Pneumonia In The Compromised Host
The potential pathogens in patients with compromised defenses are legion.
However, likely pathogens can often be predicted on the basis of the host's
immunologic defect, x-ray changes, and the pattern of clinical symptoms.
Likely pathogens based on the type of defect in host defenses are listed in
Table 73-1. Respiratory symptoms and changes on chest x-rays may be due
to a variety of processes other than infection, such as pulmonary hemorrhage,
pulmonary edema, radiation injury, pulmonary toxicity due to cytotoxic drugs,
and tumor infiltrates.
The progression rate of the disease is helpful in identifying the responsible
mechanism. In patients with acute symptoms, likely diagnoses are bacterial
infections, hemorrhage, pulmonary edema, a leukocyte agglutinin reaction,
and pulmonary emboli. A subacute or chronic presentation is more suggestive
of a fungal or mycobacterial infection, an opportunistic viral infection,
Pneumocystis carinii pneumonia, tumor, a cytotoxic drug reaction, or radiation
injury.
The pattern of changes on chest x-rays is also helpful. X-rays showing localized
disease with consolidation usually indicate an infection involving bacteria,
mycobacteria, fungi, or Nocardia sp. A diffuse interstitial pattern is more likely to
represent a viral infection, P. carinii pneumonia, drug or radiation injury, or
pulmonary edema. Diffuse nodular lesions suggest mycobacteria, Nocardia sp,
fungi, or tumor. Cavitary disease suggests mycobacteria, Nocardia sp, fungi,
or bacteria. In transplant recipients with bilateral interstitial pneumonia, the
usual cause is cytomegalovirus, or the disease is idiopathic. A pleura-based
consolidation is usually aspergillosis. In AIDS patients, bilateral pneumonia is
usually P. carinii pneumonia.
Diagnosis and Treatment
Diagnostic strategies vary according to the clinical setting and available resources.
The first tests are usually stains and culture of expectorated sputum, but these
are often diagnostically inconclusive. Because of the risk of inadequate treatment
in compromised patients, invasive procedures (usually bronchoscopy, transthoracic
needle aspiration, or open lung biopsy) are often indicated. A biopsy, which
supplies tissue for both histology and culture, may be obtained during bronchoscopy
(a transbronchial biopsy) or an open surgical procedure. Although the latter
usually requires general anesthesia and a chest tube afterward, it permits
substantial specimens to be collected from involved sites under direct visualization;
it remains the most definitive procedure with the highest diagnostic yield. However,
many of these patients are too ill to undergo an invasive biopsy procedure; in
such cases, bronchoalveolar lavage directed by bronchoscopy can often provide
diagnostic material.
Acutely ill patients who have suspected infections are often treated with drugs
selected on the basis of probabilities and on the findings with sputum Gram stain
and culture. Treatment is adjusted on the basis of more definitive diagnostic
evaluation, as described above.
Pneumonia Caused By Gram-Negative Bacilli
Gram-negative bacilli account for < 2% of community-acquired pneumonias but for
most nosocomial pneumonias, including fatal ones. The most important pathogen
is Klebsiella pneumoniae, which causes Friedländer's pneumonia. Other usual
pathogens are Pseudomonas aeruginosa, Escherichia coli, Enterobacter sp, Proteus
sp, Serratia marcescens, and Acinetobacter sp. P. aeruginosa is a common pathogen
in patients with cystic fibrosis, neutropenia, advanced AIDS, bronchiectasis, and
pneumonias acquired in intensive care. Gram-negative bacillary pneumonias are
rare in healthy hosts and usually occur in infants, the elderly, alcoholics, and
debilitated or immunocompromised hosts, especially those with
neutropenia.
The usual pathophysiologic mechanism is colonization of the oropharynx, followed
by microaspiration of upper airway secretions. Gram-negative bacilli colonize the
upper airways in patients who have serious underlying diseases with a frequency
directly correlated with disease severity.
Symptoms and Signs
Most patients with lung infections involving K. pneumoniae or other gram-negative
bacilli have a bronchopneumonia similar to other bacterial lung infections, except
for the associated high mortality. All of these organisms, especially K. pneumoniae
and P. aeruginosa, may cause abscess formation. Friedländer's pneumonia
frequently affects the upper lobes and produces sputum that looks like currant
jelly, tissue necrosis with early abscess formation, and a fulminant course.
Diagnosis
Gram-negative bacilli should be suspected in a patient with pneumonia who is in
one of the risk categories noted above, especially with neutropenia or nosocomial
pneumonia. Gram stain of sputum usually shows numerous gram-negative bacilli;
however, distinguishing the various species and genera on the basis of
morphologic characteristics is impossible. Sputum cultures usually yield the
pathogen; false-positive cultures due to organisms that colonize the upper airways
are the major problem, especially in patients previously treated with an antibiotic
for pneumonia due to other bacteria. ("Sputum superinfection" must be
distinguished from "patient superinfection.") Positive cultures from blood, pleural
fluid, or a transtracheal aspirate obtained before treatment are considered diagnostic.
Prognosis and Treatment
The mortality for gram-negative bacillary pneumonia is about 25 to 50% despite
the availability of effective antibiotics.
Most authorities prefer a cephalosporin (cefotaxime 2 g IV q 6 h or ceftazidime 2 g
IV q 8 h), imipenem 1 g IV bid, or ciprofloxacin 500 to 750 mg po bid. Each of these
drugs can be given alone or in combination with an aminoglycoside (gentamicin
or tobramycin 1.7 mg/kg IV q 8 h or 5 to 6 mg/kg once daily or amikacin
5 mg/kg q 8 h). Aminoglycosides should not be used alone. Other drugs that can
be combined with an aminoglycoside are a cephalosporin (ceftriaxone 1 to
2 g IV q 12 h or other 3rd-generation cephalosporins), an antipseudomonad
penicillin (ticarcillin 3 g IV q 4 h, ticarcillin plus clavulanic acid 3 g IV q 4 h, piperacillin
3 g IV q 4 h, or piperacillin plus tazobactam 3 g q 6 h), or a monobactam
(aztreonam 1 to 2 g IV q 8 h). A broad-spectrum cephalosporin may be used alone,
although this regimen incurs the risk of emerging resistance during treatment,
primarily with P. aeruginosa. Most infections involving P. aeruginosa are treated
with an aminoglycoside combined with an antipseudomonad penicillin, ceftazidime,
or cefoperazone, selected on the basis of in vitro sensitivity tests.
These treatment guidelines may require modification when multiple pathogens are
suspected; expectorated sputum cultures often yield a polymicrobial flora. Optimal
regimens may also require in vitro studies of synergy. Dosage recommendations noted
above apply only to adults and should be modified when renal failure is present.
Source: The Merck Manual of Diagnosis and Therapy
Preview: Viral pneumonias resulting from community outbreaks of influenza,
respiratory syncytial virus infection, and parainfluenza can be life-threatening
in elderly and immunocompromised persons. In this article, Drs Chien and Johnson
discuss the clinical characteristics of these viruses, therapeutic options, and
preventive measures.
Vaccination
Annual fall vaccination of persons at high risk is the most effective measure for
decreasing morbidity and mortality from influenza. Humoral and, to a lesser
degree, mucosal immunity is required for protection against influenza. Antibodies
against the hemagglutinin and, to a lesser degree, neuraminidase antigens
are the major determinants of host immunity.
The effectiveness of vaccine depends on the age and general health status of the
recipient and the antigenic similarity of the influenza strains in the vaccine to
those being transmitted in the community. In years when the vaccine is well
matched to circulating influenza strains, vaccine efficacy in healthy adults is in the
range of 70% to 90% (2). Adults over the age of 65 and immunocompromised
persons receive less, but still substantial, benefit. Influenza vaccination is highly
beneficial in preventing severe influenza and death in these high-risk groups.
Annual influenza vaccination is recommended for everyone over 6 months of age
in high-risk groups (table 2), workers in chronic care facilities, household contacts
of high-risk persons, and healthy persons who wish to decrease their risk for
influenza.
The optimal time for influenza vaccination in the United States is October through
mid November. The influenza season usually lasts from late December through early
March. Persons vaccinated after a community outbreak require at least 2 weeks for
effective antibody titers to develop.
Summary
Influenza, respiratory syncytial, and parainfluenza viruses usually cause mild,
self-limited illness in adults. However, elderly and immunocompromised persons
are at increased risk for development of severe pneumonia.
Clinical and radiographic features of epidemic viral pneumonias are often nonspecific.
Newer and faster methods of viral culture and viral antigen detection have
improved the capability for definitive diagnosis in recent years.
Preventive measures for influenza virus pneumonia center on limiting exposure
of high-risk patients to active cases of influenza, administering annual vaccinations,
and providing chemoprophylaxis. Prophylaxis against RSV is effective in preventing
complications. No effective vaccines have been developed against RSV or
parainfluenza.
Therapy for viral pneumonia is primarily supportive. Amantadine may be beneficial
for influenza virus pneumonia, and ribavirin may be useful for RSV and parainfluenza
virus disease. However, further definitive studies are necessary to determine their
roles in these viral pneumonias.
