Low-Dose Perfusion or CTPA for PE in Pregnancy?

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Low dose perfusion-only (LDQ) nuclear scan (the "Q" of the V/Q scan) or CTPA are the imaging studies of choice to diagnose PE in pregnant women.  LDQ (using one-third the normal technetium of V/Q) had high accuracy and lower radiation dose and may be preferred over CTPA, assuming the CXR is normal.

Why does this matter?
Pregnancy increases risk for venous thromboembolism (VTE).  The workup for PE may require imaging with radiation, which could be harmful to the developing fetus and radiation-sensitive maternal breast tissue.  Limiting radiation exposure while still accurately imaging for PE is very important.  LDQ may limit radiation exposure, but how accurate is a LDQ? Also, will it degrade diagnostic accuracy in pregnant patients with asthma?  That's what this study answered.

Low-dose Q scan or CTPA
This was a retrospective study of a single center with 322 pregnant patients who underwent PE workup. They have been doing LDQ (one-third the technetium) and some CTPA for 20 years in pregnant patients at this center. The LDQ had 100% NPV; CTPA had 97.5% NPV. Both studies had the same rate of non diagnostic imaging (9.3%). Of note, LDQ should only be performed if the CXR is negative.  The diagnostic accuracy of LDQ and CTPA held up even in a subgroup of asthmatic patients -- although the study recommends giving asthmatic patients a breathing treatment prior to either study.  Also, don't forget, if hemodynamically stable, you may want to start with ultrasound of the legs in pregnant patients to get a DVT diagnosis (in about 10%) and avoid chest imaging and radiation. The retrospective design and fact that 11% could not be followed up at 90 days means that some late PEs could have been missed.  They estimated (not measured) radiation dose for LDQ to be, "0.16 milligray, 0.47 milligray and 0.02 milligray to the maternal breast, maternal whole body and fetus. The radiation exposure from CTPA to maternal breast, maternal whole-body, and fetus is 125, 16 and 4.6 times higher respectively in comparison with LDQ, and radiation doses are three to four times higher for V/Q than for LDQ."  A milligray is basically the same as a millisievert.

Source
Performance of Low Dose Perfusion Scintigraphy and Computed Tomographic Pulmonary Angiography for Pulmonary Embolism in Pregnancy.  Chest. 2017 Aug 17. pii: S0012-3692(17)31398-3. doi: 10.1016/j.chest.2017.08.005. [Epub ahead of print]

Peer reviewed by Thomas Davis, MD.

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