Ventilation-Perfusion (VQ) Scan in Pregnancy: Diagnostic Algorithm
In pregnant women with suspected pulmonary embolism (PE), a VQ scan is recommended as the preferred imaging test when chest radiography is normal, due to lower maternal breast radiation exposure compared to CT pulmonary angiography (CTPA). 1
Initial Evaluation and Diagnostic Pathway
- A chest radiograph (CXR) should be the first radiation-associated procedure in all pregnant women with suspected PE to guide subsequent imaging selection 1, 2
- For pregnant women with signs and symptoms of deep vein thrombosis (DVT), bilateral compression ultrasound (CUS) of lower extremities should be performed first; if positive, anticoagulation can be initiated without pulmonary imaging 1, 2
- If CUS is negative or if there are no signs/symptoms of DVT, proceed with chest radiography followed by appropriate pulmonary vascular imaging 1
- D-dimer testing is not recommended to exclude PE in pregnancy due to poor specificity (D-dimer levels naturally increase during pregnancy) 1, 2
VQ Scan Indications in Pregnancy
- Normal chest radiograph: VQ scan is strongly recommended as the next imaging test rather than CTPA 1, 2
- VQ scanning delivers significantly lower radiation dose to maternal breast tissue (0.98-1.07 mGy) compared to CTPA (10-60 mGy) 1
- The lifetime relative risk of radiation-induced breast cancer in a 25-year-old woman undergoing CTPA is estimated at 1.011, making VQ scan preferable from a maternal cancer risk perspective 1, 2
- In pregnant women, unlike in non-pregnant populations, VQ scans do not have a higher rate of non-diagnostic results compared to CTPA 1
When CTPA is Preferred Over VQ Scan
- Abnormal chest radiograph: CTPA is recommended over VQ scan when CXR shows abnormalities 1
- Non-diagnostic VQ scan: When a VQ scan is non-diagnostic and further investigation is needed, CTPA is recommended rather than digital subtraction angiography 1
- Centers where VQ scanning is not readily available may use CTPA as an acceptable alternative 1
Safety and Efficacy Considerations
- Both VQ scanning and CTPA have similar false-negative rates (0.5% and 0.4% respectively) when used in pregnancy 1
- Although fetal radiation dose may be slightly higher with VQ scanning than with CTPA, both are well below the threshold for fetal radiation exposure concerns 1
- Studies show that withholding anticoagulation in pregnant women with normal or non-diagnostic VQ scans appears safe, with no reported venous thromboembolic events during follow-up 3, 4
- Recent data shows that perfusion-only imaging may be a reasonable option for pregnant patients, as the exclusion of ventilation images only minimally increases the rate of positive studies (<1%) 5
Common Pitfalls and Practical Considerations
- Despite guidelines recommending VQ scans, real-world data shows CTPA remains the most common modality used for PE diagnosis in pregnancy (55.7-70.0% across all trimesters) 6
- Non-diagnostic CTPA rates are higher in pregnancy compared to VQ scans, primarily due to suboptimal vascular opacification and respiratory motion artifacts 2, 7
- The positive scan rate for PE in pregnancy is low (3.5% compared to 8.8% in non-pregnant women), highlighting the importance of appropriate patient selection for imaging 7
- For unstable patients or when imaging studies cannot be promptly obtained, empiric anticoagulation should be considered while awaiting diagnostic results 2