Medical School · Year 1 · Respiratory · includes a quiz and discussion video

Lecture 7: Obstructive Lung Diseases

Unit 1.8: Respiratory System

Learning Objectives

By the end of this lecture, students will be able to:

1. Define obstructive lung disease and its physiologic features
2. Describe the pathophysiology of asthma and its management
3. Explain the pathophysiology of COPD including emphysema and chronic bronchitis
4. Describe bronchiectasis and its causes
5. Interpret pulmonary function tests in obstructive disease
6. Apply obstructive disease concepts to clinical management

Lecture Content

I. Overview of Obstructive Lung Disease

Obstructive lung diseases share the common feature of increased resistance to airflow, primarily affecting expiration. This airflow limitation results from various mechanisms including airway narrowing, loss of elastic recoil, and dynamic airway collapse. Understanding these diseases requires integrating knowledge of airway anatomy, respiratory mechanics, and pulmonary function testing.

The defining characteristic of obstructive disease is airflow limitation that is not fully reversible with bronchodilators. Increased resistance to expiratory airflow causes air trapping, where air remains in the lungs at the end of expiration. Over time, this hyperinflation restructures the thorax and respiratory muscles, contributing to dyspnea and exercise limitation.

Spirometric criteria establish the diagnosis of obstructive lung disease. The key finding is a reduced ratio of forced expiratory volume in one second to forced vital capacity, with a threshold typically set at 0.70 or the lower limit of normal for age. This reduced ratio reflects disproportionate impairment of expiratory flow. Forced expiratory volume in one second is reduced absolutely, while forced vital capacity may be normal or reduced. Measurements of lung volumes show normal or increased total lung capacity and increased residual volume, reflecting air trapping and hyperinflation.

The major obstructive lung diseases include asthma, chronic obstructive pulmonary disease, and bronchiectasis. Asthma features reversible airway obstruction driven by inflammation and bronchial hyperresponsiveness. Chronic obstructive pulmonary disease encompasses emphysema, characterized by parenchymal destruction, and chronic bronchitis, characterized by airway inflammation and mucus hypersecretion. Most patients with chronic obstructive pulmonary disease have features of both. Bronchiectasis involves permanent abnormal dilation of bronchi from chronic infection and inflammation.

<image>Panel A: Flow-volume loops comparing normal symmetric loop versus obstructive loop with scooped concave expiratory limb and reduced peak expiratory flow. Panel B: Spirometry tracings showing reduced slope of forced expiration in obstructive disease compared to normal. Panel C: Lung volume diagrams comparing normal versus obstructive with increased TLC, markedly increased RV from air trapping, and increased FRC from hyperinflation. Panel D: Spirometric criteria table showing FEV1/FVC less than 0.70, reduced FEV1, normal or reduced FVC, normal or increased TLC, and increased RV.</image>

II. Asthma

Asthma represents a chronic inflammatory disorder of the airways characterized by variable and reversible airflow obstruction, bronchial hyperresponsiveness to various stimuli, and underlying inflammation. The combination of these features produces the characteristic clinical pattern of episodic symptoms that often respond to treatment.

The prevalence of asthma approaches 8 percent in adults and 10 percent in children, making it one of the most common chronic diseases. While asthma may develop at any age, onset frequently occurs in childhood. Risk factors include atopy (the genetic tendency toward allergic responses), family history of asthma, and environmental exposures including allergens, respiratory infections, and air pollution.

The pathophysiology of asthma involves complex interactions among inflammatory cells, airway epithelium, and smooth muscle. In allergic asthma, initial sensitization to an allergen leads to production of allergen-specific immunoglobulin E antibodies that bind to mast cells in the airways. Upon re-exposure, allergen cross-linking of surface immunoglobulin E triggers mast cell degranulation, releasing histamine, leukotrienes, and other mediators that cause bronchoconstriction, mucosal edema, and mucus secretion. This early phase response occurs within minutes and may resolve spontaneously.

Four to eight hours after allergen exposure, a late phase response develops as eosinophils, lymphocytes, and other inflammatory cells infiltrate the airways. Eosinophils release toxic granule proteins and lipid mediators that damage epithelium and perpetuate inflammation. Type 2 helper T lymphocytes orchestrate this response through cytokines including interleukin-4, interleukin-5, and interleukin-13, which promote immunoglobulin E production, eosinophil recruitment, and mucus hypersecretion.

Chronic asthma produces airway remodeling: persistent inflammation leads to basement membrane thickening, smooth muscle hypertrophy, goblet cell hyperplasia, and angiogenesis. These structural changes may cause fixed airflow limitation that persists despite anti-inflammatory treatment, representing irreversible disease.

Clinical features include episodic wheezing, dyspnea, cough, and chest tightness. Symptoms often worsen at night or early morning. Common triggers include allergens, respiratory infections, exercise, cold air, irritants, and emotional stress. Physical examination during exacerbation reveals tachypnea, accessory muscle use, and diffuse polyphonic expiratory wheezing. Severe attacks may show reduced air entry and absent wheezing as a concerning sign of minimal airflow.

<image>Panel A: Allergen sensitization showing dendritic cells presenting to T cells, B cell IgE production, and IgE binding to mast cell surfaces. Panel B: Acute response with allergen cross-linking IgE causing mast cell degranulation releasing histamine and leukotrienes producing bronchoconstriction, edema, and mucus. Panel C: Late response showing eosinophil infiltration, epithelial damage, and Th2 cytokine release including IL-4, IL-5, and IL-13. Panel D: Airway remodeling in chronic asthma showing thickened basement membrane, smooth muscle hypertrophy, goblet cell metaplasia with cross-sectional comparison of normal versus remodeled airway.</image>

III. Asthma Classification and Treatment

Management of asthma requires accurate assessment of severity and control, guiding selection of appropriate therapy. Treatment follows a stepwise approach, increasing therapy for uncontrolled disease and decreasing when control is achieved and maintained.

Severity classification before initiating treatment categorizes asthma as intermittent, mild persistent, moderate persistent, or severe persistent based on symptom frequency, nocturnal awakening, short-acting beta-agonist use, and lung function. Intermittent asthma involves symptoms two days or fewer per week, nighttime awakening two times or fewer per month, and normal lung function between exacerbations. Mild persistent asthma involves symptoms more than two days per week, nighttime awakening three to four times per month, and lung function at 80 percent of predicted or higher. Moderate persistent asthma involves daily symptoms, nighttime awakening more than once weekly, and lung function between 60 and 80 percent of predicted. Severe persistent asthma involves symptoms throughout the day, frequent nighttime awakening, and lung function below 60 percent of predicted.

The stepwise treatment approach assigns therapy according to severity and adjusts based on response. Step 1, for intermittent asthma, uses only a short-acting beta-agonist as needed for symptoms. Step 2, for mild persistent asthma, adds low-dose inhaled corticosteroid as the preferred controller therapy. Step 3, for moderate persistent asthma, adds a long-acting beta-agonist to low-dose inhaled corticosteroid. Step 4 increases to medium or high-dose inhaled corticosteroid with long-acting beta-agonist. Step 5, for severe asthma, adds additional controllers such as oral corticosteroids or biologic therapies targeting specific inflammatory pathways.

Short-acting beta-agonists such as albuterol provide rapid bronchodilation through beta-2 receptor stimulation of airway smooth muscle relaxation. These agents relieve acute symptoms within minutes but do not address underlying inflammation. Long-acting beta-agonists such as salmeterol and formoterol provide sustained bronchodilation for maintenance therapy but should always be combined with inhaled corticosteroids due to safety concerns with monotherapy. Inhaled corticosteroids represent the cornerstone of controller therapy, reducing airway inflammation, decreasing symptoms, preventing exacerbations, and slowing decline in lung function. Leukotriene receptor antagonists such as montelukast block the effects of cysteinyl leukotrienes, which contribute to bronchoconstriction and inflammation. Biologic therapies including anti-immunoglobulin E (omalizumab) and anti-interleukin-5 (mepolizumab) target specific pathways in severe allergic or eosinophilic asthma.

Acute exacerbation management depends on severity. Mild to moderate exacerbations respond to repeated short-acting beta-agonist treatments and a course of oral corticosteroids. Severe exacerbations require intensive bronchodilator therapy, systemic corticosteroids, supplemental oxygen, and may benefit from ipratropium and intravenous magnesium sulfate. Life-threatening exacerbations with respiratory failure require intensive care and may necessitate mechanical ventilation.

<image>Panel A: Five-step staircase treatment diagram with preferred and alternative controller options at each step plus SABA reliever and step-up/step-down arrows. Panel B: Common inhaler devices with proper technique illustrations for MDI, DPI, and spacer use. Panel C: Acute exacerbation flowchart with severity assessment, treatment escalation based on response, and disposition decisions. Panel D: Key decision points including peak flow measurement, oxygen saturation monitoring, and response to initial bronchodilator therapy guiding discharge, observation, or admission.</image>

IV. COPD Overview

Chronic obstructive pulmonary disease represents a major cause of morbidity and mortality worldwide, characterized by persistent airflow limitation that is typically progressive and associated with an enhanced inflammatory response to inhaled particles and gases. Unlike asthma, the airflow limitation in chronic obstructive pulmonary disease is not fully reversible.

The disease develops from gene-environment interactions, with cigarette smoking as the dominant environmental factor accounting for 80 to 90 percent of cases. Alpha-1 antitrypsin deficiency represents the best-characterized genetic risk factor, responsible for 1 to 2 percent of cases. Occupational exposures to coal dust, silica, cadmium, and organic dust contribute. Air pollution, both outdoor and indoor from biomass fuel combustion, increases risk. Childhood respiratory infections and impaired lung development may predispose to chronic obstructive pulmonary disease in adulthood.

The disease encompasses two overlapping phenotypes that were historically described separately. Emphysema involves destruction of alveolar walls and enlargement of airspaces distal to terminal bronchioles, leading to loss of elastic recoil and impaired gas exchange. The classic emphysema patient, termed the "pink puffer," presents with severe dyspnea, pursed-lip breathing, minimal cough, thin body habitus, and relatively preserved oxygenation until late disease.

Chronic bronchitis involves inflammation of the central airways with mucous gland hyperplasia and goblet cell metaplasia leading to excessive mucus production. The clinical definition requires chronic productive cough for at least three months per year for at least two consecutive years, with other causes excluded. The classic chronic bronchitis patient, termed the "blue bloater," presents with chronic productive cough, less prominent dyspnea, stocky body habitus, hypoxemia, and earlier development of cor pulmonale.

In practice, most patients with chronic obstructive pulmonary disease manifest features of both emphysema and chronic bronchitis in varying proportions. The distinction holds conceptual value but has limited therapeutic implications.

<image>Panel A: COPD risk factors with large smoking icon showing 80-90% attribution plus smaller icons for alpha-1 antitrypsin deficiency, occupational exposure, air pollution, and childhood factors. Panel B: Phenotype spectrum bar from pure emphysema to pure chronic bronchitis with most patients in between. Panel C: Representative patient illustrations showing pink puffer as thin dyspneic with pursed lips versus blue bloater as stocky cyanotic with productive cough. Panel D: Comparison table of emphysema features (parenchymal destruction, dyspnea dominant, thin, late cor pulmonale) versus chronic bronchitis (airway inflammation, cough dominant, stocky, earlier cor pulmonale).</image>

V. Emphysema

Emphysema is defined anatomically as permanent enlargement of airspaces distal to the terminal bronchioles, accompanied by destruction of alveolar walls without obvious fibrosis. This parenchymal destruction underlies the characteristic physiological abnormalities and clinical features of emphysematous chronic obstructive pulmonary disease.

Classification by anatomical distribution describes several patterns. Centriacinar (centrilobular) emphysema affects the central portion of the acinus, destroying the respiratory bronchioles while sparing the distal alveoli. This pattern predominates in the upper lung zones and strongly associates with cigarette smoking. Panacinar (panlobular) emphysema uniformly destroys the entire acinus from respiratory bronchiole to alveoli. This pattern predominates in the lower lung zones and characterizes alpha-1 antitrypsin deficiency. Paraseptal (distal acinar) emphysema affects the peripheral acinus adjacent to pleura and interlobular septa. This pattern predisposes to formation of bullae and may cause spontaneous pneumothorax in young adults. Irregular emphysema occurs in an unpredictable distribution associated with scarring from prior inflammation.

The pathophysiology centers on loss of elastic recoil and destruction of the alveolar-capillary interface. The normal lung maintains small airway patency through radial traction from surrounding alveolar walls. When emphysema destroys these walls, small airways lack mechanical support and collapse during expiration, particularly forced expiration, trapping air distally. The loss of elastic recoil also reduces the driving pressure for expiration, further limiting expiratory flow. Destruction of alveolar septa eliminates pulmonary capillaries along with alveolar surface area, reducing the area for gas exchange and producing diffusion impairment manifest as reduced diffusing capacity for carbon monoxide.

The protease-antiprotease hypothesis explains how smoking causes emphysema. Elastase, a serine protease released primarily by neutrophils, can digest elastin and other structural proteins in alveolar walls. Normally, alpha-1 antitrypsin in lung tissue inhibits elastase, preventing parenchymal destruction. Cigarette smoke attracts neutrophils to the lung, increasing elastase burden. Smoke also oxidizes alpha-1 antitrypsin, inactivating its antiprotease function. The resulting imbalance favors proteolysis and emphysema development. Alpha-1 antitrypsin deficiency produces the same imbalance through genetic deficiency rather than oxidative inactivation.

<image>Panel A: Anatomical classification showing centriacinar destruction of central respiratory bronchioles in upper lobes from smoking, panacinar uniform destruction in lower lobes from alpha-1 antitrypsin deficiency, and paraseptal peripheral destruction with bullae. Panel B: Pathophysiology showing loss of alveolar wall attachments leading to small airway collapse during expiration, reduced elastic recoil, and air trapping with leftward-shifted compliance curve. Panel C: Protease-antiprotease imbalance with neutrophils releasing elastase increased by smoking versus alpha-1 antitrypsin inhibiting elastase decreased by oxidation or genetic deficiency. Panel D: Balance diagram tipped toward proteolysis in emphysema with elastin fiber destruction and airspace enlargement.</image>

VI. Chronic Bronchitis

Chronic bronchitis is defined clinically as chronic productive cough for at least three months in each of two consecutive years, after exclusion of other causes such as bronchiectasis or tuberculosis. This definition emphasizes the symptom of mucus hypersecretion rather than specific pathological findings.

The pathology of chronic bronchitis involves inflammatory changes primarily in the larger airways, with secondary involvement of smaller airways. Mucous gland hyperplasia represents the hallmark finding; the Reid index, measuring the ratio of mucous gland layer thickness to total bronchial wall thickness, exceeds the normal value of 0.4 to become 0.5 or greater in chronic bronchitis. Goblet cell metaplasia replaces ciliated epithelium, increasing mucus production while impairing mucociliary clearance. Inflammatory cell infiltration, predominantly neutrophils and lymphocytes, thickens the airway wall. Smooth muscle hypertrophy and submucosal edema further narrow the airway lumen. Squamous metaplasia may occur in response to chronic irritation.

Small airway disease contributes significantly to airflow limitation in chronic bronchitis. Inflammatory exudate and mucus plug the bronchioles. Fibrotic narrowing of small airways increases resistance. These changes, unlike large airway mucous gland hyperplasia, directly correlate with the severity of airflow obstruction.

Pathophysiological consequences of chronic bronchitis include increased airway resistance, ventilation-perfusion mismatch, and predisposition to infection. Mucus plugging and wall thickening narrow airways and increase resistance. Uneven distribution of mucus and inflammation creates heterogeneous ventilation while perfusion remains relatively preserved, producing low ventilation-perfusion regions and hypoxemia. Chronic mucus accumulation and impaired clearance provide an environment favorable for bacterial colonization and recurrent infections that accelerate disease progression.

Chronic hypoxemia from severe chronic bronchitis leads to pulmonary hypertension through hypoxic pulmonary vasoconstriction and eventual vascular remodeling. Right ventricular hypertrophy (cor pulmonale) develops as the right ventricle adapts to increased afterload. Eventually, right heart failure may supervene.

<image>Panel A: Cross-sections comparing normal airway with thin mucous glands versus chronic bronchitic airway with enlarged glands, Reid index greater than 0.5, goblet cell metaplasia, and intraluminal mucus. Panel B: Reid index calculation diagram with measurement technique plus small airway changes showing mucus plugging, inflammatory narrowing, and fibrosis. Panel C: V/Q diagram showing low V/Q units creating hypoxemia from uneven ventilation distribution. Panel D: Progression from chronic hypoxemia through pulmonary vasoconstriction to pulmonary hypertension to cor pulmonale with dilated thickened right ventricle.</image>

VII. COPD Diagnosis and Staging

Diagnosis of chronic obstructive pulmonary disease requires clinical suspicion based on risk factors and symptoms, confirmed by spirometry demonstrating airflow limitation that is not fully reversible. Staging systems quantify severity and guide management decisions.

Clinical suspicion arises from history of cigarette smoking or other inhalational exposures combined with characteristic symptoms. Dyspnea, initially exertional and later at rest, represents the dominant symptom. Chronic cough with or without sputum production occurs commonly. Fatigue, weight loss, and reduced exercise tolerance develop as disease progresses.

Spirometry provides the objective criterion for diagnosis. A post-bronchodilator forced expiratory volume in one second to forced vital capacity ratio below 0.70 confirms airflow limitation. The post-bronchodilator measurement distinguishes chronic obstructive pulmonary disease from asthma, as fully reversible obstruction suggests asthma. Some authorities advocate using the lower limit of normal rather than the fixed 0.70 cutoff to avoid overdiagnosis in the elderly and underdiagnosis in younger patients.

The Global Initiative for Chronic Obstructive Lung Disease (GOLD) staging system classifies airflow limitation severity by forced expiratory volume in one second as a percentage of predicted. GOLD 1 (mild) corresponds to 80 percent or greater. GOLD 2 (moderate) corresponds to 50 to 79 percent. GOLD 3 (severe) corresponds to 30 to 49 percent. GOLD 4 (very severe) corresponds to below 30 percent.

Because spirometric severity alone does not capture the full clinical picture, GOLD recommends additional assessment of symptoms and exacerbation risk. Symptom burden is assessed using standardized questionnaires: the modified Medical Research Council dyspnea scale or the COPD Assessment Test. Exacerbation risk considers the number and severity of prior exacerbations. The ABCD assessment combines these factors: Group A has few symptoms and low exacerbation risk; Group B has more symptoms but low exacerbation risk; Group C has few symptoms but high exacerbation risk; Group D has more symptoms and high exacerbation risk. This classification guides initial pharmacotherapy selection.

<image>Panel A: Diagnostic pathway from clinical features (dyspnea, cough, exposures) to spirometry with positive diagnosis criteria of post-bronchodilator FEV1/FVC less than 0.70 and obstructive pattern tracing. Panel B: GOLD grades vertical bar showing GOLD 1 at 80% or greater, GOLD 2 at 50-79%, GOLD 3 at 30-49%, GOLD 4 less than 30% color-coded green to red. Panel C: ABCD assessment grid with symptoms on horizontal axis and exacerbation risk on vertical axis creating four quadrants. Panel D: Representative patient profiles in each quadrant A through D with corresponding symptom burden and exacerbation frequency characteristics.</image>

VIII. COPD Management

Management of chronic obstructive pulmonary disease aims to reduce symptoms, improve quality of life, prevent exacerbations, and slow disease progression. Treatment combines non-pharmacologic interventions with medications tailored to disease severity and phenotype.

Smoking cessation represents the single most important intervention, as it is the only measure proven to slow the rate of lung function decline. All patients who smoke should receive counseling and pharmacotherapy support for cessation. Even in advanced disease, quitting smoking provides benefit.

Pulmonary rehabilitation programs combine exercise training, education, and behavioral modification. These programs improve exercise capacity, reduce dyspnea, and enhance quality of life, though they do not directly affect lung function. Rehabilitation provides benefit across all disease severities and is particularly valuable following exacerbations.

Vaccination against influenza annually and against pneumococcal disease reduces the risk of respiratory infections that can trigger exacerbations. COVID-19 vaccination is similarly recommended.

Long-term oxygen therapy improves survival in patients with severe resting hypoxemia, defined as arterial oxygen tension below 55 mmHg or oxygen saturation below 88 percent. The mechanism likely involves reduction of pulmonary hypertension and right ventricular strain.

Pharmacotherapy follows a stepwise approach guided by the ABCD assessment. Group A patients, with few symptoms and low exacerbation risk, require only a bronchodilator for symptom relief. Group B patients, with more symptoms, benefit from long-acting bronchodilator maintenance therapy, either a long-acting muscarinic antagonist or long-acting beta-agonist. Group C patients, with high exacerbation risk, should receive a long-acting muscarinic antagonist as initial therapy. Group D patients need combined long-acting muscarinic antagonist and long-acting beta-agonist, with consideration of adding inhaled corticosteroid if exacerbations persist, particularly in patients with eosinophilic inflammation. Phosphodiesterase-4 inhibitors such as roflumilast may benefit patients with chronic bronchitis phenotype and frequent exacerbations.

Acute exacerbation management intensifies bronchodilator therapy, adds systemic corticosteroids for five days, includes antibiotics if purulent sputum or severe symptoms suggest bacterial infection, provides supplemental oxygen titrated to saturation 88 to 92 percent, and considers non-invasive ventilation for patients with respiratory acidosis.

<image>Panel A: Non-pharmacologic foundation showing smoking cessation as most important plus pulmonary rehabilitation, vaccination, and oxygen therapy with criteria PaO2 less than 55 or SpO2 less than 88%. Panel B: Pharmacotherapy by ABCD group with A receiving bronchodilator PRN, B receiving LAMA or LABA, C receiving LAMA, D receiving LAMA plus LABA with or without ICS. Panel C: Acute exacerbation management showing increased bronchodilators, systemic steroids for 5 days, antibiotics if indicated, and oxygen targeting 88-92%. Panel D: NIV for respiratory acidosis with disposition flowchart guiding emergency department to hospital or ICU based on severity.</image>

IX. Bronchiectasis

Bronchiectasis describes permanent abnormal dilation of bronchi resulting from chronic infection and inflammation. This structural damage perpetuates a vicious cycle of impaired mucociliary clearance, bacterial colonization, and further inflammation that progressively destroys airways.

The causes of bronchiectasis are numerous, though approximately half of cases remain idiopathic despite thorough evaluation. Post-infectious bronchiectasis follows severe childhood pneumonia, tuberculosis, or nontuberculous mycobacterial infection. Immune deficiency states including hypogammaglobulinemia and human immunodeficiency virus infection predispose through impaired pathogen clearance. Genetic conditions include cystic fibrosis, the most common cause in developed countries, and primary ciliary dyskinesia, which impairs mucociliary clearance through ciliary dysfunction. Chronic aspiration damages airways through chemical and infectious injury. Autoimmune diseases including rheumatoid arthritis and inflammatory bowel disease associate with bronchiectasis through unclear mechanisms. Allergic bronchopulmonary aspergillosis causes bronchiectasis through hypersensitivity reaction to Aspergillus colonization. Focal bronchiectasis may develop distal to an obstructing lesion such as tumor or foreign body.

The pathophysiology involves a self-perpetuating cycle. Initial airway injury impairs mucociliary clearance, allowing bacterial colonization. Chronic bacterial presence stimulates persistent inflammation. Inflammatory mediators damage the bronchial wall, leading to dilation and further impaired clearance. Organisms including Pseudomonas aeruginosa, Haemophilus influenzae, and Staphylococcus aureus commonly colonize bronchiectatic airways.

Clinical features reflect the chronic purulent bronchitis that characterizes the condition. Chronic productive cough with copious purulent sputum, sometimes exceeding 30 milliliters daily, represents the cardinal symptom. Hemoptysis occurs frequently due to bronchial artery hypertrophy and rupture. Dyspnea develops as airflow obstruction and parenchymal destruction progress. Recurrent respiratory infections punctuate the chronic course as acute exacerbations. Physical examination reveals crackles and rhonchi over affected lung regions. Clubbing may develop with advanced disease.

Management focuses on breaking the vicious cycle. Airway clearance techniques including chest physiotherapy, positive expiratory pressure devices, and oscillatory devices mobilize secretions. Mucolytic agents such as hypertonic saline and N-acetylcysteine may improve secretion clearance. Antibiotics treat acute exacerbations and, in some patients with frequent exacerbations or chronic Pseudomonas colonization, chronic suppressive therapy may be warranted. Bronchodilators address the airflow obstruction component. Surgical resection may be considered for localized disease with recurrent hemoptysis or infections refractory to medical management.

<image>Panel A: Bronchiectasis causes by category including post-infectious (pneumonia, TB, NTM), immune deficiency, genetic (cystic fibrosis, PCD), aspiration, autoimmune, ABPA, obstruction, and idiopathic at 50%. Panel B: Vicious cycle showing impaired clearance leading to bacterial colonization causing inflammation damaging airway and worsening clearance with dilated bronchus containing mucus and inflammatory cells. Panel C: CT imaging showing typical findings of dilated airways with thickened walls and signet ring sign. Panel D: Management approaches including airway clearance devices, nebulized mucolytics, acute and chronic antibiotics, bronchodilators, and surgery for refractory cases.</image>

X. Pulmonary Function in Obstructive Disease

Pulmonary function testing provides objective assessment of obstructive lung diseases, confirming diagnosis, quantifying severity, evaluating reversibility, and monitoring progression or treatment response. Understanding the characteristic patterns enables accurate interpretation.

Spirometry demonstrates the hallmark abnormality: reduced ratio of forced expiratory volume in one second to forced vital capacity. This ratio falls below 0.70 in established obstructive disease. Forced expiratory volume in one second is reduced, sometimes markedly in severe disease. Forced vital capacity may be normal in mild disease but decreases in moderate to severe disease as air trapping prevents complete expiration. Assessment of bronchodilator reversibility distinguishes partially reversible obstruction (typical of asthma) from fixed obstruction (typical of chronic obstructive pulmonary disease). Significant reversibility is defined as improvement in forced expiratory volume in one second of at least 12 percent and 200 milliliters after bronchodilator administration.

Lung volume measurements reveal the consequences of air trapping. Total lung capacity is normal or increased due to hyperinflation. Residual volume is increased, sometimes dramatically, as air remains trapped at end-expiration. Functional residual capacity is increased as the resting lung volume shifts upward. The ratio of residual volume to total lung capacity increases as proportionally more air becomes trapped.

Diffusing capacity for carbon monoxide differentiates among obstructive diseases. In asthma, where the parenchyma is unaffected, diffusing capacity remains normal. In emphysema, where alveolar septa and their capillaries are destroyed, diffusing capacity decreases proportionally to the extent of parenchymal destruction. In chronic bronchitis without significant emphysema, diffusing capacity is normal or only mildly reduced. This distinction helps characterize the relative contributions of emphysema and chronic bronchitis in individual patients.

Flow-volume loops provide visual representation of obstruction. The expiratory limb shows a characteristic scooped or concave appearance as flow decreases disproportionately to volume during forced expiration. Peak expiratory flow is reduced. The inspiratory limb typically remains normal, as negative intrathoracic pressure during inspiration tends to hold airways open. Comparison of pre- and post-bronchodilator loops demonstrates any reversibility.

<image>Panel A: Flow-volume loops comparing normal versus asthma with reversible scooping versus COPD with fixed scooping and reduced peak flow showing pre- and post-bronchodilator tracings. Panel B: Lung volume bar graphs comparing normal to obstructive with increased TLC, markedly increased RV, and elevated RV/TLC ratio highlighted. Panel C: PFT comparison table across asthma, emphysema, chronic bronchitis, and bronchiectasis showing all with reduced FEV1/FVC. Panel D: Differentiating features with DLCO normal in asthma and chronic bronchitis but reduced in emphysema and reversibility significant in asthma but minimal in COPD.</image>

Summary

Obstructive lung diseases share the defining feature of reduced FEV1/FVC ratio below 0.70, reflecting increased resistance to expiratory airflow. Air trapping and hyperinflation result from premature airway closure, increasing residual volume and total lung capacity.

Asthma is a chronic inflammatory disease characterized by reversible airflow obstruction, bronchial hyperresponsiveness, and eosinophilic inflammation. The pathophysiology involves IgE-mediated mast cell degranulation in the early phase and eosinophil-driven inflammation in the late phase, with chronic disease producing airway remodeling. Treatment follows a stepwise approach with inhaled corticosteroids as the cornerstone of controller therapy and short-acting beta-agonists for symptom relief.

Chronic obstructive pulmonary disease encompasses emphysema and chronic bronchitis, with most patients manifesting features of both. Emphysema involves parenchymal destruction through protease-antiprotease imbalance, producing loss of elastic recoil and reduced diffusing capacity. Chronic bronchitis involves airway inflammation and mucous gland hyperplasia, producing chronic productive cough. Smoking cessation is the only intervention proven to slow lung function decline. Pharmacotherapy uses bronchodilators, with long-acting muscarinic antagonists and long-acting beta-agonists as maintenance therapy, and inhaled corticosteroids added for patients with exacerbations and eosinophilic inflammation.

Bronchiectasis involves permanent bronchial dilation from chronic infection and inflammation, creating a vicious cycle of impaired clearance, colonization, and progressive damage. Management emphasizes airway clearance and treatment of infections.

Pulmonary function testing confirms obstructive pattern, assesses severity and reversibility, and helps distinguish among obstructive diseases through diffusing capacity measurement, which is reduced in emphysema but preserved in asthma and chronic bronchitis.

Key Terms

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