Pulmonology Book


Ventilation Perfusion Scan

Aka: Ventilation Perfusion Scan, VQ Scan, V/Q Scan
  1. Indication
    1. Suspected Pulmonary Embolism
      1. Largely replaced by CT Pulmonary Angiogram (even in pregnancy)
      2. Used primarily when CT contrast in contraindicated and bilateral venous doppler is non-diagnostic for DVT
        1. Severe renal Impairment
        2. Anaphylaxis to CT Contrast
  2. Mechanism
    1. Evaluates air flow and Blood Flow into the lungs
      1. Ventilation (V) is evaluated with inhation of radio-isotope labeled aerosol (technetium-99m DTPA)
      2. Perfusion (P) is evaluated with IV injection of radio-labeled albumin (technetium-99m MAA)
      3. Uptake is evaluated with a gamma camera, and radiology evaluates for regions of decreased uptake
    2. Interpretation
      1. Ventilation (V) that is decreased (e.g. Pneumonia) will result in a low V/Q ratio
      2. Perfusion (Q) that is decreased (e.g. Pulmonary Embolism) will result in a high V/Q ratio
  3. Approach
    1. Determine PE Probability (Low, Moderate, High)
      1. High Sensitivity (Negative Scan almost excludes PE)
      2. Low Specificity (Positive Scan does not confirm PE)
    2. Radiologist interprets scan
      1. Two medium to large perfusion defects on V/Q mismatch
        1. Probability of PE: 90%
      2. One medium to large perfusion defect on V/Q mismatch
        1. Probability of PE: 50%
      3. Small perfusion defects
        1. Probability of PE: Low
    3. Complete by combining Clinical and Radiographic data
      1. Concordant Reading
        1. Low suspicion and Normal scan: 4% Incidence PE
        2. High suspicion and High scan: 96% Incidence PE
      2. Discordant Reading: Further workup required
        1. Only 40% of PE patients have High scan
        2. As many as 12% of PE patients have Low scan
  4. Resources
    1. Radiopedia
      1. https://radiopaedia.org/articles/vq-scan-2

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