## Restrictive Lung Disease Pattern **Key Point:** The FEV₁/FVC ratio >0.70 (here 0.82) with reduced FEV₁ and FVC in proportion defines a restrictive pattern. The markedly reduced DLCO (48%) is disproportionate and indicates parenchymal disease (interstitial fibrosis). ### Spirometric Pattern Breakdown | Parameter | Patient Value | Expected (Normal) | Interpretation | |-----------|---------------|-------------------|----------------| | FEV₁ | 68% predicted | >80% | Mildly reduced | | FVC | 62% predicted | >80% | **Moderately reduced** | | FEV₁/FVC | 0.82 | >0.70 | **Restrictive pattern** | | DLCO | 48% predicted | >80% | **Severely reduced** | **High-Yield:** In restrictive disease, both FEV₁ and FVC fall proportionally, keeping the ratio normal or elevated (>0.70). The DLCO falls out of proportion because the pathology is parenchymal (fibrosis, inflammation) affecting gas exchange, not just reducing lung volume. ### Pathophysiology in Rheumatoid Arthritis–Associated ILD **Clinical Pearl:** Rheumatoid arthritis can cause pulmonary fibrosis (RA-ILD), which: 1. Stiffens the lungs (reduces compliance) 2. Reduces total lung capacity (TLC) and vital capacity (VC) 3. Destroys alveolar-capillary membrane → severe DLCO reduction 4. Preserves airway caliber → FEV₁/FVC remains normal or high **Mnemonic — Restrictive Pattern Features:** **SMALL** - **S**piral: FEV₁/FVC >0.70 (preserved ratio) - **M**oderate to severe reduction in FVC - **A**lveolar disease → DLCO ↓↓ (disproportionate) - **L**ung volume: TLC <80% predicted - **L**ow compliance (stiff lungs) ### Why DLCO is Disproportionately Low DLCO depends on: - Alveolar-capillary surface area (destroyed in fibrosis) - Membrane thickness (thickened in fibrosis) - Hemoglobin (normal here) Fibrosis reduces surface area and thickens the membrane → DLCO falls to 48% despite only modest spirometric reduction. [cite:Harrison 21e Ch 247]
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