CHRONIC OBSTRUCTIVE PULMONARY DISEASE (COPD)
Chronic obstructive pulmonary disease (COPD) is a progressive respiratory condition characterised by persistent airflow limitation that is not fully reversible. Unlike asthma, where obstruction is episodic and largely reversible, COPD involves permanent structural damage to the airways and alveoli. The disease develops slowly over years and is most commonly associated with long-term exposure to noxious particles or gases, particularly cigarette smoke.
COPD encompasses a spectrum of pathological changes that affect both the conducting airways and the gas-exchanging regions of the lungs. These changes impair ventilation, disrupt gas exchange and place increasing strain on the respiratory and cardiovascular systems. As the disease progresses, COPD becomes a major cause of morbidity, functional limitation and reduced quality of life.
What You Need to Know
Chronic obstructive pulmonary disease is a progressive respiratory disorder characterised by persistent airflow obstruction due to increased airway resistance and loss of elastic recoil. Unlike asthma, the airflow limitation in COPD is largely irreversible because it arises from permanent structural damage rather than transient airway narrowing. The disease is driven by long-term exposure to noxious particles or gases, most commonly tobacco smoke, which leads to chronic inflammation and gradual destruction of airway and alveolar structures.
Two overlapping pathological processes dominate COPD. Chronic bronchitis primarily affects the airways, with inflammation, wall thickening, and excess mucus narrowing the bronchial lumen. Emphysema primarily affects the alveoli, where destruction of alveolar walls reduces the surface area for gas exchange and removes the elastic support that normally keeps small airways open during expiration. Most individuals with COPD have features of both processes, which together contribute to progressive airflow limitation.
At an overview level, COPD is characterised by several interconnected mechanisms:
Persistent airway narrowing and mucus plugging that increase resistance to airflow
Loss of alveolar elastic recoil, promoting airway collapse during expiration
Air trapping and hyperinflation, which increase the work of breathing
As airflow obstruction worsens, expiration becomes increasingly incomplete, causing air to remain trapped in the lungs. This leads to hyperinflation, flattening of the diaphragm, and reduced mechanical efficiency of breathing. Gas exchange becomes progressively impaired, resulting in chronic hypoxaemia and, in advanced disease, carbon dioxide retention due to inadequate alveolar ventilation. These changes underpin the gradual onset of breathlessness, reduced exercise tolerance, and systemic effects seen in long-standing COPD.
Beyond the Basics
Chronic airway inflammation and small airway disease
In chronic obstructive pulmonary disease, prolonged exposure to irritants such as tobacco smoke drives a persistent inflammatory response within the airways. This response is dominated by neutrophils, macrophages, and CD8⁺ T lymphocytes, which release proteases and inflammatory mediators that progressively damage airway walls. Unlike the inflammation seen in asthma, this process is less responsive to corticosteroids and leads to cumulative structural injury over time.
The small airways are particularly affected. Repeated inflammation causes wall thickening, fibrosis, and luminal narrowing, all of which increase resistance to airflow. At the same time, enlargement of mucus-secreting glands and an increase in goblet cells result in excessive mucus production. This mucus obstructs airflow and impairs mucociliary clearance, allowing secretions to accumulate. Because small bronchioles lack cartilage support, these changes have a disproportionate effect on expiratory airflow limitation.
Emphysema and loss of elastic recoil
Emphysema is defined by destruction of alveolar walls and the capillary beds distal to the terminal bronchioles. In COPD, protease activity outweighs antiprotease defences, leading to progressive breakdown of alveolar septa. This structural destruction permanently alters lung architecture.
Loss of alveolar walls reduces the surface area available for gas exchange and eliminates the elastic fibres that normally provide recoil during expiration. Without elastic recoil, small airways are no longer held open as air is exhaled and collapse prematurely. This dynamic airway collapse traps air within the lungs, increases residual volume, and is a central contributor to breathlessness in emphysema-predominant disease.
Air trapping, hyperinflation, and work of breathing
As expiratory airflow becomes increasingly limited, air accumulates within the lungs. Functional residual capacity and total lung capacity rise, resulting in hyperinflation. Hyperinflated lungs flatten the diaphragm, shortening its muscle fibres and reducing its ability to generate effective inspiratory force.
To compensate, greater reliance is placed on accessory muscles of respiration, increasing energy expenditure and contributing to early fatigue. Many individuals adopt prolonged expiration or pursed-lip breathing, which generates positive pressure within the airways during exhalation and helps delay airway collapse. Despite these adaptations, the increased work of breathing significantly limits exercise tolerance.
Gas exchange abnormalities and respiratory failure
Gas exchange in COPD is impaired by both airway obstruction and alveolar destruction. Ventilation–perfusion mismatch develops because poorly ventilated lung regions continue to receive blood flow, leading to chronic hypoxaemia. As disease progresses, reduced alveolar ventilation limits carbon dioxide elimination, resulting in carbon dioxide retention and respiratory acidosis.
Chronic hypoxaemia causes sustained pulmonary vasoconstriction, increasing pulmonary vascular resistance. Over time, this places strain on the right ventricle, leading to pulmonary hypertension and right ventricular hypertrophy, a process known as cor pulmonale. These changes mark advanced disease and are associated with increased morbidity and mortality.
Systemic effects of chronic obstructive pulmonary disease
COPD is increasingly recognised as a systemic inflammatory condition rather than a disease confined to the lungs. Ongoing inflammation contributes to skeletal muscle wasting, weight loss, reduced bone density, cardiovascular disease, and mood disorders such as depression and anxiety.
These systemic effects compound respiratory limitations by further reducing functional capacity and resilience. As a result, disease impact extends well beyond airflow obstruction, influencing quality of life, exercise tolerance, and long-term outcomes in individuals with COPD.
Clinical Connections
Chronic obstructive pulmonary disease commonly presents with chronic cough, daily sputum production, progressive dyspnoea, and reduced exercise tolerance that develops gradually over years. Symptoms often worsen slowly but may be punctuated by acute exacerbations, most frequently triggered by respiratory infection or environmental exposure such as air pollution. On physical examination, findings may include wheeze, prolonged expiration, hyperinflated or barrel-shaped chest, and visible use of accessory muscles, reflecting increased work of breathing and airflow limitation.
Diagnosis is confirmed with spirometry demonstrating persistent airflow obstruction, defined by a reduced forced expiratory volume in one second to forced vital capacity ratio that does not fully normalise following bronchodilator administration. This fixed limitation distinguishes COPD from asthma, where reversibility is a defining feature. Assessment commonly integrates symptom burden, spirometry severity, exacerbation history, and oxygenation status to guide management decisions and monitor disease progression.
Key clinical features that shape investigation and management include:
Chronic, progressive symptoms rather than episodic airflow limitation
Spirometry showing fixed obstruction with limited bronchodilator reversibility
Exacerbations that accelerate lung function decline and worsen prognosis
Management focuses on reducing symptom burden, slowing disease progression, and preventing exacerbations. Smoking cessation is the most effective intervention for altering disease trajectory, while bronchodilator therapy improves airflow and exercise tolerance. In selected patients, inhaled corticosteroids reduce exacerbation frequency, and pulmonary rehabilitation addresses deconditioning and breathlessness. Long-term oxygen therapy is indicated for chronic hypoxaemia and improves survival in advanced disease, making early recognition of disease severity and progression central to long-term outcomes.
Concept Check
Why is airflow obstruction in COPD considered largely irreversible?
How do chronic bronchitis and emphysema contribute differently to airflow limitation?
Why does loss of elastic recoil lead to air trapping during expiration?
How does COPD contribute to the development of pulmonary hypertension and cor pulmonale?
Why is COPD associated with systemic effects beyond the lungs?