Obstructive Lung Disease: Physiology, Phenotypes, and Treatment Levers
Start with physiology, not symptoms
Airflow obstruction is defined by a reduced post-bronchodilator FEV1/FVC ratio. The GOLD fixed cutoff of 0.70 is simple but age-dependent: it over-calls obstruction in older adults and under-calls it in younger ones, whereas the lower limit of normal (fifth percentile) is age-adjusted. Questions frequently ask which criterion is more appropriate in an older patient with a borderline ratio. Severity in COPD is graded separately by FEV1 percent predicted: GOLD 1 ≥80%, 2 = 50–79%, 3 = 30–49%, 4 <30%. Remember that the grade itself does not dictate drug choice; the ABE group (symptoms plus exacerbation history) does.
Distinguish the phenotypes
Asthma is characterized by variable, largely reversible obstruction - a bronchodilator response of ≥12% AND ≥200 mL, or variability on serial testing. COPD shows persistent obstruction, and a low DLCO with hyperinflation (increased RV/TLC, flattened hemidiaphragms) points to emphysema. Chronic bronchitis is a clinical definition: productive cough for ≥3 months in 2 consecutive years. Alpha-1 antitrypsin deficiency deserves testing in young patients, basilar-predominant emphysema, or coexisting liver disease.
Treatment
Inhaled long-acting bronchodilators are first-line; adding a long-acting muscarinic antagonist to a long-acting beta agonist improves symptoms and reduces exacerbations. Inhaled corticosteroids are not routine - they are reserved for frequent exacerbators or eosinophils ≥300 cells/µL, and they increase pneumonia risk. Smoking cessation is the only intervention proven to slow FEV1 decline. In acute exacerbation, controlled oxygen to an SpO2 of 88–92% in CO2 retainers is a classic exam trap.
Worked rule
A 68-year-old smoker has FEV1/FVC 0.55, FEV1 45% predicted, DLCO 55%: GOLD 3 COPD. If he has had no exacerbations and eosinophils are 100, give dual bronchodilator therapy - not ICS.