Chapter 26 ICU Infections

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CHAPTER 26 • ICU Infections

FIGURE 26-2. Lobar atelectasis without air bronchogram ( left ) suggests occlusion of a lobar bronchus (e.g., by mucus plug). Note the closer spacing of ipsilateral ribs ( blue arrow ) and mediastinal shift along with compenstory contralateral lung expansion ( red arrow ). The presence of an air bronchogram ( yellow arrow, right ) in conjunction with lobar volume loss indicates collapse with an open lobar passage and argues against the value of therapeutic bronchoscopy.

(CT) sharpens discrimination but may not settle the issue. An elevated level of procalcitonin, a readily measured biomarker, strongly suggests bacterial (as opposed to viral) pneumonia. Finally, widespread use of antibiotics inhibits the ability to recover a single pathogenic organism, and even when sputum cultures are positive, small numbers of colonizing bacteria are usually recovered. Causative Organisms The organisms causing pneumonia differ dramati- cally, depending on site of acquisition—community versus hospital. Common causes of community- acquired lobar pneumonia and their clinical associations are shown in Table 26-3. In the com- munity, streptococci, especially Pneumococcus , and Haemophilus influenzae, Mycoplasma, and viruses are the most common pathogens in otherwise “healthy” adults. Many underlying conditions vary this spectrum, however. In addition to the organ- isms already mentioned, patients with alcoholism, diabetes, or heart failure are predisposed to infec- tions with Klebsiella, Legionella, enteric Gram- negative rods, and Staphylococcus . When aspiration is likely (e.g., alcoholism, drug abuse, esophageal disorders), Bacteroides and other anaerobes are potential culprits. S. aureus frequently is recovered

from patients with “postinfluenza” pneumonia, and Pseudomonas species and Staphylococcus are com- mon etiologic organisms among patients with cystic fibrosis. In fact, staphylococcal disease including MRSA is now frequently encountered in patients with severe community-acquired pneumonia. Pneumonia acquired in chronic nursing care facili- ties or within 3 weeks of hospital discharge is likely to be caused by organisms usually recovered in hos- pital-acquired infections. For pneumonias that develop after the first few days in the ICU, a different, hospital-specific spec- trum predominates. Such infections are frequently polymicrobial. Gram-negative rods ( Pseudomonas aeruginosa, Klebsiella species, Enterobacter species, Acinetobacter species, E. coli, Proteus, and Serratia ) cause approximately 50% of all ICU pneumonias. Which Gram-negative organism predominates at a given hospital has a great deal to do with antibiotic pressure placed on its environment. Acinetobacter , for example, represents a significant threat in some hospitals, but by no means all. S. aureus causes another 10% to 20% of infections and its incidence appears to be higher or rising in many ICUs. The predominance of Gram-negative rods and Staphylococcus seen in the hospitalized patient is explained partially by the rapid rate at which the