Pneumonia
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Pneumonia - Medical Student Exam Overview
Sources: Harrison's Principles of Internal Medicine 22E (2025), Fishman's Pulmonary Diseases, Grainger & Allison's Diagnostic Radiology
Definition
Pneumonia is an infection of the pulmonary parenchyma. It is frequently misdiagnosed, mistreated, and underestimated despite carrying significant morbidity and mortality. - Harrison's Principles of Internal Medicine 22E, p. 1066
Classification by Acquisition Setting
| Type | Definition | Key Features |
|---|---|---|
| CAP (Community-Acquired) | Acquired outside hospital or healthcare settings | Most common; S. pneumoniae is the leading cause |
| HAP (Hospital-Acquired) | Develops ≥48 h after admission | Gram-negatives, S. aureus dominate |
| VAP (Ventilator-Associated) | Develops ≥48-72 h after endotracheal intubation | High mortality; bronchopneumonia pattern typical |
| Aspiration Pneumonia | Macroaspiration of oropharyngeal/gastric contents | Accounts for 5-15% of CAP; gravity-dependent lung zones |
The older category "healthcare-associated pneumonia (HCAP)" has been discontinued - it did not reliably predict resistant organisms and led to overuse of broad-spectrum antibiotics. - Harrison's 22E
Pathophysiology
Route of Entry
Microorganisms reach the lower respiratory tract by:
- Microaspiration of oropharyngeal organisms (most common)
- Inhalation of airborne droplets
- Hematogenous spread from a distant focus
- Direct mucosal dispersion
Positive Feedback Loop Model (Modern Concept)
The lung is not a sterile environment - it has its own microbiota. Pneumonia is not simply "invasion of a sterile space" but an emergent phenomenon:
- Inflammatory event → epithelial/endothelial injury → cytokine/chemokine/catecholamine release
- Some catecholamines selectively promote growth of S. pneumoniae and P. aeruginosa
- This creates a self-accelerating loop: more inflammation → more nutrients for bacteria → more bacterial growth → dominant pathogen emerges
- In CAP/HAP, the trigger is often a viral infection + microaspiration
Host Defense Mechanisms (that can be overcome)
- Nasal hairs, turbinates, branching airways
- Mucociliary clearance
- Gag and cough reflexes
- Alveolar macrophages, neutrophils, secretory IgA
Mediators of Damage
- IL-6, TNF → fever
- IL-8, G-CSF → neutrophil recruitment
- Neutrophil/macrophage mediators → alveolar capillary leak → impaired oxygenation, hypoxemia, infiltrates
- Some bacteria interfere with hypoxic vasoconstriction → severe hypoxemia
Pathology - Classic Stages (Lobar Pneumococcal Pneumonia)
These four stages are a high-yield exam topic:
| Stage | Histology | Key Feature |
|---|---|---|
| 1. Edema | Proteinaceous exudate + bacteria in alveoli | Vascular engorgement; serous fluid |
| 2. Red Hepatization | RBCs fill alveoli; neutrophils appear | Lung = liver-like consistency; red-red color |
| 3. Gray Hepatization | RBCs lysed; neutrophils + fibrin dominate; bacteria disappear | Infection being contained; gas exchange improves |
| 4. Resolution | Macrophages clear debris (neutrophils, bacteria, fibrin) | Return to normal |
This staging best applies to lobar pneumococcal pneumonia; VAP typically shows a bronchopneumonia pattern instead. - Harrison's 22E, p. 1067
Types by Radiographic Pattern
This is a high-yield classification for matching pathogen to X-ray appearance:
| Radiographic Pattern | Characteristic | Common Organisms |
|---|---|---|
| Lobar consolidation | Entire lobe opacified; air bronchograms; no volume loss | S. pneumoniae, Klebsiella pneumoniae |
| Bronchopneumonia | Patchy, multifocal infiltrates; peribronchial distribution | S. aureus, Gram-negative bacilli, anaerobes, S. pneumoniae |
| Interstitial pneumonia | Reticular/reticulonodular pattern; peribronchovascular infiltrate | Viruses, Mycoplasma pneumoniae |
| Cavitation | Air-fluid level within opacity | S. aureus (pneumatoceles), M. tuberculosis, Gram-negatives, anaerobes |
| Miliary | Diffuse tiny nodules | M. tuberculosis, endemic fungi, viruses |
Important caveat: No radiographic pattern is specific for any one organism. Overlap is common and differentiation based solely on CXR is unreliable. - Grainger & Allison's Diagnostic Radiology
Lobar Pneumonia mechanism: Organisms induce alveolar inflammatory edema. When consolidation fills an entire lobe, an air bronchogram appears (airway outlined by surrounding fluid-filled alveoli). Volume loss is absent or minimal in early consolidation. - Fishman's Pulmonary Diseases
Typical vs. Atypical Pneumonia
| Feature | Typical | Atypical |
|---|---|---|
| Onset | Acute (hours) | Subacute (days) |
| Presentation | Productive cough, rigors, pleuritic chest pain, high fever | Dry cough, headache, myalgias, low-grade fever |
| Pathogens | S. pneumoniae, H. influenzae, Klebsiella | Mycoplasma, Chlamydophila pneumoniae, Legionella, viruses |
| CXR | Lobar consolidation | Interstitial/patchy bilateral infiltrates |
| Sputum | Purulent, productive | Scant or absent |
| Response to beta-lactams | Yes | No (need macrolide/fluoroquinolone) |
Common Causative Pathogens
CAP Pathogens (most to least common):
- Streptococcus pneumoniae (most common overall)
- Mycoplasma pneumoniae (most common atypical; young adults)
- Haemophilus influenzae (COPD patients)
- Legionella pneumophila (atypical; hyponatremia, GI symptoms, Pontiac fever)
- Chlamydophila pneumoniae
- Respiratory viruses (influenza, RSV, SARS-CoV-2, metapneumovirus)
- Staphylococcus aureus (post-influenza; cavitation/pneumatoceles)
- Klebsiella pneumoniae (alcoholics; "currant jelly sputum"; upper lobe)
Clinical Features
Symptoms:
- Fever, chills, sweats, tachycardia
- Cough (non-productive or productive - mucoid, purulent, blood-tinged)
- Gross hemoptysis → suggests necrotizing pneumonia (think CA-MRSA)
- Dyspnea, pleuritic chest pain
- GI symptoms (nausea, vomiting, diarrhea) in up to 20% of patients
- Fatigue, headache, myalgias, arthralgias
Signs on examination:
- Tachypnea; accessory muscle use
- Dullness to percussion (consolidation/effusion)
- Increased tactile fremitus (consolidation)
- Crackles (crepitations), bronchial breath sounds, pleural friction rub
- Elderly: may present with new-onset confusion or worsening of chronic illness with few respiratory symptoms
Diagnosis
Clinical Diagnosis
Requires: compatible history (cough, fever, dyspnea, sputum) + new infiltrate on chest radiography
The sensitivity/specificity of physical examination alone is only 58% and 67% respectively.
Imaging
| Modality | Role |
|---|---|
| CXR (PA + lateral) | First-line; determines extent, detects complications (cavitation, effusion, pneumothorax) |
| CT chest (HRCT) | More sensitive; detects infiltrates up to 5 days earlier than CXR; used if CXR normal but clinical suspicion high, or to rule out alternative diagnoses |
False-negative CXR occurs in: early disease, neutropenia, hypovolemia, PCP (Pneumocystis jirovecii)
CT Images of Viral Pneumonia
CT - Influenza pneumonia (peripheral ground-glass opacities):
CT - COVID-19 pneumonia (bilateral ground-glass opacities + consolidation):
Laboratory (for hospitalized patients)
- Blood cultures (before antibiotics) - low yield (~14%) but important
- Sputum Gram stain + culture (if quality sputum obtainable)
- Urine antigen tests: S. pneumoniae antigen (high sensitivity/specificity), Legionella antigen
- Nasopharyngeal swab/PCR: influenza, SARS-CoV-2, RSV
- Inflammatory markers: CRP and procalcitonin (PCT) - PCT is insufficiently accurate alone and should NOT be used to withhold antibiotics
Microbiological gram stain (microscopy):
Sputum from pneumococcal lobar pneumonia - Gram-positive lancet-shaped diplococci with polymorphonuclear leukocytes.
Severity Scoring (Site-of-Care Decision)
CURB-65 (simpler, bedside)
| Letter | Criterion |
|---|---|
| C | Confusion (new onset) |
| U | Urea >7 mmol/L (BUN >19 mg/dL) |
| R | Respiratory rate ≥30/min |
| B | Blood pressure: systolic ≤90 or diastolic ≤60 mmHg |
| 65 | Age ≥65 years |
- Score 0 → outpatient (30-day mortality ~1.5%)
- Score 1-2 → hospitalize (unless solely due to age ≥65)
- Score ≥3 → consider ICU (mortality ~22%)
PSI (Pneumonia Severity Index)
- More validated and accurate than CURB-65 but complex (20 variables)
- Classes I-V; Class V mortality = 29.2%
Direct ICU admission criteria
- Septic shock requiring vasopressors
- Acute respiratory failure requiring mechanical ventilation
- Or ≥3 of 9 minor criteria (tachypnea, hypoxemia, multilobar infiltrates, confusion, uremia, leukopenia, thrombocytopenia, hypothermia, hypotension)
Complications (High-Yield)
| Complication | Notes |
|---|---|
| Parapneumonic effusion / Empyema | Most common with S. pyogenes, S. aureus |
| Lung abscess | Cavitation with air-fluid level; anaerobes, S. aureus, Klebsiella |
| Bacteremia | Occurs in ~25-30% of pneumococcal CAP |
| ARDS | Bilateral infiltrates; PaO2/FiO2 <300 |
| Respiratory failure | May require mechanical ventilation |
| Septic shock | End-organ failure |
High-Yield Exam Mnemonics
CURB-65 = severity scoring for CAP
Classic pathogens by setting:
- Young, healthy, walking pneumonia → Mycoplasma
- Elderly, community → S. pneumoniae
- Post-influenza, cavitation → S. aureus
- Alcoholic, upper lobe, "currant jelly" sputum → Klebsiella
- Nosocomial/ventilated → Pseudomonas, Acinetobacter, MRSA
- Hyponatremia + GI symptoms → Legionella
- Immunocompromised (HIV, CD4 <200) → Pneumocystis jirovecii
Stages of lobar pneumonia: Congestion → Red hepatization → Gray hepatization → Resolution (Come Right Go Rest)
Sources: Harrison's Principles of Internal Medicine 22E (2025) Ch. 131 | Fishman's Pulmonary Diseases and Disorders Ch. 122 | Grainger & Allison's Diagnostic Radiology Ch. 5
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