## Pathophysiology of V/Q Mismatch in COPD **Key Point:** In emphysema, the primary gas exchange defect is ventilation-perfusion (V/Q) mismatch, not true shunt. Areas of destroyed lung parenchyma have low V/Q ratios (perfusion exceeds ventilation), causing hypoxemia that is **oxygen-responsive**. ### Why Supplemental O₂ Works in V/Q Mismatch In regions with low V/Q ratios: - Blood arriving at these areas is incompletely oxygenated - Increasing the FiO₂ raises the PaO₂ of the small amount of ventilation reaching these zones - This oxygen-enriched air equilibrates with capillary blood, improving overall arterial oxygenation - The improvement is dramatic because the problem is **not absolute lack of ventilation** but rather **mismatch** ### Contrast: True Shunt vs V/Q Mismatch | Feature | V/Q Mismatch | True Shunt | |---------|-------------|----------| | Mechanism | Low V/Q areas (some ventilation present) | Zero ventilation (V/Q = 0) | | Response to O₂ | **Excellent** — PaO₂ rises significantly | Poor — minimal response to O₂ | | DLCO | Often reduced (emphysema) | Normal | | A-a gradient | Widens with O₂ | Remains wide despite O₂ | **Clinical Pearl:** The 55 mmHg improvement in PaO₂ (65→120) with modest FiO₂ increase is classic for V/Q mismatch. A true shunt would show minimal improvement even with high-flow oxygen. **High-Yield:** Emphysema causes V/Q mismatch (not shunt) because: 1. Alveolar destruction reduces ventilation to some areas 2. Pulmonary capillaries remain patent and perfused 3. Result: perfusion > ventilation in affected zones 4. Supplemental O₂ corrects the low PaO₂ in these areas [cite:West's Respiratory Physiology 10e Ch 3]
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