Chapter 26 ICU Infections

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SECTION II • Medical and Surgical Crises

oropharynx of the critically ill patient becomes colo- nized. Almost all critically ill patients are colonized with nonnative Gram-negative bacteria (many of which are antibiotic resistant) by the third hospital day. All too often, a specific pathogen cannot be iden- tified, despite good samplingmethods and symptoms compatible with acute pneumonia. Mixed polymi- crobial aerobic/anaerobic infections, Mycoplasma, Chlamydia, Legionella, and viral agents become more likely candidates under these conditions. Although fungal pneumonia ( Candida/Torulopsis species, Aspergillus, or Mucor ) must be considered in the neutropenic (<500 neutrophils/mm 3 ), dia- betic, or severely debilitated patient, it occurs only rarely in immunocompetent ones. When fungal lung involvement occurs in the immunocompetent patient, it usually is the result of hematogenous seeding with Candida in a predisposed host. For some patients with chronic destructive lung dis- eases (e.g., chronic obstructive pulmonary disease [COPD] or healed cavitary tuberculosis), Aspergillus can produce a primary invasive pneumonia. Patients infected with HIV present a unique set of problems. When the CD4 T-cell counts are normal, patients infected with HIV are suscep- tible to the same organisms as any other adult in the risk categories outlined in Table 26-3. As the

CD4 count declines (especially as it falls below 200 cells/mm 3 ), the spectrum of infecting organ- isms broadens. Although routine bacterial pathogens still predominate, Pneumocystis carinii (jiroveci), Mycobacterium tuberculosis, atypical mycobacteria, and fungal infections become more likely. There is a rough correlation between the CD4 count and the infecting organism, but the linkage is not suf- ficiently strong to forgo detailed evaluation and broad coverage. Potential pathogens also are influ- enced by the prior use of prophylactic therapy. Oral trimethoprim–sulfamethoxazole prophylaxis, for example, has dramatically reduced the incidence of Pneumocystis and Toxoplasmosis infections. Regardless of the patient substrate, the choice of initial therapy for a bacterial pneumonia is always accompanied by some uncertainty, even in the pres- ence of a Gram stain “typical” of a specific organism. Historical features can help immensely in sorting through the diagnostic possibilities. For example, the sudden onset of chills, pleurisy, rigors, and high tem- perature are characteristic features for community- acquired Pneumococcus in a young adult. On the other hand, such findings may be inconspicuous in an older person, in whom confusion or stupor often predominate. A history of seizures, drug abuse, alco- holism, or swallowing disorder focuses attention on

Table 26-3.  Clinical Associations in Community-Acquired Pneumonia Patient Characteristics Likely Organisms Healthy adult

S. pneumoniae, Mycoplasma, viruses (e.g., influenza), Chlamydia, H. influenzae

S. pneumoniae, Bacteroides, oral anaerobes

Predisposed to aspiration  Stroke  Esophageal disease  Seizures  Alcohol and drug abuse  Recent dental work

All organisms listed for healthy adult plus Klebsiella species, enteric Gram negatives, Legionella , S. aureus, Branhamella species.

Chronically ill  Diabetes  COPD  Heart failure  Low-dose steroids

Postinfluenza Cystic fibrosis

S. pneumoniae, S. aureus, H. influenzae S. aureus, Pseudomonas species.

AIDS or HIV with CD4 < 200

P. carinii, S. pneumoniae, H. influenzae, M. tuberculosis , fungal infection (geographic predilection) All organisms listed for chronically ill plus Aspergillus, Mucor , and Candida

Neutropenia