A 42-year-old woman with a history of rheumatoid arthritis presents with insidious onset of dyspnea and dry cough over 3 months. Chest X-ray shows bilateral interstitial opacities. Spirometry reveals FEV₁ = 68% predicted, FVC = 62% predicted, FEV₁/FVC ratio = 0.88, and TLC = 65% predicted. DLCO is 55% predicted. What is the most likely spirometric diagnosis?

Explanation

## Restrictive Pattern Diagnosis ### Spirometric Interpretation | Parameter | Value | Interpretation | |-----------|-------|----------------| | **FEV₁/FVC** | **0.88** | **≥ 0.70 → Preserved ratio (not obstructed)** | | **FEV₁** | 68% | Reduced, but proportionally | | **FVC** | 62% | **Reduced** | | **TLC** | **65% predicted** | **< 80% → Hallmark of restriction** | | **DLCO** | 55% | Reduced (interstitial fibrosis) | ### Diagnostic Criteria for Restriction **Key Point:** Restrictive pattern is defined by: 1. **TLC < 80% predicted** (most important criterion) 2. **FEV₁/FVC ratio ≥ 0.70** (preserved, even elevated) 3. **FEV₁ and FVC both reduced proportionally** This patient meets all three criteria. ### Clinical Context: Rheumatoid Arthritis–Associated Interstitial Lung Disease (RA-ILD) **High-Yield:** RA is the **most common connective tissue disease causing ILD** (10–15% of RA patients develop pulmonary fibrosis). **Clinical Pearl:** The combination of: - Bilateral interstitial opacities on CXR - Reduced TLC (65%) - Reduced DLCO (55%) - Preserved FEV₁/FVC ratio (0.88) ...is pathognomonic for **pulmonary fibrosis** (restrictive physiology with impaired alveolar-capillary diffusion). **Mnemonic: RSVP** — **Restrictive Spirometry pattern**: - **R**educed TLC (< 80%) - **S**pared (preserved) FEV₁/FVC (≥ 0.70) - **V**alues proportionally reduced (FEV₁ and FVC both ↓) - **P**attern = restriction ### Why DLCO Is Low Interstitial fibrosis thickens the alveolar-capillary membrane → reduced gas diffusion. DLCO is **disproportionately reduced** relative to TLC, a hallmark of ILD.