## Investigation of Choice for V/Q Mismatch Quantification ### Why V/Q Scan is the Gold Standard **Key Point:** The ventilation-perfusion scan is the gold standard investigation for detecting and quantifying regional V/Q mismatch by simultaneously imaging ventilation (using inhaled radioactive tracer) and perfusion (using intravenous radioactive tracer). **High-Yield:** In COPD, V/Q mismatch is the primary mechanism of hypoxemia. Areas with poor ventilation but preserved perfusion (low V/Q units) or areas with ventilation but no perfusion (dead space) create the characteristic hypoxemia seen in this patient. ### Mechanism of V/Q Scan 1. **Ventilation phase:** Inhaled technetium-99m (Tc-99m) diethylenetriaminepentaacetic acid (DTPA) or xenon-133 gas distributes to ventilated lung areas 2. **Perfusion phase:** Intravenous Tc-99m macroaggregated albumin (MAA) distributes to perfused lung areas 3. **Mismatch detection:** Areas with ventilation but no perfusion (V/Q > 1) or perfusion but no ventilation (V/Q < 1) are identified ### Clinical Application in COPD **Clinical Pearl:** V/Q scan helps identify which lung regions are contributing to hypoxemia and guides decisions about bronchodilator therapy, oxygen therapy, and potential lung volume reduction surgery candidacy. | Finding | V/Q Pattern | Clinical Significance | |---------|-----------|----------------------| | Emphysematous areas | Low V/Q (ventilation ↓, perfusion preserved) | Hypoxemia despite normal PaCO₂ initially | | Bullae | High V/Q (ventilation preserved, perfusion ↓) | Dead space ventilation | | Severe obstruction | Severe V/Q mismatch | Progressive CO₂ retention | **Mnemonic:** **VQ SCAN** = **V**entilation-**Q**uality assessment, **S**eparates **C**ontribution of **A**bnormal **N**etworks [cite:West Respiratory Physiology Ch 3]
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