Chapter 26 ICU Infections

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CHAPTER 26 • ICU Infections

Spinal fluid pleocytosis with a granulocytic predominance usually is documented in patients infected with bacteria. (Lymphocytic predomi- nance suggests aseptic meningitis, herpes simplex encephalitis, Lyme disease, listeriosis, tuberculo- sis, partially treated bacterial meningitis, or other nonbacterial cause.) In the absence of a traumatic tap, large numbers of erythrocytes are rarely seen in the cerebrospinal fluid (CSF) of bacterial men- ingitis and suggest such alternatives as herpes encephalitis, head trauma, and subarachnoid hem- orrhage. Eosinophils suggest a parasitic, cryptococ- cal, or coccidioidomycotic origin or a drug-related cause, such as those because of nonsteroidals, cip- rofloxacin, vancomycin, and trimethoprim–sulfa- methoxazole. With bacterial meningitis, the spinal fluid glucose level usually is less than 50% of the peripheral blood glucose value and CSF protein concentration often exceeds 100 mg/dL. Gram stain of spun spinal fluid demonstrates the organism in three of four cases of bacterial meningitis. It should be noted that seizures, tumors, trauma, and intra- cranial hemorrhage can mimic the CSF picture of meningitis. In particular, subarachnoid hemorrhage can present remarkably like bacterial meningitis. If the diagnosis of subarachnoid hemorrhage is not clear from head CT scan, it is useful to centrifuge a sample of freshly obtained spinal fluid and then examine the fluid for xanthochromia characteristic of subarachnoid hemorrhage. Culture establishes a definitive diagnosis of meningitis. Although cultures of spinal fluid are positive in more than 90% of untreated cases of bacterial meningitis, the specimen may be ren- dered sterile by even a single dose of oral antibi- otic. However, antibiotics rarely change the pattern of cells, glucose, or protein measurements in CSF for 12 to 24 hours. Leukocyte counts of 100/mm 3 , protein levels higher than 100 mg/dL, and glucose values lower than 30 mg/dL are typical in bacte- rial meningitis. If spinal fluid cultures are sterile, antigen agglutination tests may reveal the etiology, especially when Pneumococcus or H. influenzae is causative. Because of wide cross-reactivity, these agglutination tests are least helpful in establishing or ruling out Neisseria infections. Blood cultures, positive in one third of patients with bacterial meningitis, should be obtained before instituting antibiotics. After the diagnosis of bacterial menin- gitis has been established, the clinician should be

careful to exclude underlying pneumonia, abscess, or endocarditis before deciding on the dosing and duration of treatment. Viral, neoplastic, fungal, and tuberculous organisms all cause meningitis but generally present less urgently than acute bacterial

meningitis. Treatment

Although not nearly as contagious as widely feared, patients with suspected bacterial meningitis prob- ably should be isolated until the organism is identi- fied and 24 to 48 hours of antibiotic therapy have been administered. Even if spinal fluid cannot be obtained because of technical problems or concern over safety of the procedure, antibiotics should be administered as rapidly as feasible. If LP is con- traindicated, unavoidably delayed, or technically impossible, empiric therapy should be initiated as efforts are undertaken to establish a delayed diag- nosis by blood culture or antigen testing. Ideally, antibiotic therapy and its route of administration (intravenous, intrathecal) should be guided by Gram stain of centrifuged spinal fluid and modi- fied in accordance with its culture. Although men- ingeal inflammation improves the penetration of most antibiotics into the CSF, certain drugs cross much more efficiently than others. For example, penicillin, chloramphenicol, and selected third- generation cephalosporins (e.g., ceftriaxone) cross the blood–brain barrier easily, whereas aminoglyco- sides and other cephalosporins may fail to achieve effective concentrations. Penicillin has long been the drug of choice for community-acquired men- ingitis when lancet-shaped Gram-positive cocci are present unequivocally. Yet, because the risk of penicillin-resistant Pneumococcus is significant, a good empirical regimen for initial coverage is a third-generation cephalosporin (or meropenem) and vancomycin. Small Gram-negative rods sug- gest H. influenzae, making a third-generation cephalosporin (e.g., cefotaxime, ceftriaxone) the drug of choice. If Gram stain suggests an enteric (large) Gram-negative rod or if there is evidence of a parameningeal focus (e.g., sinusitis, spinal osteo- myelitis), an aminoglycoside should be added to a third-generation cephalosporin. (Aminoglycosides are never sufficient therapy alone, and even when clearly indicated for Gram-negative infections, consideration should be given to intrathecal admin- istration.) For cases in which spinal fluid cannot