Acute Bacterial Pneumonia

Chapter 20 Acute Bacterial Pneumonia



Kenneth Shelton, MD , Jeanine P. Wiener-Kronish, MD , Aranya Bagchi, MBBS




1 Define severe community-acquired pneumonia (CAP)


Patients with severe CAP have a number of characteristics:



image They generally require intensive care unit (ICU) management.


image They have a higher mortality rate than do patients with nonsevere CAP.


image Empiric antibiotic therapy in this group differs from that in patients with nonsevere CAP.


Unfortunately, it is challenging to prospectively identify this cohort of patients. Of particular concern are patients who are initially triaged as having nonsevere CAP but subsequently need ICU admission (up to 50% of ICU admissions fall under this category in some studies). Such patients tend to have a higher mortality than equally sick patients who have been directly admitted to an ICU. A number of severity of illness scores have been developed to help define severe CAP, a popular one being derived from the joint Infectious Diseases Society of America–American Thoracic Society guidelines for the management of CAP in adults (Box 20-1), which incorporates elements of the confusion, urea, respiratory rate, and blood pressure (CURB) score. By this definition, patients with one major criterion or three minor criteria are designated as having severe CAP. Another widely used score is the Pneumonia Severity Index (PSI). However, none of these scores has been prospectively validated for individual patients. Clinical judgment remains critical; do not blindly follow scores! In recent years other approaches have been explored to identify patients with severe CAP; some of these are discussed below (see answer 9 on recent developments in CAP).



Box 20-1 Criteria for Severe CAP



Minor Criteria




Respiratory rate > 30 breaths/min


PaO2/FiO2 ratio < 250


Multilobar infiltrates


Confusion or disorientation


Uremia (blood urea nitrogen level > 20 mg/dL)


Leukopenia (WBC count < 4000 cells per cubic millimeter as a result of infection alone)


Thrombocytopenia (platelet count < 100,000 cells per cubic millimeter)


Hypothermia (core temperature < 36° C)


Hypotension requiring aggressive fluid resuscitation



Major Criteria




Invasive mechanical ventilation


Septic shock with the need for vasopressors


Modified from Mandell LA, Wunderink RG, Anzueto A, et al: Infectious Diseases Society of America/American Thoracic Society consensus guidelines on the management of community-acquired pneumonia in adults. Clin Infect Dis 44(2 Suppl):S27-S72, 2007.



2 Which pathogens most commonly cause severe CAP?


The most common causes of severe CAP in ICU patients are (in order of decreasing incidence):



image Streptococcus pneumoniae


image Legionella sp


image Haemophilus influenzae


image Gram-negative rods (GNRs)


image Staphylococcus aureus


image Pseudomonas aeruginosa


Pathogens are identified in fewer than 50% of cases. Specific therapy seems to have no particular advantage over empiric therapy except in ICU patients, where every effort should be made to reach an etiologic diagnosis and tailor therapy accordingly. A typical work-up for an ICU patient would include sputum Gram stain in addition to sputum and blood cultures. In addition urinary antigen tests for Legionella and pneumococcus should be considered. More invasive tests, such as bronchoscopic bronchoalveolar lavage, may be considered in individual patients.



3 How is CAP diagnosed? Are the sputum Gram stain and culture diagnostically helpful for CAP?


CAP is diagnosed on the basis of the presence of a constellation of signs and symptoms (fever, cough, sputum production, and pleuritic chest pain) with radiographic evidence of lung infiltrates. Sputum Gram stain and culture can be obtained noninvasively and are inexpensive diagnostic tests. A sputum Gram stain specimen is considered satisfactory for interpretation when the neutrophil count is ≥ 25 and the epithelial cell count is ≤ 10 per low power field. Gram staining can have multiple benefits: the results can be used to broaden coverage to cover microorganisms that are typically not covered by empiric regimens, such as S. aureus and GNRs. Conversely, the absence of characteristic Gram stains and sputum culture is a strong argument for presumptively excluding S. aureus and GNRs as probable etiologies for the pneumonia. In addition, a positive Gram stain validates a subsequent sputum culture. Keep in mind the diagnostic limitations of sputum Gram stain and culture, including the inability to visualize atypical organisms, contamination by oral flora, and the difficulty encountered by some patients to provide adequate specimens.



4 What determines the selection of empiric antimicrobial therapy for patients with severe CAP?


The initial empiric antibiotic regimen for patients in the ICU with severe CAP is outlined in Box 20-2. Broadly speaking, the general principles of antibiotic therapy are as follows:



image Empiric treatment should cover the three most common pathogens causing severe CAP (see earlier), all atypical pathogens, and most relevant Enterobacteriaceae species. Broader coverage may be considered depending on epidemiologic considerations (see later).


image Combination therapy is better than monotherapy.


image Recent data strongly suggest that benefits of combination therapy are maximal when one of the agents is a macrolide. Therefore a macrolide should be included in all regimens unless a compelling reason exists not to do so.



Box 20-2 Recommended Empiric Antibiotics for Severe CAP in the ICU




A β-lactam (cefotaxime, ceftriaxone, or ampicillin-sulbactam)


PLUS


Either azithromycin


OR


A respiratory fluoroquinolone (levofloxacin [750 mg], moxifloxacin, or gemifloxacin)


If Pseudomonas is a consideration:


An antipneumococcal, antipseudomonal β-lactam (piperacillin-tazobactam, cefepime, imipenem, or meropenem)


PLUS


Either ciprofloxacin or levofloxacin (750 mg)


OR


The previously mentioned β-lactam plus an aminoglycoside and azithromycin


OR


The previously mentioned β-lactam plus an aminoglycoside and an antipneumococcal fluoroquinolone (for penicillin-allergic patients, substitute aztreonam for previously mentioned β-lactam)


If CA-MRSA is a consideration: Add vancomycin or linezolid.


Penicillin allergy: Substitute aztreonam for the previously mentioned β-lactams.


Modified from Mandell LA, Wunderink RG, Anzueto A, et al: Infectious Diseases Society of America/American Thoracic Society consensus guidelines on the management of community-acquired pneumonia in adults. Clin Infect Dis 44(2 Suppl):S27-S72, 2007.



5 What risk factors would prompt broader coverage?


Risk factors that would prompt broader antimicrobial coverage can be conveniently considered by the type of organism to be covered:



image Pseudomonas: Long-term oral steroids (> 10 mg prednisone per day), underlying bronchopulmonary disease (bronchiectasis), severe chronic obstructive pulmonary disease, alcoholism, frequent antibiotic use. Note that the strongest justification for beginning antipseudomonal coverage is the presence of a consistent Gram stain of blood or sputum.


image Community-acquired methicillin-resistant S. aureus (CA-MRSA): Patients with cavitary lesions, patients who have had influenza, patients receiving long-term dialysis, intravenous (IV) drug abusers, and patients who have had recent antibiotic treatment (particularly with fluoroquinolones). Although a consistent sputum Gram stain is a strong reason to cover for S. aureus, a blood Gram stain may be falsely positive because of contamination.


image Anaerobes: Aspiration in the setting of alcohol or drug intoxication or in the presence of gingival disease or esophageal dysmotility.


image Drug-resistant S. pneumoniae (DRSP): Age > 65 years, alcoholism, immunosuppression, exposure to antibiotics in the last 3 months (class-specific resistance), comorbidities, and exposure to children attending day care. In most cases, typical empiric therapy for CAP in the ICU (Box 20-2) should cover DRSP.



6 When should antibiotics be initiated, and what is the optimal duration of treatment?


Although earlier guidelines recommended initiation of antibiotics within 4 hours of diagnosis of CAP, more recent evidence has prompted a subtle shift in the current guidelines. They still recommend prompt initiation of antibiotics but do not specify a window of greatest benefit. The guidelines do recommend that patients admitted through the emergency department (ED) should receive their first dose of antibiotics while in the ED.


Patients with CAP should be treated for a minimum of 5 days, should be afebrile for 48 to 72 hours, and should not have more than one CAP-associated sign of clinical instability (Box 20-3) before stopping treatment.



Box 20-3 Criteria for Clinical Stability in Resolving CAP




Temperature < 37.8° C


Heart rate < 100 beats/min


Respiratory rate < 24 breaths/min


Systolic blood pressure > 90 mm Hg


Arterial oxygen saturation > 90% or PO2 > 60 mm Hg with room air


Modified from Mandell LA, Wunderink RG, Anzueto A, et al: Infectious Diseases Society of America/American Thoracic Society consensus guidelines on the management of community-acquired pneumonia in adults. Clin Infect Dis 44(2 Suppl):S27-S72, 2007.

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Jul 7, 2016 | Posted by in CRITICAL CARE | Comments Off on Acute Bacterial Pneumonia

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