A 68-year-old woman with idiopathic pulmonary fibrosis (IPF) and a 35-year-old man with moderate persistent asthma both present for pulmonary function testing. Both show reduced FEV₁ (60% predicted). Which spirometric feature best distinguishes the restrictive pattern of IPF from the obstructive pattern of asthma?

Explanation

## Obstructive vs. Restrictive: FEV₁/FVC as the Discriminator **Key Point:** The FEV₁/FVC ratio is the single most reliable spirometric parameter to distinguish obstructive from restrictive airway disease, even when both show reduced absolute FEV₁. ### Pathophysiologic Basis **Restrictive Disease (IPF):** - Both FEV₁ and FVC are reduced **proportionally**. - The lungs are stiff; expiration is **not obstructed** — it is simply limited by reduced lung volume. - FEV₁/FVC ratio remains **normal or elevated (>0.80)** because the patient can empty the reduced lung volume efficiently. - Example: FEV₁ 60%, FVC 70% → FEV₁/FVC = 0.86 (normal). **Obstructive Disease (Asthma, COPD):** - FEV₁ is reduced **disproportionately** to FVC due to airway collapse during forced expiration. - FVC may be normal or only mildly reduced (because total lung capacity is normal or increased). - FEV₁/FVC ratio is **reduced (<0.70)** because the patient cannot expel air in the first second. - Example: FEV₁ 60%, FVC 90% → FEV₁/FVC = 0.67 (obstructed). ### Comparison Table | Parameter | Restrictive (IPF) | Obstructive (Asthma) | |-----------|-------------------|----------------------| | **FEV₁/FVC ratio** | **>0.80 (normal/elevated)** | **<0.70 (reduced)** | | FEV₁ | Reduced | Reduced | | FVC | Reduced | Normal or mildly reduced | | TLC | Reduced | Normal or increased | | RV | Normal or reduced | Elevated | | RV/TLC | Normal | Elevated | **High-Yield:** **FEV₁/FVC ratio is the gold standard discriminator.** If both FEV₁ and FVC are reduced but the ratio is >0.80, think **restrictive**. If FEV₁ is reduced out of proportion to FVC (ratio <0.70), think **obstructive**. **Clinical Pearl:** In IPF, the patient has a "small but efficient" lung — reduced volume but normal airway mechanics. In asthma, the patient has a "normal-sized but obstructed" lung — normal capacity but impaired flow. **Mnemonic:** **FERN** — **F**EV₁/FVC **E**levated → **R**estrictive; **N**ormal airway mechanics.