Low Dose Perfusion Scan for PE in Pregnancy
Sheen et al. Performance of Low-Dose Perfusion Scintigraphy and CT Pulmonary Angiography for Pulmonary Embolism in Pregnancy. CHEST; 2018
Pulmonary embolism is a low incidence, high morbidity diagnosis with a relatively non-specific presentation in pregnant women. In the general population, the d-dimer remains the screening laboratory value for pulmonary embolism, but this test loses its value in pregnancy. A retrospective cohort study at the Montefiore system asked if low-dose perfusion scanning (LDQ) could replace CT pulmonary angiography (CTPA) in excluding pulmonary embolism. The primary outcome was negative predictive outcome of imaging for PE diagnosis at 90 days.
The study was conducted over 5 years at multiple sites in the Motefiore system. Women with a negative CXR underwent imaging with LDQ. 2 sites in the system with L&D used CTPA as their imaging modality of choice, and the women in this cohort tended to be further along in their pregnancy, which was a notable difference between groups. 2/81 women in the CTPA negative group had a 90 day event, while 0 women in the LDQ group had a negative event. It’s important to note that these two events were after delivery, which is its own risk factor for VTE. The primary endpoint showed equivalent NPV of 100% for the LDQ group and 97.5% for the CTPA group. At face value, this suggests that low dose perfusion scans could be utilized in place of CTPA for ruling out PE.
However, all imaging studies (VQ, LDQ and CTPA) deliver safe in utero fetal radiation doses, and nuclear medicine is rarely available 24 hours per day. The study authors recommend temporary anticoagulation until nuclear medicine studies can be obtained, which imparts a significant risk to both the patient and fetus that outweighs the risk of radiation that a CTPA creates. The lack of availability of nuclear medicine makes this study very difficult to generalize to many emergency departments.
This study was very underpowered, yet findings were consistent with Cochrane review literature. The authors do not recommend utilizing a positive LDQ as confirmatory for CTPA, which would require additional imaging prior to treatment. However, if your department has access to nuclear medicine, this study found non-inferiority in LDQ scans for ruling out PE, in a study that provides less radiation to maternal tissue. This imaging study can be utilized as an alternative to CTPA in the right clinical setting, but is unlikely to be practice changing in most departments.