Community-Acquired
Pneumonia
Pneumonia is an acute lower respiratory tract (LRT) illness, usually
due to infection, associated with fever, focal chest symptoms
( ± signs) and new shadowing
on chest X-ray (CXR) (Fig. 36a). Table 1 lists microorganisms and
pathological insults that cause pneumonia.
Classification
In the clinical situation, microbiological
classification of pneumonia is not practical as causative organisms may not
be identifie or diagnosis takes several days. Likewise, anatomical (radiographical)
appearance (e.g. lobar pneumonia (affecting one lobe) or bronchopneumonia
(widespread, patchy involvement)) gives little practical in- formation about
cause. The following classificatio is widely accepted:
· Community-acquired pneumonia (CAP): describes LRT infections occurring within 48 hours
of hospital admission in patients who have not been hospitalized for more than
14 days. The most frequently identifie
organism is Streptococcus pneumoniae (20-75%). Mycoplasma
pneumoniae, Chlamydia pneumoniae and Legionalla spp., the
'atypical' bacterial pathogens (2-25%) and viral infections (8-12%) are
relatively common causes. Haemophilus influenzae and Moraxella
catarrhalis are associated with chronic obstructive pulmonary disease
(COPD) exacerbations, and staphylococcal infection may follow influenza
Alcoholic, diabetic and nursing home patients are prone to staphylococcal,
anaerobic and Gram-negative organisms.
· Hospital-acquired (nosocomial) pneumonia (Chapter 37): any LRT infections developing more
than 2 days after hospital admission. Likely organisms are Gram-negative
bacilli (∼70%) or staphylococcus (∼15%)
· Aspiration/anaerobic pneumonia: bacteroides and other anaerobic infections follow
aspiration of oropharyngeal contents (e.g. CVA).
· Opportunistic pneumonia (Chapter 39): immunosuppressed patients (e.g.
steroids, chemotherapy and HIV) are susceptible to viral, fungal and
mycobacterial infections, in addition to other bacterial organisms.
· Recurrent pneumonia: due to aerobic and anaerobic organisms occurs in
cystic fibrosi and bronchiectasis.
Epidemiology
Annual incidence: 5-11 cases per 1000 adult population; 15-45%
require hospitalization (1-4 cases per 1000) of whom 5-10% are treated in ICU.
Incidence is highest in the very young and elderly. Mortality: 5-12% in
hospitalized patients; 25-50% in ICU patients. Seasonal variation: with
peaks (e.g. Mycoplasma in autumn, Staphylococcus in spring) and
annual cycles occur (e.g. 4-yearly Mycoplasma epidemics). Frequent viral
infections increase CAP in winter.
Risk factors
Factors associated with increased risk
of CAP are listed in Table 2. Specific risk factors include age (e.g.
Mycoplasma in young adults), occupation (e.g. brucellosis in
abattoir workers and Q fever in sheep workers), environment (e.g.
psittacosis with pet birds and erlichiosis due to tick bites) or geographical
(e.g. coccidiomycosis in southwest USA). Epidemics of Coxiella burnetti (Q
fever) or Legionella pneumophila are often localized (e.g. Legionnaire's
disease may involve a specifi hotel due to air conditioner contamination).
Diagnosis
The aims are to establish the diagnosis,
identify complications, assess severity and determine classification
to aid antibiotic choice.
Clinical features
These are inaccurate without a CXR and
cannot predict causative organisms (i.e. 'atypical' pathogens do not have
characteristic presentations). Symptoms may be general (e.g. malaise,
fever, rigors and myalgia) or chest-specifi (e.g. dyspnoea, pleurisy, cough and
haemop- tysis). Signs include cyanosis, tachycardia and tachypnoea; with
focal dullness, crepitations, bronchial breathing and pleuritic rub on chest examination.
In young or old patients and atypical pneumonias (e.g. Mycoplasma), non-respiratory
features (e.g. confusion, rashes and diarrhoea) may predominate. Complications
are shown in Fig. 36e.
Investigations
Routine blood tests: white cell count (WCC) and C-reactive protein
confir infection; haemolysis and cold agglutinins occur in approximately 50% of
Mycoplasma infection; abnormal liver function tests suggest Legionella
or Mycoplasma infection. Blood gases: identify respiratory failure.
Microbiology: no microorganism is isolated in approximately 33-50% of
patients due to previous antibiotic therapy or inadequate specimen collection.
Blood cultures in severe CAP, and sputum, pleural fluid and bronchoalveolar
lavage samples, with appropriate staining, culture and assessment of antibiotic
sensitivity, may determine the pathogen and effective therapy. Serology: identifie
Mycoplasma infection but long processing times limit clinical value.
Rapid antigen detection tests for Legionella (e.g. urine) and
pneumococcus (e.g. serum and pleural f uid) are more useful. Radiology: CXR
(Fig. 36a) and CT scans aid diagnosis and detect complications.
Severity assessment
The following features are associated
with increased mortality and indicate the need for monitoring in ICU: Clinical:
age more than 60 years; respiratory rate more than 30/min; diastolic blood
pressure less than 60 mmHg; new atrial f brillation; confusion; multilobar
involvement; and coexisting illness. Laboratory: urea more than 7 mmol/L;
albumin less than 35 g/L; hypoxaemia Po2 less than 8 kPa;
leucopenia (WCC <4 x 109/L); leucocytosis (WCC >20 x 109/L);
and bacteraemia. Severity scoring: CRB-65 and CURB-65 scores, allocate
points for confusion; urea more than 7 mmol/L; respiratory
rate more than 30/min; low systolic (<90 mmHg) or diastolic (<60
mmHg) blood pressure and age more than 65 years, to stratify
patients into mortality groups suitable for different management pathways (Fig.
36c and d).
Management
Supportive measures: include oxygen to maintain Pao2
of more than 8 kPa (Sao2 <90%) and
intravenous flui ( ± inotrope)
resuscitation to ensure haemodynamic stability. Ventilatory support: non-invasive
(e.g. continuous positive airway pressure (CPAP)) or mechanical ventilation may
be required in respiratory failure (Chapter 42). Physiotherapy and
bronchoscopy: aid sputum clearance.
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Initial antibiotic therapy: represents the 'best guess', according to
pneumonia classificatio and likely organisms, as microbiological results are
not available for 12-72 hours. Therapy is adjusted when results and antibiotic
sensitivities become available. The American and British Thoracic Societies
(ATS, BTS) recommend the following initial antibiotic protocols for CAP:
· Non-hospitalized patients: usually respond to oral therapy with amoxicillin
(BTS) or an advanced macrolide (e.g. clarithromycin) or doxycycline (ATS).
Patients with severe symptoms or at risk for drug- resistant S. pneumoniae (e.g.
recent antibiotics and comorbidity) are treated with a β-lactam plus a
macrolide or doxycycline, or an antipneumococcal fluoroquinolon (e.g.
moxifloxacin alone.
· Hospitalized patients: initial therapy must cover 'atypical' organisms
and S. pneumoniae. An intravenous macrolide is combined with a β-lactam
or an antipneumococcal fluoroquinolon (ATS/BTS) or cefuroxime (BTS). If not
severe, combined ampicillin and macrolide (oral or i.v.) may be adequate (BTS).
Staphylococcal infection following influenz and H. influenzae in COPD
should be covered.
