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Pneumonia: Complete Overview

1. Definition and Epidemiology

Pneumonia is an inflammation of the lung parenchyma, most often infectious, characterized by fever, cough, sputum production, pleuritic chest pain, and a pulmonary infiltrate on imaging. It is the most common infectious cause of hospitalization and death in the United States, accounting for about 7 annual health care visits per 1000 young adults but rising to 96 visits per 1000 adults aged 85 and older. Overall, ~7 million or more CAP cases occur annually in the US, and along with influenza, it remains among the top 10 leading causes of death. The mortality rate in outpatients is usually <5%, but ranges from 12 to 40% in hospitalized patients.

Goldman-Cecil Medicine, p. 992; Harrison's Principles of Internal Medicine 22E, p. 1069

2. Classification

Type	Setting	Common Pathogens
Community-Acquired Pneumonia (CAP)	Onset outside hospital or within 48h of admission	S. pneumoniae, H. influenzae, M. pneumoniae, Legionella, viruses
Hospital-Acquired (HAP)	>48h after admission	P. aeruginosa, MRSA, gram-negative bacilli
Ventilator-Associated (VAP)	After intubation	MDR organisms
Aspiration Pneumonia	Impaired gag reflex, alcohol, neurological disease	Anaerobes, gram-negatives
Immunocompromised	HIV, transplant, steroids	Pneumocystis jirovecii (PCP), fungi, CMV, unusual bacteria

3. Etiology by Age/Patient Population

Population	Likely Pathogens
Neonates	Group B streptococci, gram-negative bacilli
Infants 3-19 weeks	Chlamydia trachomatis
Children <5 years	RSV, parainfluenza (viral predominates - up to 90%)
Children >5 years	M. pneumoniae, C. pneumoniae, S. pneumoniae
Adults (outpatient, no comorbidities)	S. pneumoniae, M. pneumoniae, C. pneumoniae, H. influenzae, respiratory viruses
Adults with cardiopulmonary disease	All above + DRSP, enteric gram-negatives, possibly anaerobes
Severe CAP (ICU)	S. pneumoniae, Legionella, S. aureus, enteric gram-negatives, P. aeruginosa
Alcoholism	K. pneumoniae, oral anaerobes, S. pneumoniae, M. tuberculosis
COPD/smoking	H. influenzae, P. aeruginosa, M. catarrhalis

Fishman's Pulmonary Diseases and Disorders, p. 2202; Harrison's, Table 131-2

4. Pathophysiology

The most common route of infection is microaspiration of oropharyngeal organisms. Pneumonia impairs oxygenation through two key mechanisms: increased shunting and increased perfusion to low V/Q ratio units. In mild to moderate disease (PaO₂ ~74 mmHg), shunt increases to ~7.5% and low V/Q regions to ~4.2%. In severe disease requiring mechanical ventilation, shunt can exceed 22% and low V/Q regions >10%. With lobar pneumonia, the inflammatory exudate begins in distal air spaces adjacent to the visceral pleura and spreads via collateral routes (pores of Kohn) to produce uniform homogeneous opacification.

5. Clinical Features

Symptoms:

Fever, chills, rigors
Productive cough (purulent, blood-streaked, or "rusty" sputum in S. pneumoniae)
Pleuritic chest pain
Dyspnea, tachypnea
Fatigue, malaise

Signs:

Tachycardia, tachypnea, fever (may be absent in elderly)
Dullness to percussion (consolidation)
Increased tactile fremitus
Bronchial breathing, crepitations (crackles) on auscultation
Egophony ("E" sounds like "A")
Decreased breath sounds (effusion)
Confusion in elderly (may be the only sign)

Atypical presentation (Mycoplasma, Legionella, Chlamydia): gradual onset, dry cough, extrapulmonary features (headache, myalgia, GI symptoms, rash), chest radiograph often worse than clinical picture.

6. Chest X-Ray Findings with Pictures

The chest radiograph is the primary imaging tool for diagnosing and classifying pneumonia. Three main radiographic patterns are recognized:

6A. Lobar / Segmental Pneumonia

The exudate fills alveoli uniformly, producing a homogeneous opacity that respects lobar or segmental boundaries. Air bronchograms (air-filled bronchi seen against a background of opacified lung) are a hallmark feature. Caused most commonly by S. pneumoniae, but also Klebsiella, Legionella, and H. influenzae.

Staphylococcal pneumonia - frontal chest X-ray showing lobar pattern

Radiograph showing staphylococcal pneumonia with lobar consolidation - from ROSEN's Emergency Medicine

6B. Round Pneumonia

A discrete, well-marginated spherical consolidation that can mimic a pulmonary mass. Resolves with antibiotics.

Round pneumonia - CXR and CT showing mass-like left upper lobe consolidation with air bronchogram on CT

Fig. 5.2 Round Pneumonia in a 64-year-old man. (A) CXR shows mass-like consolidation in left upper lobe (arrow). (B) CT shows discrete fairly well-marginated opacity containing small areas of low attenuation; confirmed as S. pneumoniae - from Grainger & Allison's Diagnostic Radiology

6C. Bronchopneumonia (Lobular Pneumonia)

Histologically peribronchiolar inflammation. Radiographically shows patchy, multifocal consolidations with poorly defined margins, predominantly in lower lobes. Air bronchogram is usually absent. Centrilobular ill-defined nodules and "tree-in-bud" opacities on HRCT. Caused by S. aureus, H. influenzae, P. aeruginosa, and anaerobes.

Bronchopneumonia CT - focal right lower lobe consolidation with air bronchogram and tree-in-bud opacities

Fig. 5.3 Bronchopneumonia (H. influenzae). Coronal CT shows right lower lobe consolidation with air bronchogram and poorly defined margins (arrows); tree-in-bud opacities visible (arrowhead) - from Grainger & Allison's Diagnostic Radiology

6D. RSV Bronchiolitis and Pneumonia (Before and After Treatment)

Chest X-ray A (before) and B (after treatment) showing lower lobe opacities then improvement

Before (A) and after (B) antibiotic therapy - Murray & Nadel's Respiratory Medicine. Bilateral lower lobe opacities in pneumonia resolving after treatment.

6E. Bilateral Diffuse Infiltrates (Severe/Interstitial Pneumonia)

Bilateral diffuse infiltrates - typical of severe/atypical pneumonia, ARDS, or PCP. From Symptom to Diagnosis, 4th Ed.

Summary of CXR Findings

Finding	Description	Association
Lobar/segmental consolidation	Homogeneous opacity, lobe-shaped	S. pneumoniae, K. pneumoniae
Air bronchogram	Air-filled bronchi visible within opacity	Lobar consolidation
Patchy infiltrates	Multifocal poorly-defined opacities	Bronchopneumonia
Interstitial pattern	Bilateral diffuse small opacities	Atypical/viral, Mycoplasma, PCP
Pleural effusion	Blunting of costophrenic angle	Parapneumonic effusion, empyema
Cavitation	Air-fluid level within opacity	S. aureus, K. pneumoniae, anaerobes, TB
Bulging fissure sign	Lobar expansion with bulging fissure	K. pneumoniae (classic)
Normal CXR	Up to 10% of confirmed PCP	Pneumocystis pneumonia

7. Diagnosis

7A. Clinical Diagnosis

Pneumonia is diagnosed by clinical signs + abnormal chest radiograph showing pulmonary infiltrates. No single test is diagnostic; a combination of history, examination, and investigations is used.

7B. Laboratory Investigations

Test	Finding/Purpose
CBC	Leukocytosis (bacterial); normal or low WBC (viral/atypical)
CRP / Procalcitonin	Elevated in bacterial; helps guide antibiotic stewardship
Blood cultures	Before antibiotics in hospitalized patients; positive in ~10-15%
Sputum Gram stain & culture	Guides narrowing of therapy
Urinary antigen	Legionella (sensitivity ~74-80%), pneumococcal (sensitivity ~70-80%)
Serology	Mycoplasma, Chlamydia (cold agglutinins, titres)
Respiratory multiplex PCR	Viral panel (influenza, RSV, COVID-19, adenovirus)
HIV test	If PCP or atypical pneumonia suspected
ABG / SpO₂	Severity assessment, hypoxemia
BUN, creatinine, electrolytes	Part of CURB-65 and severity scoring
LFTs	Especially for Legionella (hyponatremia, raised transaminases)
Bronchoscopy/BAL	Immunocompromised patients, non-responding CAP

7C. Severity Scores for Admission Decisions

CURB-65 (1 point each):

Confusion
Urea >7 mmol/L (BUN >20 mg/dL)
Respiratory rate ≥30/min
Blood pressure: systolic <90 or diastolic ≤60 mmHg
65: age ≥65 years

Score	Mortality	Recommendation
0-1	Low	Outpatient treatment
2	Intermediate	Consider hospital admission
3-5	High	Hospitalize; ICU if ≥4

Pneumonia Severity Index (PSI) is the more detailed alternative (20 variables), stratifying into classes I-V; classes I-III indicate likely safe outpatient management.

7D. ATS/IDSA Criteria for Severe CAP (ICU admission indicated if 1 major or 3 minor criteria):

ATS/IDSA Severe Pneumonia Criteria and ICU Treatment Algorithm

Figure: ATS/IDSA 2007 criteria for severe pneumonia and treatment algorithm. ICU care if 1 major or 3 minor criteria - from Fishman's Pulmonary Diseases and Disorders

Major criteria:

Invasive mechanical ventilation required
Septic shock requiring vasopressors

Minor criteria:

RR ≥30/min
PaO₂/FiO₂ ≤250
Multi-lobar infiltrates
Confusion/disorientation
BUN ≥20 mg/dL
WBC <4000/mm³
Platelet count <100,000/mm³
Core temperature <36°C
Hypotension requiring aggressive fluid resuscitation

8. Treatment

8A. Antibiotic Treatment by Site of Care

Outpatient (No Comorbidities, Low Severity)

Monotherapy for 5 days:

Amoxicillin (first-line, β-lactam monotherapy now acceptable in guidelines for low-risk)
Doxycycline
Azithromycin (where macrolide resistance is low)

Outpatient with Comorbidities (COPD, diabetes, heart/liver/renal disease, or recent antibiotics)

Respiratory fluoroquinolone (levofloxacin or moxifloxacin) monotherapy, OR
Amoxicillin/clavulanate or cefpodoxime or cefuroxime + azithromycin/clarithromycin/doxycycline

Inpatient, Non-Severe CAP (No ICU, No MRSA/Pseudomonas Risk)

Respiratory fluoroquinolone monotherapy (levofloxacin, moxifloxacin), OR
β-lactam (ampicillin-sulbactam, ceftriaxone, cefotaxime) + macrolide or doxycycline
Newer options: omadacycline or lefamulin monotherapy (noninferior to fluoroquinolone)
Note: Fluoroquinolone caution due to risk of C. difficile, tendinopathy, neuropathy, QT prolongation

Severe CAP - ICU

β-lactam + macrolide (preferred), OR
β-lactam + respiratory fluoroquinolone
Add MRSA coverage (vancomycin or linezolid) if: prior MRSA isolation, hospitalization + IV antibiotics in last 90 days, or locally validated risk factors
Add Pseudomonas coverage (piperacillin-tazobactam, cefepime, or carbapenem) if prior isolation or recent hospitalization with IV antibiotics

Aspiration Pneumonia

CAP or HAP-directed therapy (anaerobic coverage not routinely needed per current IDSA/ATS 2019 guidelines; gram-negatives predominate)
Clindamycin or metronidazole if lung abscess or empyema present

MRSA Pneumonia (CA-MRSA, post-influenza, necrotizing)

Linezolid (preferred - also inhibits toxin production), OR
Vancomycin ± clindamycin (to inhibit exotoxin production)

Atypical Pneumonia (Mycoplasma, Chlamydia)

Macrolide (azithromycin) or doxycycline or respiratory fluoroquinolone

Pneumocystis jirovecii Pneumonia (PCP)

Trimethoprim-sulfamethoxazole (first-line)
Adjunct corticosteroids if PaO₂ <70 mmHg
Goldman-Cecil Medicine, p. 993; Fishman's Pulmonary Diseases and Disorders, p. 2202-2204

8B. Duration of Antibiotic Therapy

Outpatient CAP: 5 days (per current IDSA/ATS 2019 and updated guidelines)
Inpatient non-severe: 5-7 days
Severe CAP: 7-10 days (longer for Legionella, bacteremic pneumococcal disease, necrotizing pneumonia, or lung abscess)
Switch IV to oral when clinically stable: temperature ≤37.8°C, HR <100, RR <24, SpO₂ ≥90%, BP stable, normal mental status

8C. Supportive Treatment

Oxygen therapy to maintain SpO₂ ≥94% (88-92% in COPD patients)
IV fluids for dehydration, sepsis
Analgesia for pleuritic pain (NSAIDs, paracetamol)
Antipyretics
Chest physiotherapy in patients with excessive secretions
NIV/mechanical ventilation for respiratory failure (PaO₂/FiO₂ <250 or severe hypoxemia)
DVT prophylaxis
Corticosteroids - adjunctive use in select cases of severe CAP (evidence remains limited but may reduce treatment failure in severe disease)

8D. Complications to Watch For

Complication	Features
Parapneumonic effusion / Empyema	Most common; develops in ~3-5% of CAP; requires thoracentesis ± drainage
Lung abscess	Cavitation, prolonged fever, anaerobic organisms; treat with prolonged antibiotics
Sepsis / Septic shock	Multi-organ dysfunction; requires ICU, vasopressors
Acute respiratory failure / ARDS	PaO₂/FiO₂ <200; mechanical ventilation needed
Cardiac events	Atrial fibrillation, heart failure exacerbation, MI - occur in ~20-25% of hospitalized patients
Necrotizing pneumonia	CA-MRSA, K. pneumoniae; multiple cavities
Meningitis/bacteremia	S. pneumoniae bacteremia in 10-15%

9. Prevention

Pneumococcal vaccines: PCV13 (Prevnar 13) - 85% protection against serotype-specific pneumococcal pneumonia; PPSV23 for adults ≥65 or high-risk groups
Influenza vaccine: annual; reduces secondary bacterial pneumonia after influenza
COVID-19 vaccine
Smoking cessation
Good oral hygiene (reduces aspiration risk)
Follow-up CXR: at 6 weeks post-discharge in those meeting lung cancer screening criteria to confirm resolution (per NICE NG250, 2025 and ATS guidelines)

10. Follow-Up

Clinical improvement (fever, WBC, respiratory rate) is expected within 48-72 hours of appropriate antibiotics. Radiographic clearance lags behind clinical improvement - may take 4-6 weeks to fully resolve on CXR, especially in elderly patients. Persistent consolidation at 6 weeks warrants CT scan and bronchoscopy to exclude an underlying lesion such as pneumonic-type lung adenocarcinoma. The NICE NG250 guideline (September 2025) specifically recommends this follow-up pathway.

Sources:

Goldman-Cecil Medicine International Edition (2-Volume Set), Chapter 85
Harrison's Principles of Internal Medicine, 22nd Edition (2025), Chapter 131
Fishman's Pulmonary Diseases and Disorders, Chapter 125
Grainger & Allison's Diagnostic Radiology, Chapter 5
Murray & Nadel's Textbook of Respiratory Medicine
ROSEN's Emergency Medicine: Concepts and Clinical Practice
Symptom to Diagnosis: An Evidence-Based Guide, 4th Ed.
NICE NG250 Pneumonia: Diagnosis and Management (September 2025)

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