Infectious Diseases in Critical Care Medicine

presenting with influenza pneumonia A may have an unremarkable CXR early, even with

hypoxemia present. Bilateral segmental interstitial infiltrates may appear in 48 hours and are

accompanied by severe hypoxemia.S. pneumoniaeandH. influenzaeCAP following influenza A

pneumonia sequentially after improvement are not, unlike S. aureus, accompanied by

cavitation. However, if influenza pneumonia A presents simultaneously with focal/segmental

infiltrates and rapid cavitation in<72 hours, the likely pathogen isS. aureus[methicillin-

susceptibleS. aureus(MSSA) or methicillin-resistantS. aureus(MRSA)] (Tables 13 and 14).

Avian influenza (H5N1) pneumonia and swine influenza (H1N1) pneumonia have not been

complicated by simultaneous subsequent bacterial pneumonia.

Table 13 MRSA Terminology

MRSA strain Epidemiology and microbiology Antibiotic therapy

HA-MRSAa Strainsoriginate within the hospital,have
SCCmec,I,II<III genes, PVL gene
(rare), and elaborate severalS. aureus
toxins.

Resistant to most antibiotics. Only
vancomycin, quinupristin/dalfopristin,
minocycline, linezolid, tigecycline,
or daptomycin arereliablyeffective.
CO-MRSAa Strainsoriginate in the hospital and later
present (onset) from the community,
have SCCmecI, II, III genes,
PVL gene (rare), and elaborate several
S. aureustoxins.

CO-MRSA hassame susceptibilityas

HA-MRSA andshould be treated

asHA-MRSA.

CA-MRSA SCCmecIV, V genes, PVL gene,
(common). CA-MRSA PVL-strains are
clinically indistinguishablefrom MSSA
or CO-MRSA. Elaborate the usual
S. aureustoxins plus 18 other toxins.
CA-MRSA (PVLþ) present as MRSA
pneumonia (with influenza)
or severe pyomyositis.Other MRSA
from the community should be treated
as considered as CO-MRSA.

CA-MRSA susceptible to clindamycin,

TMP-SMX, and doxycycline.

Antibiotics for CO-MRSA/HA-MRSA

are also effective against CA-MRSA,

butnotvice versa.

aAdapted from Refs. 34 and 35.

Abbreviations: CA-MRSA, community-acquired MRSA; CO-MRSA, community-onset MRSA; HA-MRSA, hospital-
acquired MRSA; PVL, Panton–Valentine leukocidin; SCC, staphylococcal cassette chromosome.

Table 14 Diagnostic Approach to the Clinical Presentations of Severe Human Seasonal Influenza A Pneumonia

Initial presentation of acute
human seasonal influenza A pneumonia Likely pathogens Empiric antimicrobial therapy

Severe hypoxemia (A–a gradient>35) None
.No infiltrates (early) or bilateral diffuse
infiltrates (later)on CXR/chest CT scan

Influenza A

(human, avian, swine) Oseltamiviramantadine

orrimantadine

.Focal/segmental infiltrateson CXR/chest

CT scan with rapid cavitation (<72 hours)

Influenza AMSSA/

CA-MRSA

Avian influenza (H5N1)

Swine influenza (H1N1)

if also MSSA/CA-MRSA CAP;

tigecyclineorlinezolid

Initial presentation of human seasonal influenza A pneumonia followed by interval of improvement followed by
bacterial CAP (5-–7 days)

Mild/moderate hypoxemia(A–a gradient<35)
.Focal/segmental infiltrates onCXR/chest
CT scan

S. pneumoniae Levofloxacinormoxifloxacin

orceftriaxone

H. influenzae

Abbreviations: CAP, community-acquired pneumonia; CXR, chest X Ray; MSSA, methicillin-susceptible
S. aureus; CA-MRSA, community-acquired methicillin-resistantS. aureus; CT, computed tomography.

Severe Community-Acquired Pneumonia in Critical Care 173