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PA25.3 | Obstructive Airway Disease & Bronchiectasis — SDL Guide

Learning Objectives

Distinguish obstructive from restrictive lung disease using spirometry patterns (FEV1/FVC ratio)
Define emphysema and describe its four morphological types with their distinct etiologies
Explain the protease-antiprotease and oxidant-antioxidant imbalance in COPD pathogenesis
Define chronic bronchitis clinically and histologically (Reid index); contrast with emphysema
Describe the immunological basis and airway morphology of asthma including Curschmann spirals and Charcot-Leyden crystals
Define bronchiectasis, enumerate its causes, and trace the obstruction-infection cycle
List complications of COPD and bronchiectasis including cor pulmonale

INSTRUCTIONS

Obstructive airway diseases -- COPD, asthma, and bronchiectasis -- rank among the leading causes of chronic morbidity worldwide and are a high-yield area in final MBBS examinations and clinical postings. Building on your Year-1 knowledge of respiratory physiology and anatomy, this module uses the spirometric FEV1/FVC ratio as an anchor to organise the COPD spectrum, then extends to asthma and bronchiectasis. You will move from mechanism to morphology to clinical picture -- exactly the framework examiners expect.

References

Robbins & Cotran Pathologic Basis of Disease, 10th ed., Ch 15 -- The Lung (textbook)
Harsh Mohan Textbook of Pathology, 8th ed., Ch 16 -- Respiratory System (textbook)

Version 2.0 | NMC CBUC 2024

CLINICAL SCENARIO

A 58-year-old retired carpenter arrives breathless after climbing one flight of stairs. Chest X-ray shows hyperinflated lungs with a flattened diaphragm. Spirometry: FEV1/FVC = 0.52 (normal >0.70). He smoked 40 pack-years and has never had wheeze. Meanwhile, the patient in the next bed -- also breathless -- has a productive cough every winter for the past four years, a barrel chest, and a sputum culture growing Haemophilus influenzae. Same spirometry pattern, entirely different disease process. By the end of this session you will know exactly why.

WHY THIS MATTERS

COPD affects over 250 million people globally and is the third leading cause of death. Asthma affects 350 million. Bronchiectasis is common in post-tubercular lungs -- highly relevant in South Asian practice. These diseases appear in Short Answer Questions (SAQs), clinical case vignettes, and OSCE spirometry interpretation stations. Understanding the FEV1/FVC framework lets you classify any obstructive disease quickly -- a skill you will use every clinical posting.

RECALL

Before continuing, recall from your Year-1 respiratory physiology:

FEV1 = forced expiratory volume in 1 second. Reflects airway resistance.
FVC = forced vital capacity. Reflects total lung volume.
Normal FEV1/FVC ratio >= 0.70.
In obstruction, airways narrow -> FEV1 falls disproportionately -> ratio <0.70.
In restriction, both FEV1 and FVC fall equally -> ratio preserved (>0.70) but absolute volumes are small.
Surfactant (Year-1 biochemistry): produced by Type II pneumocytes; reduces alveolar surface tension.

Obstructive vs Restrictive Pattern -- The Gateway Distinction

A three-panel medical diagram compares obstructive and restrictive lung disease using lung mechanics, spirometry ratios, and a gateway classification table. — **Panel A:** Obstructive lung disease: narrowed bronchi, mucus plug, collapsed small airway during expiration, trapped air, hyperinflated lung, flattened diaphragm; Restrictive lung disease: small stiff lung, fibrotic interstitium, reduced expansion, decreased TLC, raised diaphragm.. **Panel B:** Spirometry distinction showing FEV1, FVC, FEV1/FVC < 0.70 for obstructive disease, and reduced FVC with FEV1/FVC >= 0.70 for restrictive disease.. **Panel C:** Gateway table comparing FEV1/FVC, FVC, TLC, cause, and examples for obstructive versus restrictive patterns, with obstructive column highlighted as the module pathway..

Obstructive lung disease is defined by a reduced FEV1/FVC ratio (<0.70). Airflow is impeded during expiration because airways are narrowed, collapsed, or plugged. Total lung capacity (TLC) is normal or increased (air trapping). Examples: COPD (emphysema + chronic bronchitis), asthma, bronchiectasis.

Restrictive lung disease shows a reduced FVC with a preserved or elevated FEV1/FVC ratio. The lung cannot fully expand. Examples: pulmonary fibrosis, sarcoidosis, pneumoconiosis, pleural restriction, neuromuscular weakness.

Feature	Obstructive	Restrictive
FEV1/FVC	< 0.70	>= 0.70
FVC	Normal or increased	Decreased
TLC	Increased (air-trapped)	Decreased
Cause	Airway narrowing	Lung/chest wall stiffness

This table is your single gateway. Every subsequent disease in this module falls in the obstructive column.

Three side-by-side flow-volume loop diagrams compare normal, obstructive, and restrictive spirometry patterns with FEV1, FVC, TLC, and curve-shape changes annotated. — **Panel A:** Normal flow-volume loop showing peak expiratory flow, smooth expiratory limb, inspiratory limb, FEV1 bracket, FVC bracket, and normal FEV1/FVC.. **Panel B:** Obstructive flow-volume loop showing reduced peak expiratory flow, scooped-out expiratory limb, increased TLC, air trapping with increased RV, reduced FEV1, FVC bracket, and decreased FEV1/FVC.. **Panel C:** Restrictive flow-volume loop showing small preserved-shape loop, reduced TLC, reduced FVC, proportionally reduced FEV1, and normal or increased FEV1/FVC..

COPD -- Overview and Pathogenesis

A four-panel diagram summarizes COPD as smoking-related emphysema and chronic bronchitis driven by protease-antiprotease and oxidant-antioxidant imbalance, ending in airflow obstruction and tissue destruction. — **Panel A:** COPD overview showing cigarette smoke exposure, lungs, emphysema, chronic bronchitis, frequent overlap, reduced expiratory airflow, respiratory failure, and cor pulmonale.. **Panel B:** Protease-antiprotease imbalance showing neutrophils, macrophages, elastase, matrix metalloproteinases, alpha1-AT, oxidation, unopposed elastase, and alveolar septal destruction.. **Panel C:** Oxidant-antioxidant imbalance showing cigarette smoke oxidants, reactive oxygen species, activated neutrophils, alpha1-AT inactivation, surfactant damage, and airway inflammation.. **Panel D:** Final tissue outcomes showing normal alveoli versus emphysematous enlarged airspaces with alveolar wall destruction, plus bronchial mucous gland hyperplasia and mucus plugging..

COPD (Chronic Obstructive Pulmonary Disease) is an umbrella term for two related but distinct conditions that frequently overlap -- emphysema and chronic bronchitis. Both reduce expiratory airflow, both are primarily caused by smoking, and both progress to respiratory failure and cor pulmonale.

Pathogenesis -- Two Imbalances:

Protease-antiprotease imbalance: Cigarette smoke recruits neutrophils and macrophages into alveoli. These release elastase (and matrix metalloproteinases) that digest the alveolar wall. Normally, alpha-1-antitrypsin (alpha1-AT) inhibits elastase. Smoking inactivates alpha1-AT by oxidation -> unopposed elastase -> alveolar wall destruction.

Oxidant-antioxidant imbalance: Cigarette smoke itself contains oxidants; activated neutrophils generate reactive oxygen species. These oxidants inactivate alpha1-AT, damage surfactant, and trigger airway inflammation directly.

Key link: Both imbalances converge on the same outcome -- destruction of alveolar walls and/or mucous gland hyperplasia depending on which zone of the lung is affected.

Emphysema -- Definition, Types, and Morphology

A five-panel pathology diagram defines emphysema and compares centriacinar, panacinar, paraseptal, and irregular patterns of acinar destruction. — **Panel A:** Normal acinus versus emphysema; terminal bronchiole, respiratory bronchiole, alveolar duct, alveolar sac, enlarged distal airspace, destroyed alveolar septa, absence of fibrosis.. **Panel B:** Centriacinar emphysema; proximal acinus and respiratory bronchioles destroyed, distal alveoli relatively spared, smoking association, upper lobe predominance.. **Panel C:** Panacinar emphysema; uniform destruction of entire acinus from proximal to distal parts, alpha-1 antitrypsin deficiency, lower lobe predominance.. **Panel D:** Paraseptal emphysema; distal acinar involvement near pleura and interlobular septa, subpleural blebs and bullae, spontaneous pneumothorax risk.. **Panel E:** Irregular emphysema; patchy acinar destruction adjacent to healed scar tissue, scar-associated morphology..

Emphysema is defined as permanent abnormal enlargement of airspaces distal to the terminal bronchiole, accompanied by destruction of alveolar walls, without fibrosis.

Three elements are non-negotiable in the definition: permanent enlargement, wall destruction, absence of fibrosis.

Four morphological types:

Type	Zone destroyed	Cause	Note
Centriacinar (centrilobular)	Proximal acinus -- respiratory bronchioles	Smoking (>95%)	Most common; upper lobe predominance
Panacinar (panlobular)	Entire acinus -- proximal to distal	alpha1-antitrypsin deficiency; also late smoking	Lower lobe predominance
Paraseptal (distal acinar)	Distal acinus near septa/pleura	Idiopathic; young adults	Causes spontaneous pneumothorax
Irregular (scar emphysema)	Irregular; around scars	Post-inflammatory fibrosis	No clinical syndrome

Morphology:
- Gross: lungs voluminous, pale, fluffy, don't collapse on opening chest; bullae (>1 cm) in paraseptal type.
- Micro: enlarged airspaces with thinned, destroyed alveolar walls; pink puffer phenotype -- pursed-lip breathing to maintain positive end-expiratory pressure, thin, barrel-chested, markedly dyspnoeic but relatively pink (hyperventilates to maintain PaO2).

A four-panel comparison diagram shows centriacinar, panacinar, paraseptal, and irregular emphysema with the destroyed acinar zones and typical lung lobe distributions highlighted. — **Panel A:** Centriacinar emphysema: terminal bronchiole, respiratory bronchiole, proximal acinar zone of destruction, preserved distal alveolar ducts and sacs, upper-lobe predominance inset.. **Panel B:** Panacinar emphysema: uniform destruction of respiratory bronchiole, alveolar ducts, and alveolar sacs throughout the acinus, lower-lobe predominance inset, alpha-1-antitrypsin deficiency note.. **Panel C:** Paraseptal emphysema: preserved proximal acinus, distal alveolar duct and sac destruction near pleura and interlobular septum, subpleural bullae tendency, upper-zone predominance inset.. **Panel D:** Irregular emphysema: patchy acinar destruction around fibrotic scar tissue, irregular preserved and destroyed airspaces, no consistent lobe predominance inset..

SELF-CHECK

A 45-year-old non-smoker with no respiratory symptoms is found to have lower-lobe predominant emphysema on CT. Which enzyme deficiency is most likely responsible?

A. Myeloperoxidase

B. alpha-1-Antitrypsin

C. Superoxide dismutase

D. Surfactant protein B

Reveal Answer

Answer: B. alpha-1-Antitrypsin

alpha-1-Antitrypsin (alpha1-AT) deficiency causes panacinar (panlobular) emphysema with lower-lobe predominance in non-smokers -- classically presenting in the 4th-5th decade. alpha1-AT normally inhibits neutrophil elastase; without it, alveolar walls are unprotected. Myeloperoxidase generates oxidants but its deficiency doesn't cause emphysema. Surfactant protein B deficiency causes neonatal respiratory distress, not adult emphysema.