Community-Acquired Pneumonia & Lower Respiratory Tract Infections

Community-Acquired Pneumonia & Lower Respiratory Tract Infections

Etiology and Pathogenesis across the Lifespan

Community-Acquired Pneumonia (CAP) is an acute infection of the pulmonary parenchyma acquired outside hospital settings. Microorganisms enter the lower respiratory tract via microaspiration of oropharyngeal secretions, aerosol inhalation, or hematogenous spread.

Typical Bacterial Pathogens

• Streptococcus pneumoniae (Pneumococcus): Encapsulated gram-positive diplococci. The most common cause of CAP across all adult age groups (up to 50% of culture-positive cases), classically causing lobar consolidation.
• Haemophilus influenzae: Gram-negative coccobacilli, common in patients with COPD, bronchiectasis, or smoking history.
• Moraxella catarrhalis: Gram-negative diplococci in underlying lung disease.
• Staphylococcus aureus: Gram-positive cocci in clusters. MSSA and MRSA cause necrotizing post-viral influenza pneumonia.
• Gram-Negative Bacilli (Klebsiella, Pseudomonas aeruginosa): Seen in severe alcoholism, structural lung disease, or recent antibiotic use.

Atypical Pathogens

Atypical pathogens lack peptidoglycan cell walls and resist beta-lactam antibiotics:

• Mycoplasma pneumoniae: Classic "walking pneumonia" in young adults, military recruits, and college students. Presents with gradual headache, low-grade fever, malaise, and dry coughing.
• Legionella pneumophila: Acquired from contaminated water systems or cooling towers. Features high fever, hyponatremia, elevated liver enzymes, and watery diarrhea alongside confusion.
• Chlamydia pneumoniae: Causes mild lower respiratory symptoms.

Viral Etiologies

Account for up to 30% of cases: Influenza A/B, SARS-CoV-2, RSV, and Human Metapneumovirus. Viral mucosal disruption predisposes to secondary bacterial superinfections.

Clinical Presentation and Atypical Geriatric Manifestations

Younger adults present with acute high fever, rigors, productive cough with purulent or rust-colored sputum, pleuritic chest pain, dyspnea, and tachycardia. Physical exam shows consolidation: crackles, bronchial breath sounds, egophony ("E-to-A"), and dullness to percussion.

Geriatric Nuances and Atypical Presentation

Older adults frequently lack classic fever, cough, or leukocytosis. Pneumonia presents subtly with:

• Acute confusion, delirium, or altered mental status
• Lethargy, weakness, or anorexia
• Acute functional decline, loss of mobility, or new-onset falls
• Decompensation of chronic conditions (worsening heart failure, glycemic instability)
• Mild tachypnea (respiratory rate > 24 breaths/min is the most reliable sign of infection in frail elders)

A low threshold for diagnostic chest radiography is essential in older adults experiencing functional decline.

Risk Stratification: CURB-65 vs. Pneumonia Severity Index (PSI)

Objective scoring tools determine disposition (outpatient vs. medical ward vs. ICU).

The CURB-65 Score

Assign 1 point for each criterion:

• Confusion: New orientation impairment.
• Uremia: BUN > 19 mg/dL (7 mmol/L).
• Respiratory Rate: >= 30 breaths/min.
• Blood Pressure: Systolic BP < 90 mmHg or Diastolic BP <= 60 mmHg.
• Age >= 65 years.

Disposition Guidelines:

• Score 0–1: Low risk (0.6%–1.5% mortality); outpatient treatment.
• Score 2: Intermediate risk (4.8%–9.2% mortality); short inpatient admission or close observation.
• Score >= 3: High risk (15%–40% mortality); inpatient admission (scores 4–5 evaluate for ICU).

Pneumonia Severity Index (PSI / PORT Score)

PSI evaluates 20 variables including age, nursing home status, comorbidities (cancer, liver, CHF, cerebrovascular, renal disease), physical exam findings (altered mental status, RR >= 30, SBP < 90, temp < 35°C or >= 40°C, pulse >= 125), and lab/imaging data (pH < 7.35, BUN >= 30, sodium < 130, glucose >= 250, hematocrit < 30%, PaO2 < 60, pleural effusion).

PSI Risk Classes:

• Class I–II (Score <= 70): Outpatient care.
• Class III (Score 71–90): Outpatient vs. brief observation unit stay.
• Class IV–V (Score > 90): Inpatient hospital admission (Class V requires ICU).

Guideline Note: ATS/IDSA guidelines explicitly prefer PSI over CURB-65 for hospitalization decisions because PSI incorporates comorbid illness more accurately, avoiding unnecessary admissions in young patients while capturing high-risk elderly outpatients.

2026 ATS/IDSA Guideline-Based Empiric Antibiotic Regimens

Outpatient WITHOUT Comorbidities

For healthy outpatients without risk factors for drug resistance:

• First-Line: Amoxicillin 1 gram orally TID (targets S. pneumoniae).
• Alternative: Doxycycline 100 mg orally BID.
• Conditional Alternative: Macrolide monotherapy (Azithromycin 500 mg day 1, then 250 mg daily) ONLY IF local pneumococcal macrolide resistance is < 25%.

Outpatient WITH Comorbidities

For patients with chronic heart, lung, liver, or renal disease; diabetes; alcoholism; malignancy; or asplenia:

• Combination Therapy (Preferred): Beta-lactam (Amoxicillin-clavulanate 875/125 mg BID or 2000/125 mg ER BID; Cefpodoxime 200 mg BID; or Cefuroxime 500 mg BID) PLUS Macrolide (Azithromycin) OR Doxycycline 100 mg BID.
• Monotherapy Alternative: Respiratory Fluoroquinolone (Levofloxacin 750 mg daily or Moxifloxacin 400 mg daily). Reserved due to FDA black-box warnings (tendonitis/rupture, neuropathy, QT prolongation, aortic aneurysm).

Inpatient Treatment Regimens

• Non-Severe Inpatient: IV Beta-lactam (Ceftriaxone 1–2 g daily or Ampicillin-sulbactam 1.5–3 g q6h) PLUS Macrolide (Azithromycin) or Doxycycline; OR IV Respiratory Fluoroquinolone.
• Severe ICU Inpatient: IV Beta-lactam (Ceftriaxone 2 g daily) PLUS IV Azithromycin OR IV Respiratory Fluoroquinolone.
• MRSA / Pseudomonas Risk: Add Vancomycin (15–20 mg/kg q8–12h) or Linezolid (600 mg q12h) for MRSA. Add Piperacillin-tazobactam (4.5 g q6h) or Cefepime (2 g q8h) for Pseudomonas.

Pneumococcal Immunization Recommendations (Updated 2026 ACIP Guidelines)

Recommended for all adults aged >= 65 years and adults aged 19–64 with underlying risk conditions (COPD, asthma, diabetes, heart failure, CKD, cirrhosis, immunocompromising conditions, active smoking, alcoholism).

2026 Vaccination Options

1. PCV20 Alone (Preferred Single-Dose Option): 1 dose of Pneumococcal Conjugate Vaccine 20-valent (PCV20). No subsequent PPSV23 booster is needed.
2. PCV15 followed by PPSV23: 1 dose of PCV15, followed by 1 dose of PPSV23 at least 1 year later (8 weeks later for immunocompromised patients).
3. PCV21 (Capvaxive): Option for adults meeting ACIP pneumococcal indications.

Follow-up, Radiographic Clearing, and Complications

Clinical improvement is expected within 48 to 72 hours. Minimum treatment duration is 5 days; patients must be afebrile for 48–72 hours before stopping antibiotics.

Radiographic Follow-Up Protocol

Radiographic resolution lags behind clinical cure, taking 6 to 12 weeks. Routine repeat chest X-rays are unnecessary in young healthy outpatients recovering promptly. However, a follow-up chest X-ray at 6 to 12 weeks is recommended for patients aged > 50 years, smokers, or those with persistent symptoms to confirm clearing and exclude underlying endobronchial malignancy.