## Diagnostic Investigation for PE-Induced V/Q Mismatch ### V/Q Scan as Diagnostic Gold Standard for PE **Key Point:** The ventilation-perfusion scan is the most specific investigation for diagnosing pulmonary embolism by demonstrating characteristic **perfusion defects with preserved ventilation** (high V/Q mismatch), which is pathognomonic for PE. **High-Yield:** In PE, thrombotic obstruction of pulmonary arteries causes **regional perfusion defects while ventilation remains intact** — the classic "mismatched" V/Q pattern. This is fundamentally different from COPD or pneumonia where both ventilation and perfusion are reduced together (matched defects). ### Pathophysiology of V/Q Mismatch in PE ```mermaid flowchart TD A[Pulmonary embolism]:::outcome --> B[Arterial obstruction]:::outcome B --> C[Perfusion defect]:::outcome C --> D{Ventilation status?}:::decision D -->|Preserved| E[High V/Q mismatch]:::outcome E --> F[Hypoxemia without CO₂ retention]:::outcome D -->|Reduced| G[Matched defect]:::outcome G --> H[Other lung disease]:::outcome ``` ### V/Q Scan Interpretation in PE | V/Q Pattern | Ventilation | Perfusion | Interpretation | Clinical Diagnosis | |-------------|-------------|-----------|-----------------|-------------------| | **Mismatched** | Normal/preserved | Defect | High V/Q | **PE (pathognomonic)** | | Matched | Defect | Defect | Normal V/Q ratio | COPD, pneumonia, atelectasis | | Reversed mismatch | Defect | Normal | Low V/Q | Shunt lesions, pneumonia | **Clinical Pearl:** A high-probability V/Q scan (multiple segmental or larger perfusion defects with normal ventilation) has >90% specificity for PE and can confirm diagnosis without further imaging in appropriate clinical context. **Mnemonic:** **PE = PV mismatch** = **P**erfusion defect with **V**entilation preserved [cite:Harrison 21e Ch 297]
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