Which of the following best describes the spirometric finding in obstructive lung disease?

Explanation

## Obstructive Lung Disease Spirometry Pattern **Key Point:** Obstructive disease is characterized by airway narrowing, leading to reduced FEV₁ with relatively preserved FVC, resulting in a **reduced FEV₁/FVC ratio (<70%)** and **elevated RV/TLC ratio** (air trapping). ### Pathophysiology Airway obstruction causes: - **Reduced expiratory flow** → FEV₁ falls disproportionately - **Preserved vital capacity** → FVC remains near-normal (patient can still inhale fully) - **Air trapping** → residual volume (RV) increases, TLC may increase Common causes: - COPD (emphysema, chronic bronchitis) - Asthma - Bronchiectasis - Cystic fibrosis ### Diagnostic Spirometric Criteria | Finding | Obstructive | Restrictive | |---------|-------------|-------------| | FEV₁ | ↓ | ↓ | | FVC | Normal or ↓ | ↓ | | FEV₁/FVC | **<70%** | >80% | | RV/TLC | **>35–40%** | <35% | | TLC | Normal or ↑ | ↓ | | Response to bronchodilator | Improvement (≥12% & ≥200 mL) | No improvement | **Mnemonic:** **OAFS** — Obstruction = Air Flow reduced, FEV₁/FVC <70%, air trapping (elevated RV/TLC). **High-Yield:** The FEV₁/FVC ratio <70% is the sine qua non of obstruction. Combined with elevated RV/TLC (>35–40%), it confirms air trapping and rules out restriction. **Clinical Pearl:** Post-bronchodilator spirometry showing ≥12% and ≥200 mL improvement in FEV₁ suggests asthma (reversible obstruction); lack of response suggests COPD (fixed obstruction). [cite:Harrison 21e Ch 246]