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Obstructive Lung Diseases: A Complete Clinical Guide

Sources: Robbins & Kumar Basic Pathology; Murray & Nadel's Textbook of Respiratory Medicine; Fishman's Pulmonary Diseases and Disorders; Katzung's Basic and Clinical Pharmacology; Textbook of Family Medicine

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What Defines "Obstructive"?

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PART I: COPD

Definition

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Pathophysiology

Emphysema

• Enlargement of air spaces distal to terminal bronchioles due to destruction of alveolar elastic support structures
• Key mechanism: Proteases (especially elastase from neutrophils) overwhelm antiprotease defenses (especially α₁-antitrypsin) → alveolar wall destruction → loss of elastic recoil
• Consequences:
  • Reduced elastic recoil → dynamic airway collapse during expiration → air trapping
  • Reduced alveolar surface area → low DLCO (diffusing capacity) — key distinguishing feature from chronic bronchitis
  • Increased static lung compliance (CstL)
  • Barrel chest from chronic hyperinflation

Chronic Bronchitis

• Clinical definition: Persistent productive cough for ≥ 3 consecutive months in ≥ 2 consecutive years
• Pathogenesis:
  • Mucus overproduction in proximal airways → hyperplasia of mucous glands + goblet cell metaplasia
  • MUC5AC concentration increases up to 10-fold, MUC5B 3-fold in severe COPD
  • Airway obstruction from small airway inflammation (chronic bronchiolitis), not from mucus in large airways
  • Acrolein (from cigarette smoke) is a potent inducer of MUC5AC
  • Ciliary dysfunction reduces mucociliary clearance → persistent infection with H. influenzae
• DLCO is normal or near-normal (alveoli preserved)
• Leads to hypoxemia + hypercapnia ("Blue Bloater" phenotype)

Inflammatory Cell Profile

• Predominantly neutrophilic (unlike asthma which is eosinophilic)
• Also: CD8+ T-lymphocytes, macrophages
• Lymphoid follicles increasingly recognized as central to COPD pathogenesis

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Epidemiology & Risk Factors

• 3rd most common cause of death in the US, costing >$40 billion/year
• Smoking is the dominant risk factor (15–30% of smokers historically, though radiographic changes seen even in smokers with normal spirometry)
• α₁-antitrypsin deficiency — panacinar emphysema, onset in 4th–5th decade, accelerated by smoking
• Occupational exposures: silica dust, coal dust, biomass fuel smoke
• Air pollution, recurrent respiratory infections, low birth weight

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Diagnosis

Clinical Presentation

• Progressive dyspnea (initially exertional, then at rest)
• Chronic productive cough
• Wheeze, especially on exertion or with exacerbations
• Reduced exercise tolerance

Spirometry (Required for Diagnosis)

• Post-bronchodilator FEV₁/FVC < 0.70 confirms airflow obstruction

GOLD Classification (Severity by FEV₁ % predicted)

Additional Diagnostic Tests

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Treatment

Step 1: Non-Pharmacological (All Stages)

• Smoking cessation — single most effective intervention, slows FEV₁ decline
• Pulmonary rehabilitation
• Vaccination: influenza (annual), pneumococcal, COVID-19, pertussis
• Nutritional support
• Avoidance of occupational/environmental triggers

Step 2: Bronchodilators (Mainstay of Pharmacotherapy)

LAMAs and LABAs are equally preferred as first-line maintenance therapy; combining them provides additive benefit.

Step 3: Inhaled Corticosteroids (ICS)

• Less central than in asthma; associated with increased risk of bacterial pneumonia
• Indicated when:
  • Severe airflow obstruction (GOLD 3–4)
  • History of frequent exacerbations
  • Blood eosinophils ≥ 300 cells/µL → reasonable response to ICS
  • Blood eosinophils < 100 cells/µL → low likelihood of ICS benefit
  • History of asthma independent of smoking
• Triple therapy (LABA + LAMA + ICS) for patients with persistent exacerbations despite dual therapy

Step 4: Other Pharmacotherapy

Step 5: COPD Exacerbation Management

• Defined as acute worsening of respiratory symptoms beyond normal day-to-day variation
• Triggered predominantly by viral infections; bacterial infections (H. influenzae, S. pneumoniae, M. catarrhalis) also contribute
• Management:
  • Increase SABA/SAMA (nebulized)
  • Systemic corticosteroids — prednisone 40 mg/day × 5 days
  • Antibiotics — β-lactams, doxycycline, or azithromycin (routine in moderate-severe exacerbations, unlike asthma)
  • Controlled oxygen: target SpO₂ 88–92% (avoid hyperoxia → hypercapnic drive blunted)
  • NIV (BiPAP) if pH < 7.35 with hypercapnia — reduces intubation, mortality
  • ICU/invasive ventilation if NIV fails

Step 6: Long-Term Oxygen Therapy (LTOT)

• Indicated if:
  • PaO₂ ≤ 55 mmHg, OR
  • SpO₂ ≤ 88%, OR
  • PaO₂ 56–59 mmHg with cor pulmonale, polycythemia, or pulmonary hypertension
• Goal: SpO₂ ≥ 90%, ≥ 15 hours/day
• Only intervention (besides smoking cessation) proven to improve survival in COPD

Surgical Options

• Lung volume reduction surgery (LVRS): upper-lobe emphysema with low exercise capacity after rehabilitation
• Bronchoscopic valve placement (endobronchial valves): for severe emphysema
• Lung transplantation: end-stage COPD; BODE index guides listing

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Complications

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PART II: ASTHMA

Definition

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Pathophysiology

Atopic (Allergic) Asthma — Most Common

• Th2-mediated IgE response to environmental allergens
• Early-phase reaction (within minutes): IgE cross-linking → mast cell degranulation → histamine, leukotrienes (LTC4, LTD4), prostaglandins → bronchospasm, mucus secretion, vascular leakage
• Late-phase reaction (4–8 hours): Eosinophils, T-lymphocytes, cytokines → sustained inflammation
• Key cytokines: IL-4 (IgE class switching), IL-5 (eosinophil recruitment), IL-13 (mucus, AHR), IL-33, TSLP (alarmin signals from epithelium)
• Eosinophils are the key inflammatory cells across nearly all asthma subtypes; eosinophil products (major basic protein, eosinophil cationic protein) cause epithelial damage

Non-Atopic (Intrinsic) Asthma

• Triggers: viral URIs, cold air, exercise, aspirin/NSAIDs, stress, air pollution
• Not IgE-mediated; mechanism less well defined

Airway Remodeling (Chronic Asthma)

• Subbasement membrane thickening (collagen deposition)
• Hypertrophy and hyperplasia of bronchial smooth muscle
• Goblet cell metaplasia and mucous gland hypertrophy
• Angiogenesis
• This adds an irreversible component to airway obstruction in long-standing disease

Aspirin-Exacerbated Respiratory Disease (AERD)

• Aspirin/NSAIDs block COX-1 → arachidonic acid shunted to lipoxygenase → excess leukotrienes → severe bronchospasm
• Classic triad: asthma + nasal polyps + aspirin sensitivity (Samter's triad)

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Diagnosis

Clinical Features

• Episodic wheeze, dyspnea, chest tightness, cough (often nocturnal/early morning)
• Symptoms worsen with triggers (allergens, exercise, cold air, URIs)
• Reversibility: symptoms and airflow obstruction improve with bronchodilators

Spirometry

• FEV₁/FVC < 0.70
• ≥ 12% and ≥ 200 mL improvement in FEV₁ after bronchodilator = significant reversibility (hallmark of asthma vs. COPD)
• Methacholine challenge test: confirms airway hyperresponsiveness when spirometry is normal

Severity Classification (Before Treatment)

Labs & Investigations

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Treatment

Step-Up Therapy (GINA Guidelines)

GINA 2023 no longer recommends SABA-only therapy at any step due to risk of fatal exacerbations without background ICS.

Biologics for Severe Asthma (Step 5)

Acute Severe Asthma (Status Asthmaticus)

1. Oxygen: titrate to SpO₂ ≥ 94%
2. SABA (albuterol/salbutamol) — continuous or q20min nebulization
3. Ipratropium — add in moderate-severe attacks
4. Systemic corticosteroids — prednisolone 40–50 mg or IV hydrocortisone; do not delay
5. Magnesium sulfate IV (2g over 20 min) — for severe/life-threatening attacks unresponsive to bronchodilators
6. Heliox — reduces airway resistance in refractory cases
7. NIV — limited evidence; avoid if deteriorating rapidly
8. Intubation — as last resort; high risk; use lung-protective strategy (low RR, long expiratory time)

Danger Signs in Acute Asthma

• Silent chest (no wheeze = no airflow = imminent arrest)
• Normal or rising PaCO₂ despite tachypnea (fatigue, impending failure)
• SpO₂ < 92% despite O₂
• Inability to speak in sentences
• Altered consciousness

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Complications

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PART III: BRONCHIECTASIS

Definition & Pathophysiology

• Acquired disorder: abnormal irreversible dilation of bronchi and bronchioles from recurrent infection and inflammation
• Vicious cycle: impaired mucociliary clearance → bacterial colonization → neutrophilic inflammation → proteolytic damage to bronchial walls → further dilation

Causes (Mnemonic: ABCDEF)

• Allergic bronchopulmonary aspergillosis (ABPA)
• Bronchoobstructive (foreign body, tumor)
• Cystic fibrosis (most common cause in children/young adults)
• Dyskinetic cilia (Primary Ciliary Dyskinesia / Kartagener syndrome)
• Effect of necrotizing pneumonia (TB, pertussis, measles — delayed treatment)
• Failure of immunity (hypogammaglobulinemia, HIV, IgA deficiency)
• Also: MAC (Mycobacterium avium complex) — especially in tall, thin post-menopausal women (Lady Windermere syndrome)

Clinical Features

• Chronic productive cough with large volumes of thick, mucopurulent, tenacious sputum (3-layer appearance on standing)
• Dyspnea, wheeze
• Hemoptysis (can be massive from hypertrophied bronchial arteries)
• Recurrent lower respiratory tract infections
• Clubbing (significant bronchiectasis)
• Crackles/coarse rhonchi on auscultation

Diagnosis

Treatment

• Airway clearance: chest physiotherapy, oscillatory PEP devices, mucolytic nebulization (hypertonic saline, DNase in CF)
• Antibiotics for exacerbations: guided by sputum culture; Pseudomonas coverage with IV antibiotics if needed
• Long-term azithromycin (3×/week): reduces exacerbation frequency in colonized patients
• Treat underlying cause: immunoglobulin replacement, antifungals (ABPA), CFTR modulators (CF)
• Surgery: resection for localized disease or life-threatening hemoptysis
• No curative treatment exists for diffuse disease other than lung transplantation

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PART IV: COMPARISON TABLE — Obstructive Diseases at a Glance

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PART V: ALPHA-1 ANTITRYPSIN DEFICIENCY

• Autosomal codominant — most common deficiency allele: PiZ (E342K mutation in SERPINA1)
• PiZZ homozygote: serum AAT < 11 µmol/L — highest risk
• A1AT is a serine protease inhibitor (serpin) produced by the liver
• Deficiency → inadequate inhibition of neutrophil elastase → lung destruction
• Liver disease: misfolded PiZ protein accumulates in hepatocytes → PAS-positive diastase-resistant globules → cirrhosis
• Treatment: Augmentation therapy (weekly IV pooled human A1AT) — slows lung density decline in PiZZ patients with emphysema
• Screen all COPD patients < 45 years, non-smokers, panacinar pattern, family history, or liver disease

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PART VI: KEY DIFFERENTIATING LABS & NUMBERS TO REMEMBER

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