Pneumonia Study Guide

📖 Core Concepts Pneumonia = inflammation of alveoli → fluid‑filled, impairs gas exchange. Epidemiology: 450 M cases/yr (≈7 % of world population); 4 M deaths, highest in very young, very old, and chronically ill. Classification – by acquisition (community‑, health‑care, hospital‑, ventilator‑, aspiration) and by radiographic pattern (lobar, bronchopneumonia, interstitial). Common pathogens: Bacterial: Streptococcus pneumoniae (50 % CAP), H. influenzae (≈20 %), M. pneumoniae, Legionella, S. aureus. Viral: influenza, RSV, rhinovirus, coronavirus (incl. SARS‑CoV‑2). Aspiration: anaerobes + gastric flora. Key signs: cough (productive or dry), fever, pleuritic chest pain, dyspnea. Children < 5 yr: fever + cough + fast/difficult breathing. Elderly: confusion may be the dominant presentation; fever can be absent. Severity scores: CURB‑65 (Confusion, Urea, RR ≥ 30, BP low, Age ≥ 65) → guides admission. Management principle: early antibiotics + supportive care (oxygen, fluids) → lower mortality. --- 📌 Must Remember Top bacterial cause: S. pneumoniae ≈ 50 % of community‑acquired bacterial pneumonia. CURB‑65 points: each component = 1 point; 0‑1 = outpatient, 2 = short admission/close follow‑up, 3‑5 = hospitalize (often ICU). First‑line outpatient antibiotics: UK: amoxicillin. North America: amoxicillin OR doxycycline OR macrolide (azithromycin) if atypicals suspected. Hospitalized CAP: β‑lactam + macrolide (e.g., cefazolin + azithromycin). Duration: 3–5 days for most CAP (both adults & children) if clinically stable; longer for MRSA, Legionella, or complications. Vaccines: annual influenza; PCV13 (children < 2 yr) + PPSV23 (adults ≥ 50 yr or high‑risk). Procalcitonin cut‑offs: ≥0.25 µg/L → start antibiotics; <0.10 µg/L → discourage antibiotics. Oxygen target: SpO₂ 92–96 % (adults). Fluid resuscitation for septic shock: 30 mL/kg crystalloid bolus. --- 🔄 Key Processes Infection pathway Upper‑respiratory infection → micro‑aspiration (sleep) → bacteria reach alveoli → neutrophil influx → consolidation. CURB‑65 calculation Assess Confusion, Urea >7 mmol/L, RR ≥30, SBP <90 mm Hg or DBP ≤60 mm Hg, Age ≥65 → sum points. Antibiotic selection algorithm (outpatient) No comorbidities & low risk → amoxicillin. Recent antibiotic use or risk for atypicals → add macrolide or use doxycycline. Hospital admission decision CURB‑65 ≥ 2 or severe hypoxemia, mental status change, or hemodynamic instability → admit. Supportive care escalation O₂ → high‑flow nasal cannula → non‑invasive ventilation → invasive ventilation (lung‑protective, low tidal volume). --- 🔍 Key Comparisons Typical vs. Atypical bacterial pneumonia Typical (e.g., S. pneumoniae): abrupt high fever, productive rust‑colored sputum, lobar consolidation. Atypical (e.g., M. pneumoniae, Legionella): gradual fever, dry cough, extra‑pulmonary symptoms (GI, CNS), interstitial pattern. Community‑acquired vs. Hospital‑acquired pneumonia CAP: onset outside health‑care settings; most common pathogens are S. pneumoniae, H. influenzae. HAP/VAP: ≥48 h after admission/intubation; higher risk of MDR organisms (Pseudomonas, MRSA). Aspiration pneumonitis vs. aspiration pneumonia Pneumonitis: chemical injury from gastric acid → no bacteria → no antibiotics. Pneumonia: bacterial infection after aspiration → requires antibiotics. Chest X‑ray vs. Lung ultrasound CXR: widely available, good for lobar consolidation; may miss early or small infiltrates. Ultrasound: bedside, higher sensitivity for consolidations & pleural effusions when performed by trained operator; no radiation. --- ⚠️ Common Misunderstandings Fever must be present → Not true; elderly, malnourished, or severe disease may be afebrile. All viral pneumonias need antibiotics → Only if bacterial superinfection is suspected; routine antibiotics are discouraged. A normal chest X‑ray rules out pneumonia → Viral pneumonias can appear normal; early disease may be occult. Procalcitonin >0.25 µg/L always means bacterial infection → It’s a guide; clinical context remains essential. --- 🧠 Mental Models / Intuition “Air‑bag” model: Think of the alveolus as a balloon. Infection fills it with fluid and cells → the balloon loses its spring → impaired gas exchange = dyspnea. “Pathogen‑setting” rule: Community → S. pneumoniae unless risk factors (alcoholism → anaerobes/MRSA; recent flu → secondary bacterial). Hospital → broaden to MDR (Pseudomonas, MRSA). Severity ladder: Each CURB‑65 point adds a rung; once you reach the “hospital” rung (≥2), stop climbing at home. --- 🚩 Exceptions & Edge Cases Immunocompromised hosts: May present with atypical pathogens (Pneumocystis, fungal) and atypical radiographs; low yield of routine cultures. Legionella: Often presents with GI symptoms and hyponatremia; urine antigen test needed. Klebsiella (“currant‑jelly” sputum): Common in alcoholics, diabetics, or aspiration; tends to cause necrotizing pneumonia → consider early imaging for abscess. MRSA risk: Recent influenza, prior MRSA colonization, or severe skin/soft‑tissue infection. --- 📍 When to Use Which | Clinical Scenario | Preferred Test / Treatment | |-------------------|-----------------------------| | Suspected CAP in otherwise healthy adult | Chest X‑ray + amoxicillin (or doxycycline/macrolide if atypical concern) | | Severe dyspnea, SpO₂ < 90 % | Admit, give supplemental O₂, obtain arterial blood gas, start IV antibiotics (β‑lactam + macrolide) | | Elderly with confusion, no fever | Do not dismiss pneumonia; obtain CXR, consider CURB‑65, start empiric antibiotics if suspicion high | | Ventilator‑associated pneumonia | Obtain quantitative BAL culture; start broad‑spectrum coverage (e.g., carbapenem + vancomycin) pending results | | Influenza season, flu‑like illness | Test with rapid antigen or PCR; start oseltamivir ≤48 h if confirmed or high suspicion | | Aspiration event with chemical injury only | Provide supportive care, no antibiotics | | Pregnant patient | Use macrolide (azithromycin) for atypicals; avoid doxycycline & fluoroquinolones | | Pneumocystis pneumonia (HIV, CD4 < 200) | Trimethoprim‑sulfamethoxazole + adjunctive steroids if PaO₂ < 70 mm Hg | --- 👀 Patterns to Recognize Rusty sputum + lobar consolidation → classic S. pneumoniae. “Currant‑jelly” sputum + alcohol use → Klebsiella (often necrotizing). GI symptoms + hyponatremia → think Legionella. Bilateral basal infiltrates in a bedridden patient → aspiration pneumonia. Wheezing + normal CXR in flu season → viral pneumonia (often RSV or influenza). Rapid rise in RR with normal auscultation in a child → early pneumonia; look for chest indrawing. --- 🗂️ Exam Traps “All patients with pneumonia need a 10‑day antibiotic course.” – Modern guidelines accept 3–5 days if clinically stable. “Procalcitonin >0.25 µg/L guarantees bacterial infection.” – It’s a guide; clinical judgment overrides the number. “Absence of fever rules out pneumonia.” – Elderly, immunocompromised, and severe cases may be afebrile. “Macrolide monotherapy is always appropriate for CAP.” – Only when atypical pathogens are likely; beware of macrolide resistance in S. pneumoniae. “A normal chest X‑ray excludes pneumonia.” – Early viral pneumonias and small infiltrates can be missed; consider clinical context. ---