In a patient with obstructive airway disease, which spirometric pattern is characteristic?

Explanation

## Obstructive Pattern — Spirometric Hallmark **Key Point:** The hallmark of obstructive airway disease is a **reduced FEV₁/FVC ratio** (typically <70%), with relatively preserved or increased total lung capacity due to air trapping. ### Mechanism 1. Airway obstruction causes **slow expiration** → FEV₁ falls disproportionately 2. Air trapping occurs → residual volume increases 3. Total lung capacity remains **normal or elevated** (unlike restriction) 4. FEV₁/FVC ratio drops below 70% (normal ≥80%) ### Spirometric Features of Obstruction | Feature | Obstructive | Restrictive | |---------|-------------|-------------| | **FEV₁/FVC** | ↓ (<70%) | Normal or ↑ (≥80%) | | **FEV₁** | ↓ | ↓ | | **FVC** | Normal or ↓ | ↓ | | **TLC** | Normal or ↑ | ↓ | | **RV** | ↑ (air trapping) | Normal | **High-Yield:** The **FEV₁/FVC ratio is the gold standard** for detecting obstruction. A ratio <70% with reduced FEV₁ = obstruction, regardless of absolute FVC value. **Clinical Pearl:** In severe obstruction, FVC may also be reduced (due to air trapping limiting expiration time), but TLC remains elevated — this distinguishes it from restriction. [cite:Harrison 21e Ch 246]