What is Community-Acquired Pneumonia (CAP) and its medical management?

Community-Acquired Pneumonia (CAP) and Its Medical Management

Community-acquired pneumonia (CAP) is an infection of the lung parenchyma that occurs outside of the hospital setting and requires prompt diagnosis and targeted antibiotic therapy based on severity assessment to reduce morbidity and mortality.

Definition and Epidemiology

• CAP is one of the most common acute infections requiring hospital admission and the sixth leading cause of death in the United States 1
• It is the number one cause of death from infectious diseases in the US, with approximately 5.6 million cases annually and 1.1 million requiring hospitalization 1
• Mortality rates range from 1-5% in outpatients, averaging 12% in hospitalized patients, and approaching 40% in ICU patients 1

Pathophysiology and Risk Factors

• The increasing age of the population and presence of comorbidities (COPD, diabetes, renal insufficiency, heart failure, malignancy, etc.) contribute to higher risk of CAP 1
• Common risk factors include advanced age, smoking, and chronic medical conditions 1
• Aspiration is a common event, occurring in up to half of all adults during sleep, and can lead to aspiration pneumonia 1

Common Pathogens

• Streptococcus pneumoniae remains the predominant bacterial pathogen 2
• Other common pathogens include Haemophilus influenzae, Moraxella catarrhalis, Mycoplasma pneumoniae, Chlamydophila pneumoniae, and Legionella pneumophila 1, 3
• Drug-resistant Streptococcus pneumoniae (DRSP) and community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) are emerging concerns 1

Diagnosis

• Diagnosis is based on clinical symptoms, physical examination findings, and radiographic confirmation 2
• Empiric antibiotic therapy should be initiated in adults with clinically suspected and radiographically confirmed CAP regardless of initial serum procalcitonin level 2

Severity Assessment

• Severity assessment should guide the decision between outpatient versus inpatient treatment, with tools like CURB-65 helping identify patients who can be safely treated as outpatients 2, 4
• For patients admitted through the emergency department, the first antibiotic dose should be administered while still in the ED to minimize time to treatment 2

Treatment Approach

Outpatient Treatment

• For previously healthy outpatients with no recent antibiotic therapy: a macrolide (azithromycin) or doxycycline 2
• For outpatients with comorbidities or recent antibiotic therapy: an advanced macrolide or a respiratory fluoroquinolone 2
• For suspected aspiration with infection: amoxicillin-clavulanate or clindamycin 2

Non-Severe Inpatient Treatment

• Combined oral therapy with amoxicillin and a macrolide (erythromycin or clarithromycin) is recommended 4
• Most non-severe inpatients can be adequately treated with oral antibiotics 4
• A respiratory fluoroquinolone (levofloxacin) may provide a useful alternative for those intolerant of penicillins or macrolides 4, 5

Severe Inpatient Treatment

• Patients with severe pneumonia should be treated immediately after diagnosis with parenteral antibiotics, typically a β-lactam plus macrolide or β-lactam plus respiratory fluoroquinolone 4
• For Pseudomonas infection risk: use an antipneumococcal, antipseudomonal β-lactam (piperacillin-tazobactam, cefepime, imipenem, or meropenem) plus either ciprofloxacin or levofloxacin 1, 4
• For community-acquired MRSA infection risk: add vancomycin or linezolid 1, 4

Duration of Therapy

• Patients with CAP should be treated for a minimum of 5 days 4
• Patients should be afebrile for 48–72 hours and have no more than 1 CAP-associated sign of clinical instability before discontinuation of therapy 4
• For severe pneumonia where legionella, staphylococcal, or Gram-negative enteric bacilli are suspected or confirmed, treatment should be extended to 14-21 days 2

Switching from IV to Oral Therapy

• Criteria for switch include improvement in cough and dyspnea, afebrile (<100°F) on two occasions 8 hours apart, decreasing white blood cell count, and functioning gastrointestinal tract with adequate oral intake 1
• Even if the patient is febrile, switch therapy can occur if other clinical features are favorable 1
• If the patient has met criteria for switch, oral therapy can be started and the patient discharged on the same day, if other medical and social factors permit 1

Management of Treatment Failure

• Up to 10% of all CAP patients will not respond to initial therapy 1
• Initial antibiotic therapy should not be changed in the first 72 hours unless there is marked clinical deterioration 1
• For patients who fail to improve, a diagnostic evaluation is necessary to look for drug-resistant or unusual pathogens, non-pneumonia diagnoses, or pneumonia complications 1

Prevention

• Pneumonia can be prevented by pneumococcal and influenza vaccines in appropriate at-risk populations 1
• Smoking cessation should be promoted in all patients to eliminate an important risk factor for CAP 1

Follow-up

• Clinical review should be arranged for all patients at around 6 weeks, either with their general practitioner or in a hospital clinic 2
• A chest radiograph should be arranged at follow-up for patients with persistent symptoms or physical signs, or who are at higher risk of underlying malignancy 2

Special Considerations

• The healthcare-associated pneumonia (HCAP) categorization should be abandoned as it led to overuse of broad-spectrum antibiotics without improved outcomes 1
• Instead, clinicians should only cover empirically for MRSA or P. aeruginosa if locally validated risk factors for either pathogen are present 1