VQ Scan

Summary

• VQ scan is still the recommended initial imaging test for the workup of PE (pulmonary embolism) although many institutions have replaced VQ with CT because of uncertainty with how to manage nondiagnostic VQ scans.
• CT is an ideal initial test in those patients with high clinical suspicion in whom you may choose to perform CT first or in those with a specific abnormality on CXR in whom CT may diagnose the abnormality, explain the patient’s symptoms and exclude pulmonary embolism (PE).

  Figure: Normal ventilation-perfusion scan with homogenous distribution of ventilation and perfusion. Top row of images is always ventilation, following row always perfusion.

• The caveat of this approach is that a negative CT in these patients will require follow-up V/Q or duplex since CT cannot rule-out PE.
• Nondiagnostic (low and intermediate probability) scans are not an acceptable endpoint for workup of PE except in patients with low probability V/Q scan, MRI scan and low clinical suspicion who have only a 4% risk of PE and thus could be safely discharged home.
• The management of nondiagnostic scans depends entirely on the pretest clinical suspicion which must be determined before V/Q scan results.
• Patients with nonhigh prestest probability and nonhigh V/Q scans may be candidates for serial duplex scans as outpatients or can be admitted for angiography depending on the patient’s stability and patient/physician preferences.
• Patients with high pretest probability and a low probability V/Q scan result or a low pretest probaility and a high probability V/Q scan should receive CT followed by angiography if CT is negative.
• Patients with high pretest probability and intermediate V/Q scans have a 66% probability of PE and are typically given the presumptive diagnosis of PE.

Modified PIOPED Scan Criteria

• High Probability

  
  Figure: High probability lung scan. Large perfusion defects noted without matching ventilation defects.
  

  

  Figure: Intermediate probability scan with small mismatched perfusion defects not large enough to qualify as a high probability scan.

• Two or more large (>75% of a segment) mismatched segmental perfusion defects or the arithmetic equivalent in moderate (25-75% of a segment) or large and moderate defects.
• Intermediate Probability
• Two large mismatched segmental perfusion defects or the arithmetic equivalent in moderate or large and moderate defects.
• One matched V/Q defect plus a normal CXR
• Difficult to categorize as low or high or not described as low or high.
• Low probability
• Any perfusion defect with a substantially larger CXR abnormality
• >One matched VQ defects plus some normal perfusion plus normal CXR
• Any number of small perfusion defects plus normal CXR
• Nonsegmental perfusion defects (e.g., pleural effusion, cardiomegaly, enlarged mediastinal structures, raised hemidiaphragm)
• Normal
• No perfusion defects seen

Indeterminate Results

• The main criticism of the VQ scan is the high percentage (60-70%) of indeterminate results (low probability or intermediate probability).
• Unless the V/Q scan is low probability and the clinical suspicion is low, the diagnosis of PE cannot be excluded or made based on a nondiagnostic study.
• A reasonable approach to nondiagnostic VQ scans in patients with nonhigh pretest probability is serial duplex scans of the lower extremities (repeat duplex over a 1-2 wk period) which has a lower false negative rate than pulmonary angiogram.

  

  Figure: Low probability scan with marked nonhomogenous ventilation consistent with chronic pulmonary disease.