A 25 y F g1p0 @8wks pregnant presents to ED complaining of SOB. Clinical suspicion for PE with a positive dimer to 0.68. Assuming lower extremity dopplers and chest x ray are unremarkable. 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 (i.e. moderate probability scans) 7-25% of the time whereas CTA imaging in pregnant patients was found to be non-diagnostic of PE 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 of PE with the same or less frequency than CTA.
It is important to realize that V/Q scans are used strictly to determine the presence or absence of PE, while CTA may identify numerous different pathologies.
Thus, while the preference of many clinicians is CTA, which may well be appropriate, evaluate the risks and give consideration to V/Q scan. Also important to keep in mind is that this information is with respect to chest scans and the work up of PE in a pregnant patient should likely begin with lower extremity dopplers.