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Pneumonia - community acquired

Updated February 2007 by Charles Gomersall

Definition

Acute infection of pulmonary parenchyma that is associated with at least some symptoms of acute infection and is accompanied by presence of an acute infiltration on CXR or auscultatory findings consistent with pneumonia and that occurs in patient who is not hospitalized or residing in a long term care facility for ≥14 days prior to onset of symptoms.

Aetiology

See table 1

Pneumococcal infection is by far the commonest cause and most frequent cause of severe pneumonia and of death. Prevalence probably underestimated and most cases in which no organism is identified are probably due to pneumococcal infection.
Other causes of community acquired pneumonia are relatively uncommon. Include Mycoplasma pneumoniae, Haemophilus influenzae, Legionella spp., influenza, RSV and other viruses, Staph. aureus, Chlamydia pneumoniae, Strep. pyogenes and Moraxella catarrhalis, Klebsiella pneumoniae, Neisseria meningitides
Haemophilus influenzae can cause pneumonia in previously fit patients as well as those with chronic lung disease.
Pneumonia due to both Mycoplasma pneumoniae and influenza virus tends to occur in epidemics. Mycoplasma epidemics tend last for 3 winters while influenza epidemics tend to occur annually. Mycoplasma tends to affect teenagers and young adults but may occur in older patients.
Influenza virus infection commonly associated with secondary bacterial infection, usually Strep pneumoniae or, more seriously, Staph aureus. 50% of all staphylococcal pneumonias occur in association with influenza infection.
Gram negative infection is unusual in community-acquired cases, except, possibly, in the elderly and those with chronic lung disease, especially bronchiectasis
Legionella and Staph. aureus are much more common in patients with CAP that is severe enough to require mechanical ventilation
Elderly residents of old-age homes may be infected with similar organisms to patients with hospital acquired pneumonia

Clinical features

systemic and respiratory manifestations.
fever, sweats, rigors, cough, sputum production, pleuritic chest pain, dyspnoea, tachypnoea, pleural rub and inspiratory crackles common
classic signs of consolidation in < 25%
other organ systems may be involved depending on the type and severity of pneumonia
diagnosis may be more difficult in the elderly. Although the vast majority have respiratory symptoms and signs, > 50% may also have non-respiratory symptoms and > 1/3 may have no systemic signs of infection.

Investigation

Should not delay administration of antibiotics. Delays of as little as 8 hours are associated with worse outcome.

Important investigations in all patients

Chest X-ray
Arterial blood gases or oximetry
Full blood count
Urea & electrolytes
Liver function tests
Blood cultures (x2)
Sputum (if immediately available) for urgent gram stain, culture +/- pneumococcal antigen. NB. Pulmonary secretions should be transported to laboratory and processed within 2 h so that any fastidious organisms that may be present (eg Strep. pneumoniae) do not die
Pleural fluid (if present) for gram stain, culture, pH and leukocyte count - all patients with a pleural effusion >1 cm thick on a lateral decubitus film
Urinary Legionella antigen (specific but only moderately sensitive). Note that currently available tests detect only serogroup 1.
Pneumococcal urinary antigen
- used to augment standard diagnostic methods
- rapid result (~15 mins)
- sensitivity 50-80%, specificity ~90%
- may identify some cases of pneumonia due to pneumococcus that are not detected by other methods
HIV status

Other investigations to consider

Chlamydia pneumoniae Ig M by microimmunofluorescence
- titre of 1:16 or greater considered positive
influenza virus rapid antigen assay
fibreoptic bronchoscopy combined with the use of the protected specimen brush and broncho-alveolar lavage is useful in some cases but its exact role is not well defined. Recommended for patients with a fulminant course or who have pneumonia unresponsive to therapy. The procedure for obtaining samples and complications of the procedure are outlined in table 3. These techniques need to be combined with quantitative culture to minimize the effect of contamination of the sampling channel of the bronchoscope with upper respiratory tract organisms.

Treatment

General

oxygen
fluid to correct dehydration and provide maintenance requirements
organ support

Antimicrobials

Administration of antibiotics should not be delayed until microbiological results are available and therefore treatment should be started on a "best guess" basis.
Ideally each unit should have its own regimes tailored to the local flora and antibiotic resistance patterns. In their absence those in figure 1 may be useful.

Figure 1. Antibiotic regimes for treatment of severe community acquired pneumonia (CAP) in critically ill patients. Advanced macrolides include azithromycin and clarithromycin. Non anti-pseudomonal 3^rd generation cephalosporins include cefotaxime and cetriaxone. Respiratory fluoroquinolones include moxifloxacin and levofloxacin. Note that fluoroquinolones may not be appropriate drugs for treatment of community acquired pneumonia in many parts of Asia because of the high minimum inhibitory concentrations exhibited by pneumococcal isolates from many Asian countries.
There is controversy regarding the appropriate change to empiric therapy based on microbiological findings. There are concerns that changing to more narrow spectrum antimicrobial cover may result in inadequate treatment of the 5-38% of patients with polymicrobial infection. Furthermore dual therapy may be more effective than monotherapy even when the identified pathogen is sensitive to the agent chosen (ref).
Drug resistant Streptococcus pneumoniae (DRSP):
- penicillin MIC <4 mg/L - regimes in figure 1 may be appropriate
- MIC ≥4 mg/L - antipneumococcal fluoroquinolone, clindamycin, vancomycin, teicoplanin or linezolid
Role of antivirals in severe influenza pneumonia is unclear. Early treatment (<48 hours from onset of symptoms) of less severe influenza pneumonia with zanamivir or oseltamivir is associated with a shorter duration of symptoms
- Oseltamivir is recommended as first-line therapy for patients with suspected avian influenza A/H5N1
Empiric antibiotic therapy for suspected bacterial superinfection in patients with influenza pneumonia should cover Strep. pneumoniae, Staph. aureus, Haemophilus influenzae
Pneumonia due to varicella zoster virus and herpes simplex virus should be treated with acyclovir
Treatment for Legionnaires' disease (with azithromycin or a fluoroquinolone) is indicated when there is epidemiological evidence of the disease, even in the face of negative diagnostic tests
- risk factors for Legionnaires':
  - recent travel with overnight stay outside of home
  - exposure to spas
  - recent changes in domestic plumbing
  - renal or hepatic failure
  - diabetes
  - systemic malignancy
Click here for IDSA recommended antimicrobial therapy for other pathogens

Duration of therapy

No good data
Courses as short as 5 days may be sufficient but antibiotics should be continued until:
- patient has been afebrile for 48-72 hours
- organ dysfunction has largely resolved
Short courses may be suboptimal for:
- bacteraemic Staph aureus pneumonia
- meningitis or encephalitis complicating pneumonia
- infection with less common organisms (eg Burkholderia pseudomallei), fungi or Pseudomonas aeruginosa

Response to therapy

Average time to resolution of fever depends on severity, age of patient and organism
- 7 days in elderly patients
- 2.5 in young patients with pneumococcal pneumonia
- 6-7 in bacteraemic patients with pneumococcal pneumonia
- 1-2 in patients with M. pneumoniae pneumonia
- 5 in patients with Legionella pneumonia
Blood and sputum cultures usually negative within 24-48 h although P. aeruginosa and M. pneumoniae may persist in sputum despite effective treatment
CXR changes lag behind clinical changes. Only 20-30% of elderly patients, patients with underlying illness or patients with extensive pneumonia on presentation have a clear CXR by 4 weeks

Failure to respond

Wrong diagnosis
Wrong drug or dose or route of administration
Underlying host factors (eg immunocompromise, mechanical obstruction of bronchus)
Complication (eg empyema, hospital acquired pneumonia, bronchiolitis obliterans organizing pneumonia)
Drug resistant organism
Multiple organisms
Concurrent infection at another site
Non-infectious cause of fever

ICU admission

Indications:

Need for advanced organ support (eg invasive mechanical ventilation, vasopressors)
ICU admission should also be considered for patients with ≥3 of:
- pH<7.3
- PaO₂/FiO₂ ratio ≤250 (PaO₂ in mmHg) or ≤33 (PaO₂ in kPa)
- hypothermia (core temperature) <36ºC
- need for aggressive fluid resuscitation
- confusion
- urea >7 mmol/l (20 mg/dl)
- leucopaenia
- thrombocytopaenia
- multi-lobar infiltrates