A 42-year-old woman with a history of rheumatoid arthritis presents with progressive dyspnea and dry cough over 3 months. High-resolution CT chest shows diffuse interstitial infiltrates. Spirometry reveals FEV₁ = 65% predicted, FVC = 58% predicted, FEV₁/FVC ratio = 0.88, and TLC = 62% predicted. Which lung volume compartment is most significantly reduced in this patient?

Explanation

## Restrictive Lung Disease Pattern **Key Point:** This patient demonstrates a **restrictive pattern** with: - FEV₁/FVC ratio = 0.88 (normal, > 0.70) - Both FEV₁ and FVC reduced proportionally (65% and 58% predicted) - TLC = 62% predicted (reduced) ### Clinical Context The combination of: - Rheumatoid arthritis (connective tissue disease) - Diffuse interstitial infiltrates on HRCT - Restrictive spirometry ...indicates **Rheumatoid Arthritis-associated Interstitial Lung Disease (RA-ILD)**, a common extra-articular manifestation. ### Lung Volume Compartments in Restrictive Disease ```mermaid flowchart TD A[Restrictive Lung Disease]:::outcome --> B[Reduced TLC]:::outcome B --> C[All compartments reduced proportionally]:::action C --> D[RV reduced]:::action C --> E[ERV reduced]:::action C --> F[IRV reduced]:::action C --> G[VC reduced]:::action H[Mechanism: Loss of elastic recoil<br/>Pulmonary fibrosis<br/>Reduced lung compliance]:::outcome --> B ``` ### Lung Volume Compartments | Compartment | Definition | Status in Restrictive Disease | |-------------|-----------|-------------------------------| | **TLC** | VC + RV | ↓↓ (diagnostic) | | **VC** | IRV + TV + ERV | ↓ | | **RV** | Air remaining after maximal expiration | ↓ (proportionally) | | **FRC** | ERV + RV | ↓ | | **IRV** | Air that can be inspired after normal inspiration | ↓ | | **ERV** | Air that can be expired after normal expiration | ↓ | **High-Yield:** In restrictive disease, **all lung volumes are reduced proportionally** because the lungs are mechanically stiff and cannot expand normally. RV is NOT preserved (as in obstruction); it is reduced along with everything else. **Mnemonic: RESTRICTIVE = REDUCE ALL (RAA)** — Reduced All compartments, including RV, ERV, IRV, TLC, and FRC. ### Contrast with Obstructive Disease | Feature | Restrictive | Obstructive | |---------|------------|-------------| | **TLC** | ↓ | ↑ or normal | | **RV** | ↓ | ↑↑ (air trapping) | | **FEV₁/FVC** | Normal or ↑ | ↓ | | **Mechanism** | Stiff lungs, reduced compliance | Air trapping, loss of elastic recoil | | **Cause** | Fibrosis, ILD, chest wall disease | COPD, asthma, emphysema | **Clinical Pearl:** The preserved FEV₁/FVC ratio (0.88) in this patient is a key discriminator. In restrictive disease, the ratio is normal because both FEV₁ and FVC fall together. The diagnosis of restriction is made by **reduced TLC on body plethysmography**, not spirometry alone. [cite:Harrison 21e Ch 297]