A 25 y F g1p0 @8wks pregnant presents to ED complaining of SOB. Clinical suspicion for PE with a positive dimer to 0.68. How will you further workup this patient?
CTA or V/Q Scan?
For us recent med school grads the new teaching has been that V/Q scanning produces a greater radiation risk to the fetus. While CTA does deliver a slightly lower radiation dose to the fetus in comparision to V/Q scanning (0.003 to 0.131 mGy versus 0.32 to 0.74 mGy), according to the National Council of Radiation Protection and Measurements the risk of radiation-associated abnormalities is negligible at levels below 50 mGy. Additionally, the maternal dose of radiation for CTA vs V/Q is 7.3 mGy vs 0.9 mGy respectively and V/Q scanning results in 150 fold less irradiation to breast and lung tissue.
In addition, the rate of non-diagnostic imaging increases in pregnant patients undergoing CTA and decreases in patients undergoing V/Q. CTA scanning is thought to be more frequently non-diagnostic in pregnancy because there is an increase pressure from the IVC (esp. in the 3rd trimester) which interrupts blood flow from the SVC (where contrast enters). V/Q scans improve because pregnant patients are generally younger and healthier.
V/Q scans in pregnant patients were found to be non-diagnostic 7-25% of the time whereas CTA imaging in pregnant patients was found to be non-diagnostic in 17-28% of patients.
Comparative analysis between the 2 imaging modalities has shown varying results, but overall, studies indicate V/Q is non-diagnostic with the same or less frequency than CTA.
Thus, think twice when ordering CTA in pregnancy given the possible increased risk of non-diagnostic reads as well as significantly increased radiation exposure to the mother.