Acute infection of lung parenchyma producing alveolar inflammation and consolidation

Combination of respiratory symptoms + systemic inflammatory features

Severity varies from mild ambulatory illness to life-threatening respiratory failure

⸻

Typical Clinical Presentation

Fever with chills and rigors

Productive cough with purulent sputum

Pleuritic chest pain worsened by breathing or coughing

Dyspnea due to impaired gas exchange

Fatigue and generalized weakness

⸻

Respiratory Manifestations

Cough initially dry → later productive

Sputum yellow, green, or rust-colored (pneumococcal)

Shortness of breath proportional to extent of lung involvement

Pleuritic pain suggests pleural inflammation

⸻

Systemic Features

Malaise and anorexia from cytokine response

Myalgia and headache

Sweating and chills

Confusion or delirium in elderly patients

⸻

Atypical Presentation (Common in Elderly & Immunocompromised)

Absence of high fever

Confusion or functional decline as primary presentation

Mild cough despite significant pneumonia

Unexplained hypoxia

⸻

Clinical Pattern Variations

Typical bacterial pneumonia → sudden onset, high fever, productive cough

Atypical pneumonia → gradual onset, dry cough, prominent systemic symptoms

Aspiration pneumonia → foul-smelling sputum, dependent lung involvement

Complications arise from uncontrolled infection, inflammatory injury, or systemic spread

• Parapneumonic pleural effusion — inflammatory fluid accumulation adjacent to infected lung.

• Empyema thoracis — pus collection in pleural cavity requiring drainage.

• Lung abscess — localized necrosis with cavitation, common in aspiration or anaerobic infection.

• Acute respiratory failure — severe gas exchange impairment → hypoxia ± ventilation support.

• Acute Respiratory Distress Syndrome (ARDS) — diffuse alveolar damage due to cytokine storm.

• Sepsis — systemic inflammatory response with organ dysfunction.

• Septic shock — persistent hypotension despite fluid resuscitation.

• Bacteremia — hematogenous spread of infection.

• Metastatic infections — meningitis, endocarditis, septic arthritis (especially pneumococcal).

• Exacerbation of chronic diseases — COPD, asthma, heart failure destabilization.

• Delirium / encephalopathy — common in elderly patients.

• Pleural fibrosis or restrictive lung defect — late complication after severe infection.

• Death — risk increased in elderly, immunocompromised, or delayed treatment.

Conditions that mimic pneumonia clinically or radiologically must be excluded

• Acute bronchitis — cough without focal consolidation on imaging.

• Viral upper respiratory infection — systemic symptoms but clear lung fields.

• Pulmonary tuberculosis — chronic cough, weight loss, apical infiltrates/cavitation.

• COVID-19 infection — bilateral ground-glass opacities, viral prodrome.

• Influenza pneumonia — seasonal, myalgia prominent.

• Pulmonary embolism — sudden dyspnea, pleuritic pain, normal or wedge-shaped opacity.

• Acute pulmonary edema (heart failure) — bilateral perihilar shadows, cardiomegaly.

• Aspiration pneumonitis — dependent lung involvement, altered consciousness history.

• Lung malignancy with post-obstructive infection — persistent localized opacity.

• Interstitial lung disease flare — diffuse interstitial pattern without infection signs.

• Sarcoidosis — bilateral hilar lymphadenopathy, chronic course.

• Hypersensitivity pneumonitis — exposure history + diffuse infiltrates.

Infection occurs when microorganisms overcome airway defense mechanisms and reach alveoli

Most commonly caused by bacteria, followed by viruses and atypical organisms

⸻

1. Bacterial Causes (most common in adults)

Streptococcus pneumoniae — most common classical cause

Haemophilus influenzae — common in smokers & COPD patients

Moraxella catarrhalis — elderly and chronic lung disease

Staphylococcus aureus — post-influenza pneumonia

Gram-negative bacilli (Klebsiella, E. coli) — alcoholics, diabetics, debilitated patients

⸻

2. Atypical Bacterial Causes

Mycoplasma pneumoniae — young adults, mild “walking pneumonia”

Chlamydophila pneumoniae — prolonged dry cough illness

Legionella pneumophila — contaminated water systems, severe systemic illness

⸻

3. Viral Causes

Influenza virus — major seasonal cause

SARS-CoV-2 (COVID-19)

Respiratory Syncytial Virus (RSV)

Adenovirus, Parainfluenza, Human metapneumovirus

⸻

4. Aspiration-Related Causes

Oropharyngeal anaerobic flora entering lung during impaired consciousness

(alcohol intoxication, stroke, seizures, elderly dysphagia)

⸻

5. Host Predisposing Factors

Impaired mucociliary clearance (smoking, pollution)

Chronic lung disease (COPD, bronchiectasis)

Diabetes mellitus

Malnutrition or advanced age

Immunosuppression

⸻

Core Mechanism

Microaspiration of pathogens + impaired host defense → alveolar infection → inflammatory exudate → consolidation

Diagnosis is clinical supported by radiological and microbiological evidence

Investigations confirm pneumonia, identify severity, and detect causative organism

⸻

Basic Investigations (First Line)

Chest X-ray — confirms lung consolidation

• Lobar consolidation → typical bacterial pneumonia

• Patchy/interstitial infiltrates → atypical organisms

• Helps exclude TB, malignancy, pulmonary edema

Complete Blood Count (CBC)

• Leukocytosis with neutrophilia → bacterial infection

• Normal or mild elevation → viral/atypical causes

CRP / ESR

• Elevated inflammatory markers support infection severity

• Useful for monitoring treatment response

Pulse oximetry

• Detects hypoxia early

• Guides need for oxygen therapy or admission

⸻

Microbiological Investigations

Sputum Gram stain and culture

• Identifies bacterial pathogen

• Helps antibiotic selection

Blood cultures (before antibiotics in moderate–severe cases)

• Detect bacteremia (poor prognostic indicator)

Nasopharyngeal PCR panel

• Influenza, RSV, SARS-CoV-2, atypical pathogens

Urinary antigen tests

• Streptococcus pneumoniae antigen

• Legionella pneumophila antigen

⸻

Biochemical Tests (Severity Assessment)

Renal function tests (urea, creatinine)

• Used in CURB-65 severity scoring

Liver function tests

• Baseline before therapy; systemic involvement

Serum electrolytes

• Hyponatremia may suggest Legionella infection

⸻

Advanced / Hospital Investigations

Arterial Blood Gas (ABG)

• Assesses respiratory failure

• Indicates need for ICU care

Procalcitonin (optional)

• Helps differentiate bacterial vs viral infection

• Guides antibiotic duration

⸻

When Further Imaging Needed

CT chest

• Non-resolving pneumonia

• Suspected abscess, malignancy, or complications

Pneumonia is primarily an alveolar disease, not airway disease.

Gas exchange failure occurs due to alveolar filling → ventilation–perfusion mismatch.

Fever + cough + focal chest signs together are more diagnostic than imaging alone.

Crepitations represent reopening of fluid-filled or collapsed alveoli during inspiration.

Bronchial breath sounds occur when consolidated lung transmits sound better than aerated lung.

Normal chest X-ray early does NOT exclude pneumonia.

Radiological changes may lag 24–48 hours behind symptoms.

Hypoxia severity depends more on lung area involved than organism type.

Elderly patients often present WITHOUT fever.

Confusion, fall, or weakness may be the only presentation.

Persistent fever after antibiotics → think complication, not treatment failure.

(Abscess, empyema, resistant organism, wrong diagnosis)

Right lower lobe pneumonia commonly results from aspiration due to bronchial anatomy.

Viral infection often precedes bacterial pneumonia.

Epithelial injury → impaired mucociliary clearance → secondary bacterial invasion.

Clinical improvement usually precedes radiological resolution.

X-ray clearing may take weeks.

Overuse of antibiotics in viral infections promotes resistance and microbiome disruption.

Severe pneumonia causes systemic inflammation → sepsis risk.

Infection begins when pathogens bypass upper airway defenses and reach terminal bronchioles & alveoli

Microaspiration during sleep is common — disease occurs when host immunity fails to clear organisms

⸻

Alveolar macrophages recognize pathogens via pattern-recognition receptors (TLRs)

Release of pro-inflammatory cytokines (IL-1, IL-6, TNF-α) → neutrophil recruitment

Neutrophils migrate into alveoli → phagocytosis + release of reactive oxygen species

⸻

Increased capillary permeability → protein-rich exudate fills alveoli

Alveolar spaces become filled with inflammatory cells, fibrin, and debris → consolidation

Gas exchange impaired → ventilation-perfusion mismatch → hypoxemia

⸻

Lobar Pneumonia Pattern (e.g., Streptococcus pneumoniae)

Rapid spread through pores of Kohn → uniform lobe involvement

Classical stages: congestion → red hepatization → grey hepatization → resolution

⸻

Bronchopneumonia Pattern

Patchy inflammation centered around bronchioles

More common in elderly, hospitalized, or immunocompromised

⸻

Atypical Pneumonia Pattern

Primarily interstitial inflammation

Minimal alveolar exudate → less consolidation

Dry cough + diffuse infiltrates

⸻

Severe Disease

Excess cytokine response → systemic inflammatory response (SIRS)

Bacteremia → sepsis

ARDS in severe viral or bacterial pneumonia

⸻

Core Mechanism

Pathogen invasion + exaggerated inflammatory response → alveolar exudation → impaired oxygen diffusion

1. Sputum Examination

• *Gram stain of sputum*

• Rapid bedside identification of bacterial type

• Gram-positive diplococci → Streptococcus pneumoniae

• Gram-negative bacilli → Klebsiella, H. influenzae

• Mixed flora → aspiration pneumonia

• *Sputum culture & sensitivity*

• Confirms causative organism

• Guides antibiotic selection

• Best sample: early morning deep cough specimen

• *Quality criteria (important clinically)*

• Many neutrophils

• Few epithelial cells → true lower respiratory sample

⸻

2. Blood Tests (Microbiological pathology)

• *Blood culture*

• Detects bacteremia

• Important in severe CAP and sepsis

• Positive in pneumococcal pneumonia, staphylococcal infection

⸻

3. Antigen Detection Tests

• *Urinary pneumococcal antigen*

• Useful even after antibiotics started

• *Legionella urinary antigen*

• Rapid diagnosis of Legionella pneumonia

⸻

4. Molecular Tests

• *PCR respiratory panel*

• Detects viral pathogens (Influenza, RSV, SARS-CoV-2, HMPV)

• Detects atypical bacteria (Mycoplasma, Chlamydia)

• *RT-PCR*

• Gold standard for viral pneumonia diagnosis

⸻

5. Bronchoscopy Specimens (Severe / unclear cases)

• *Bronchoalveolar lavage (BAL)*

• Culture, cytology, PCR analysis

• Used in ventilated or immunocompromised patients

• *Protected specimen brush sampling*

• Reduces contamination from upper airway flora

⸻

6. Pleural Fluid Analysis (if effusion present)

• *Thoracentesis fluid analysis*

• Cell count, protein, LDH

• Gram stain & culture

• Detects:

• Parapneumonic effusion

• Empyema

⸻

7. Histopathology (Rarely required)

• Lung biopsy (only when diagnosis uncertain) shows:

• *Neutrophilic alveolar exudates* (bacterial pneumonia)

• *Interstitial lymphocytic infiltration* (viral pneumonia)

• *Organizing pneumonia pattern*

⸻

Clinical Reasoning Note (Important)

Pathological tests identify the organism → imaging identifies distribution → clinical signs determine severity.

CAP diagnosis is therefore clinicoradiological, while pathology confirms etiology.

Prevention focuses on reducing infection exposure, improving host immunity, and preventing aspiration

• Annual influenza vaccination — prevents secondary bacterial pneumonia.

• Smoking cessation — improves mucociliary clearance and airway defense.

• Adequate nutrition and hydration — supports immune function.

• Hand hygiene and respiratory etiquette reduce transmission.

• Elevate head end of bed in debilitated patients to prevent aspiration.

• Reduce alcohol excess — improves cough reflex and immunity.

• Proper management of swallowing disorders and stroke patients.

1. Chest X-ray (Primary Investigation)

Used for diagnosis, localization, severity assessment, and follow-up.

Typical Findings

• *Lobar consolidation*

• Homogeneous opacity involving one lobe

• Air bronchogram sign present

• Classical pneumococcal pneumonia

• *Bronchopneumonia pattern*

• Patchy, multifocal opacities

• Peribronchial distribution

• Common in staphylococcal or gram-negative infections

• *Interstitial infiltrates*

• Reticular or ground-glass appearance

• Suggests viral or atypical pneumonia

• *Air bronchogram*

• Air-filled bronchi visible within consolidation

• Strong indicator of alveolar disease

⸻

2. Distribution Clues (Clinical Correlation)

• Upper lobe predominance

→ TB, aspiration (upright), Klebsiella

• Lower lobe involvement

→ Typical CAP organisms

• Bilateral diffuse infiltrates

→ Viral pneumonia / ARDS

• Dependent segment involvement

→ Aspiration pneumonia

⸻

3. Complication Detection on X-ray

• *Pleural effusion*

• *Empyema* (loculated fluid)

• *Lung abscess* — cavity with air-fluid level

• Atelectasis

⸻

4. Chest CT Scan

Indications (NOT routine):

• Poor response to treatment

• Diagnostic uncertainty

• Immunocompromised patient

• Suspected complications

CT Findings

• Ground-glass opacities → viral/atypical pneumonia

• Segmental consolidation

• Cavitation

• Tree-in-bud pattern (endobronchial spread)

• Abscess formation

⸻

5. Lung Ultrasound (Increasing Clinical Use)

Bedside ICU tool

Findings:

• *B-lines* → interstitial involvement

• Subpleural consolidation

• Dynamic air bronchograms

• Pleural effusion detection

Useful in children, ICU, and radiation-avoidance settings.

⸻

Clinical Reasoning Pearl

• Symptoms + Signs → Suspect pneumonia

• X-ray → Confirms pneumonia

• Pathology tests → Identify organism

Radiology defines extent and complications, not the causative agent.

CAP is classified clinically based on causative organism, pathology pattern, and host factors

1. Etiology-based subtypes

• Typical bacterial pneumonia — Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis.

• Atypical pneumonia — Mycoplasma pneumoniae, Chlamydophila pneumoniae, Legionella pneumophila.

• Viral pneumonia — Influenza, RSV, SARS-CoV-2, Parainfluenza, HMPV.

• Aspiration pneumonia — anaerobic oral flora due to impaired airway protection.

• Mixed infection pneumonia — viral infection followed by bacterial superinfection.

⸻

2. Anatomical / Pathological pattern

• Lobar pneumonia — consolidation of an entire lobe (classically pneumococcal).

• Bronchopneumonia — patchy peribronchial infection affecting multiple lobules.

• Interstitial pneumonia — diffuse interstitial inflammation (often viral/atypical).

• Necrotizing pneumonia — tissue destruction and cavitation (e.g., Staphylococcus aureus).

⸻

3. Host-based classification

• Immunocompetent CAP — otherwise healthy individuals.

• Elderly-associated CAP — reduced immunity and aspiration risk.

• Immunocompromised CAP — HIV, malignancy, transplant, steroids.

• Chronic lung disease–associated CAP — COPD, bronchiectasis.

⸻

4. Severity classification (clinical use)

• Mild CAP — outpatient management possible.

• Moderate CAP — hospitalization required.

• Severe CAP — respiratory failure or sepsis; ICU care.

Tachypnea due to reduced lung compliance and hypoxia

Tachycardia secondary to fever and systemic stress

Hypoxia (↓SpO₂) caused by alveolar consolidation

⸻

General Examination

Ill-appearing patient with malaise

Increased respiratory effort

Use of accessory respiratory muscles in moderate–severe disease

Cyanosis in severe hypoxemia

⸻

Vital Signs

Temperature elevation (>38 °C)

Respiratory rate increased (>20–24/min in adults)

Tachycardia (>100/min)

Low oxygen saturation

⸻

Respiratory System Examination

Reduced chest expansion on affected side

Dullness to percussion over consolidated lung

Bronchial breath sounds over areas of consolidation

Inspiratory crackles (crepitations) due to alveolar fluid

Increased vocal resonance / egophony

Whispering pectoriloquy (sound transmission through consolidated lung)

⸻

Severe Disease Indicators

Hypotension (sepsis)

Altered mental status

Signs of respiratory distress

Possible pleural rub if pleuritis present

Community-acquired pneumonia (CAP) is an acute infection of lung parenchyma acquired outside hospital or healthcare facilities

Characterized by alveolar inflammation and consolidation leading to impaired gas exchange

Clinically presents with fever, cough, dyspnea, and radiological infiltrates

Represents infection reaching alveoli beyond normal airway defense mechanisms

⸻

CAP differs from bronchitis because:

Bronchitis → airway inflammation

Pneumonia → alveolar involvement

⸻

Common cause of morbidity and mortality especially in elderly, children, and comorbid patients

Acute onset fever due to systemic inflammatory response

Productive cough caused by alveolar exudate formation

Purulent sputum reflects neutrophilic airway inflammation

Pleuritic chest pain occurs due to pleural surface irritation

Dyspnea develops from impaired gas exchange

Fatigue and malaise due to cytokine-mediated systemic illness

⸻

Respiratory Symptoms

Cough — may be dry (atypical) or productive (typical bacterial)

Sputum production — yellow, green, or rust-colored

Shortness of breath on exertion or at rest

Chest discomfort worsened by deep inspiration

⸻

Systemic Symptoms

Fever with chills or rigors

Sweating and generalized weakness

Loss of appetite (anorexia)

Myalgia and headache

⸻

Atypical Presentation (Common in Elderly)

Confusion or altered mental status

Minimal fever or absent cough

General functional decline

⸻

Children

Poor feeding

Irritability

Tachypnea may be predominant symptom

Treatment is based on severity, probable organism, age, and comorbid conditions

General Principles

• Early empiric antibiotic therapy reduces mortality — start after clinical diagnosis (do not delay for reports).

• Severity assessment guides treatment site — outpatient vs inpatient vs ICU.

• Supportive care is equally important as antibiotics.

⸻

Outpatient Management (Mild CAP)

• Amoxicillin — first-line for typical bacterial CAP.

• Doxycycline — alternative covering atypical organisms.

• Macrolide (azithromycin/clarithromycin) — where pneumococcal resistance is low.

• Oral hydration and rest.

• Antipyretics (paracetamol).

• Follow-up within 48–72 hours.

⸻

Inpatient (Moderate CAP)

• β-lactam + macrolide combination

(e.g., ceftriaxone + azithromycin).

• Alternative: respiratory fluoroquinolone (levofloxacin/moxifloxacin).

• Oxygen therapy if SpO₂ < 92%.

• IV fluids if dehydrated.

⸻

Severe / ICU CAP

• Broad-spectrum IV antibiotics immediately.

• β-lactam + macrolide or β-lactam + fluoroquinolone.

• Consider MRSA coverage (vancomycin/linezolid) if risk factors present.

• Consider antipseudomonal therapy in structural lung disease.

• Non-invasive or mechanical ventilation when required.

• Hemodynamic support for septic shock.

⸻

Supportive Management

• Oxygen supplementation — maintain adequate oxygenation.

• Bronchodilators if bronchospasm present.

• Chest physiotherapy and mobilization.

• Adequate nutrition and hydration.

• DVT prophylaxis in hospitalized patients.

⸻

Special Situations

• Aspiration pneumonia → add anaerobic coverage (amoxiclav, piperacillin-tazobactam).

• Immunocompromised patients → organism-directed therapy.

• Viral pneumonia → supportive ± antivirals (e.g., influenza).

⸻

Treatment Duration

• Usually 5–7 days if clinical improvement occurs.

• Longer duration for complications or resistant organisms.