (e.g., perirectal abscess), and atypical pulmonary infiltration.