## Distinguishing Obstructive from Restrictive Lung Disease on Spirometry ### Key Pathophysiology **Key Point:** The FEV₁/FVC ratio is the single best discriminator between obstructive and restrictive patterns because it reflects the fundamental difference in airway mechanics. - **Obstructive disease**: Air trapping and loss of elastic recoil cause disproportionate reduction in FEV₁ (forced expiratory volume in 1 second) relative to FVC (forced vital capacity). The ratio falls below 70% (or 0.70). - **Restrictive disease**: Both FEV₁ and FVC are reduced proportionally because the lungs are stiff and cannot expand fully. The ratio remains normal or may even be elevated (>70%). ### Comparative Spirometric Features | Feature | Obstructive | Restrictive | |---------|-------------|-------------| | **FEV₁/FVC ratio** | **↓ <70%** | **Normal or ↑ (>70%)** | | FEV₁ | ↓↓ (marked) | ↓ (mild to moderate) | | FVC | Normal or ↓ (mild) | ↓↓ (marked) | | TLC | ↑ (air trapping) | ↓ | | RV | ↑↑ (air trapping) | Normal or ↓ | | DLCO | ↓ (emphysema) or normal (asthma, chronic bronchitis) | ↓ (fibrosis) or normal | ### Why This Ratio Works **Clinical Pearl:** In obstructive disease, the patient cannot empty the lungs quickly—FEV₁ drops dramatically while FVC remains relatively preserved. In restrictive disease, both volumes are small but the patient *can* empty what little volume is present—hence the ratio stays normal. **High-Yield:** The FEV₁/FVC ratio is the **gold standard discriminator** because it is independent of patient effort and body size (it is a ratio, not an absolute value). ### Additional Confirmatory Tests - **Lung volumes (plethysmography)**: TLC is elevated in obstruction (air trapping) and reduced in restriction. - **DLCO**: Often reduced in restrictive disease (fibrosis reduces gas exchange surface); variable in obstruction. **Mnemonic: OBSTRUCTIVE = OBSTRUCT the ratio (FEV₁/FVC ↓)** [cite:Harrison 21e Ch 246]
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