II. Indication
- Suspected Pulmonary Embolism
- Largely replaced by CT Pulmonary Angiogram (even in pregnancy)
- Used primarily when CT contrast in contraindicated and bilateral venous doppler is non-diagnostic for DVT
- Severe renal Impairment
- Anaphylaxis to CT Contrast
III. Mechanism
- Evaluates air flow and Blood Flow into the lungs
- Ventilation (V) is evaluated with inhation of radio-isotope labeled aerosol (technetium-99m DTPA)
- Perfusion (P) is evaluated with IV injection of radio-labeled albumin (technetium-99m MAA)
- Uptake is evaluated with a gamma camera, and radiology evaluates for regions of decreased uptake
- Interpretation
- Ventilation (V) that is decreased (e.g. Pneumonia) will result in a low V/Q ratio
- Perfusion (Q) that is decreased (e.g. Pulmonary Embolism) will result in a high V/Q ratio
IV. Approach
- Determine PE Probability (Low, Moderate, High)
- High Sensitivity (Negative Scan almost excludes PE)
- Low Specificity (Positive Scan does not confirm PE)
- Radiologist interprets scan
- Two medium to large perfusion defects on V/Q mismatch
- Probability of PE: 90%
- One medium to large perfusion defect on V/Q mismatch
- Probability of PE: 50%
- Small perfusion defects
- Probability of PE: Low
- Two medium to large perfusion defects on V/Q mismatch
- Complete by combining Clinical and Radiographic data