## Obstructive Pattern Recognition on Spirometry **Key Point:** An obstructive pattern is defined by a reduced FEV₁/FVC ratio (typically <0.70), reflecting disproportionate loss of expiratory flow relative to vital capacity. ### Characteristic Features of Obstruction | Feature | Finding | Mechanism | |---------|---------|----------| | FEV₁/FVC | **Decreased** (<0.70) | Air trapping and flow limitation | | FEV₁ | Variable (mild–severe) | Depends on disease stage | | Inspiratory curve | **Normal** | Obstruction is expiratory | | Expiratory curve | **Scooped/concave** | Dynamic airway collapse | | RV and TLC | **Increased** | Air trapping | | PEF | **Decreased** | Loss of expiratory power | **High-Yield:** The FEV₁/FVC ratio is the **gold standard discriminator** between obstructive and restrictive patterns. In obstruction, the ratio *always* falls; in mild disease, FEV₁ may still be >80% predicted, but the ratio remains low. ### Why Option 4 is Wrong An **increased or normal FEV₁/FVC ratio** (e.g., 0.80 or higher) is **not consistent** with an obstructive pattern. This ratio is the defining criterion for obstruction; if it is preserved or elevated, the pattern is either: - **Restrictive** (both FEV₁ and FVC reduced proportionally, ratio normal or high) - **Normal** In mild obstructive disease, FEV₁ may be preserved (>80% predicted), but the FEV₁/FVC ratio **must still be low** (<0.70) to diagnose obstruction. ### Why Options 1, 2, 3 Are Correct - **Option 1 (Increased RV/TLC):** Classic finding in obstruction due to air trapping and dynamic hyperinflation. - **Option 2 (Decreased PEF, scooped curve):** Hallmark of expiratory flow limitation; the curve becomes concave during forced expiration. - **Option 3 (Decreased DLCO in emphysema):** Emphysematous COPD shows reduced DLCO due to alveolar destruction; asthma typically preserves DLCO. **Clinical Pearl:** A patient with FEV₁ 62% predicted but an FEV₁/FVC ratio of 0.68 has **moderate obstruction**. The low ratio confirms the diagnosis; the preserved FEV₁ (relative to severity) suggests either asthma or early COPD. [cite:West Respiratory Physiology 10e Ch 9]
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