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Community Acquired Pneumonia


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.

Community Acquired Pneumonia

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.

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