pathogens causing CAP. It is a common clinical misconception that because a patient is
immunocompromised, the pathogen range is extensive. Excluding the usual CAP pathogens
also seen in normal hosts, the range of pathogens in compromised hosts is defined and limited
by the immune defect, i.e., CAP patients with multiple myeloma are prone to CAP due to
typical encapsulated bacterial pathogens not viruses,Rickettsia, or parasites. If the clinician has
determined by history/laboratory tests that the patient has multiple myeloma, then the
pathogens are predictable and not extensive or unusual. The clinical approach, therefore, rests
on the relationship between the disorders, which is the determinant of the immune defect,
which, in turn determines the potential pathogen. The range of potential pathogens determines
what constitutes appropriate empiric antimicrobial coverage in normal/immunocompromised
patient with severe CAP (1,2,8,9,10).
Disorders Associated with Impaired B-Lymphocyte/Humoral Immunity (HI)
The disorders associated with impaired B-lymphocyte function are those that decrease
humoral immunity (HI). The pathogens predisposed to by impaired B-lymphocyte function
are the same regardless of the underlying disorder. CAP pathogens associated with impaired
HI are the encapsulated pulmonary pathogens, i.e.,S. pneumoniae, H. influenzae.The conditions
associated with decreased HI commonly encountered in clinical practice include disorders
with hyposplenia/asplenia, multiple myeloma, cirrhosis, systemic lupus erythematosus (SLE),
and chronic lymphocytic leukemia (CLL) [a combined B-/T-lymphocyte disorder—HI>CMI
(cell-mediated immunity)]. The degree of hyposplenism may be inferred from the CBC by
noting the concentration of Howell–Jolly bodies (percentage) in the peripheral smear. The
number of Howell–Jolly bodies is inversely proportional to the degree of splenic dysfunction.
The most common clinical presentation of CAP associated with hyposplenism is an
“apparently normal” host with good cardiopulmonary function that presents as
otherwise unexplained severe CAP. Severe CAP always has an underlying cardiopulmonary/
virulent pathogens aside, immunologic explanation. Patients presenting with CAP and
hypotension/shock have either impaired splenic function, influenza alone or with
Table 4 Clinical Approach to Severe CAP
I. Presumptive diagnosis of severe CAP
.Initial CXR
.Assess clinical severity
.Sputum/blood cultures
.Ventilatory and vital organ support
II. Consider disorders that mimic severe CAP
.Chest CT scan to r/o CAP and limit differential diagnostic mimics of severe CAP
.Treat medical disorders mimicking severe CAPIII. Consider severe CAP presentation due to noninfectious mimics of CAP
superimposed on mild/moderate CAP
.Initiate therapy for treatable medical disorders mimicking severe CAP
.Also,treat underlying mild/moderate CAP
IV. Empiric therapy of severe bacterial CAP
.Initiate empiric antibiotic therapy
.Order additional diagnostic laboratory tests to identify specific CAP pathogens
.Select an agent active against both typical/atypical CAP pathogens
Moxifloxacin or levofloxacin
Tigecycline
Ceftriaxoneplusdoxycycline or azithromycin
.If influenza A pneumonia with early focal/segmental infiltrates and rapid cavitation<72 hours (MSSA/
CA-MRSA CAP)
Linezolid
Tigecycline
.If influenza A pneumonia with improvement after 5–7 days, followed by late focal/segmental infiltrates
Moxifloxacin or levofloxacin
Ceftriaxone
Abbreviations: CAP, community-acquired pneumonia; CXR, chest X ray; CT, computed tomography.
Severe Community-Acquired Pneumonia in Critical Care 167