## Distinguishing Obstructive from Restrictive Patterns ### Key Spirometric Discriminators **Key Point:** The hallmark of obstruction is a reduced FEV₁/FVC ratio (< 0.70) *despite* normal or increased total lung capacity (TLC). In restriction, both FEV₁ and FVC fall proportionally, preserving or even raising the FEV₁/FVC ratio. ### Comparison Table: Obstructive vs Restrictive | Feature | Obstructive | Restrictive | | --- | --- | --- | | **FEV₁/FVC** | ↓ (< 0.70) | Normal or ↑ (≥ 0.70) | | **TLC** | Normal or ↑ | ↓ | | **FVC** | Normal or mildly ↓ | ↓ | | **FEV₁** | ↓ | ↓ | | **RV** | ↑ | Normal or ↓ | | **Mechanism** | Air trapping (↑ RV) | Loss of lung parenchyma or chest wall stiffness | **High-Yield:** The *combination* of reduced FEV₁/FVC *with* normal or elevated TLC is pathognomonic for obstruction. This reflects air trapping (elevated residual volume) despite preserved total lung volume. ### Why This Patient Has Obstruction His FEV₁/FVC of 0.62 is reduced, and his normal TLC indicates his lungs are not shrunken—they are hyperinflated due to air trapping from small-airway disease (COPD). If he had restriction (e.g., pulmonary fibrosis), his TLC would be reduced, and his FEV₁/FVC would remain ≥ 0.70. **Clinical Pearl:** In early COPD, FVC may be nearly normal, but FEV₁ drops disproportionately, widening the gap. TLC is often normal or increased because of emphysematous destruction and loss of elastic recoil. ### Why Other Options Are Wrong - **Reduced FEV₁/FVC alone:** Both obstructive and restrictive patterns can show reduced FEV₁/FVC if restriction is severe and RV is preserved (e.g., neuromuscular disease). Not discriminatory on its own. - **Reduced FVC:** Occurs in both obstruction (mild-to-moderate) and restriction. Not specific. - **Reduced DLCO:** While COPD with emphysema often shows reduced DLCO, restrictive diseases (pulmonary fibrosis) also show reduced DLCO. Not a discriminator.
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