10
Severe Community-Acquired Pneumonia
in Critical Care
Burke A. Cunha
Infectious Disease Division, Winthrop-University Hospital, Mineola, New York,
and State University of New York School of Medicine, Stony Brook, New York, U.S.A.INTRODUCTION
Community-acquired pneumonia (CAP) may present as mild, moderate, or severe pneumonia.
Patients with severe CAP require hospital admission and usually are admitted to the critical
care unit (CCU). Patients with severe CAP in the CCU usually are those with compromised
respiratory function requiring ventilatory support. In immunocompetent patients, severe CAP
is clinically severe primarily because of the underlying cardiopulmonary status of the patient.
While some pathogens are inherently more virulent than others, e.g.,Legionellais more virulent
than Moraxella catarrhalis, clinical severity is primarily determined by host rather than
microbial factors. A patient with Legionnaire’s disease and good cardiopulmonary function
may present with severe CAP just as a patient with severe chronic obstructive pulmonary
disease (COPD) withM. catarrhalisCAP. Patients with various degrees of hyposplenism often
present with severe CAP (1–7).
CAP may occur in normal or compromised patients, the clinical approach to determine
the cause of severe CAP depends on assessing the cardiopulmonary status, degree of splenic
dysfunction, and identifying the disorders associated with specific immune defects. Analysis
of host defense defects by history is combined with the chest X Ray (CXR)/CT scan’s
distribution of infiltrates and degree of hypoxemia (1,8). After noninfectious causes of severe
CAP are ruled out (i.e., mimics of CAP), the physician should then consider those patients who
might have CAP and a noninfectious disorder (8–10).
Empiric therapy depends upon knowing the usual pathogens associated with specific
immune defects. A cardinal principle of empiric antimicrobial therapy is that severe CAP
should be treated the same way as non-severe CAP in terms of antibiotic selection. However,
patient with severe CAP may have a longer length of stay (LOS), strong clinical course, and
may require prolonged antibiotic therapy. Therapy is continued as the diagnostic workup is in
progress. If the causative pathogen is identified, there is no rationale for changing the
antibiotics to one with a narrower spectrum. Antibiotic resistance potential is related to specific
antibiotics and is not related to antibiotic class. Changing to a narrow-spectrum antibiotic has
no effect on antibiotic resistance, i.e., withStreptococcus pneumoniaeCAP, there is no rationale to
change from ceftriaxone to penicillin because of a narrower spectrum. Therapy of severe CAP
is usually for two to three weeks in total (10–12).
DETERMINANTS OF SEVERE CAP
Microbial Virulence
The clinical spectrum ofS. pneumoniaeCAP ranges from mild in young ambulatory adults, to
fulminating, overwhelming sepsis in asplenics. Because of advanced lung disease, even low-
virulence organisms, e.g.,M. catarrhalis, may precipitate borderline, already-compromised
respiratory function.M. catarrhalis, in a patient with severe COPD and may present as severe
CAP. More than microbial virulence, host factors are the key determinants of the clinical
presentation of severe CAP (Table 1).
Secondary bacteremias associated with CAPs are reflective of the bacteremic potential of
the organism, and are not per se a marker of clinical severity. Bacteremia frequently
accompaniesS. pneumoniaeorHemophilus influenzaeCAP and is part of the clinical presentation
and is not related to CAP severity (1,13–20).