## Interpretation of Spirometry and Lung Volumes **Key Point:** The combination of reduced FEV₁/FVC ratio (<0.70) with elevated TLC and markedly elevated RV is pathognomonic for obstructive airway disease with air trapping, characteristic of COPD. ### Pattern Recognition | Parameter | Finding | Interpretation | |-----------|---------|----------------| | FEV₁/FVC | 0.58 (<0.70) | **Obstructive pattern** | | TLC | 125% predicted | **Hyperinflation** | | RV | 180% predicted | **Severe air trapping** | | FVC | 68% predicted | Reduced due to air trapping limiting expiration | **High-Yield:** In COPD, the RV/TLC ratio is typically >35–40% (normal <35%), reflecting inability to empty the lungs due to small airway collapse during expiration. ### Pathophysiology of Air Trapping 1. Loss of elastic recoil in emphysema → airways collapse during expiration 2. Residual air remains in alveoli → RV increases disproportionately 3. Patient adopts pursed-lip breathing to maintain positive airway pressure 4. TLC increases as lungs become hyperinflated **Clinical Pearl:** The clinical triad of diminished breath sounds + prolonged expiration + hyperinflation on imaging confirms obstructive disease. Smoking history and chronic productive cough are classic for COPD. ### Why This Is COPD, Not Asthma - **Asthma** typically shows reversibility of airflow obstruction after bronchodilators; COPD shows minimal reversibility - **Asthma** does not cause permanent TLC elevation or RV elevation unless severe acute exacerbation - This patient's stable, progressive course over 6 months favors COPD **Mnemonic:** **TRAP** = **T**LC elevated, **R**V elevated, **A**irflow obstruction, **P**rogressive in COPD.
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