The correct answer is:

Obtain a pulmonary CT angiogram

Educational Objective:

Discuss the diagnostic workup of pulmonary embolism in pregnancy.

Key Point:

CT angiogram or V/Q scanning, not compression ultrasonography, are the recommended definitive imaging studies in the diagnosis of pulmonary embolism in a pregnant patient WITHOUT signs and symptoms of a deep vein thrombosis (DVT). In patients WITH signs and symptoms of a DVT, compression ultrasonography would be the first study.

Explanation:

Thromboembolic disease accounts for almost 20% of obstetric mortality, making it the leading cause of death in pregnancy. The risk of venous thromboembolism (VTE) increases during pregnancy to five or six times that of nonpregnant women, and is especially high in patients with histories of obesity, smoking, age >35 years, and/or pre-existing hypercoagulable state. 

Anticoagulation should be given only after the diagnosis of VTE is confirmed. In such cases, heparin or low-molecular weight heparin is appropriate, as warfarin is contraindicated in pregnancy (due to teratogenic effects and high risk of fetal hemorrhage). 

In BOTH pregnant and nonpregnant patients, clinical signs of PE (pain, tenderness, and swelling in the lower extremities, tachycardia, dyspnea, and pleuritic pain) are so nonspecific that they do not allow for a purely clinical diagnosis of PE. D-dimer by itself is not recommended in pregnancy to exclude VTE, as it is insufficiently sensitive in pregnant patients. Radiation studies are a concern in pregnant patients due to the inherent radiation exposure to the fetus and the maternal breast tissue.

In 2019, a prospective, multicenter study suggested that the YEARS algorithm, using adapted Wells criteria, could avoid unnecessary pulmonary CT angiograms in pregnant patients who presented with possible PE. Similar to the Wells algorithm, the YEARS algorithm first of all determines risk of PE on the basis of various criteria (in the case of the YEARS algorithm, these are: hemoptysis, clinical evidence of DVT, high index of suspicion for PE, and D-dimer). The authors reported that use of this modified algorithm avoided CT angiography in up to 65% of pregnant patients, an important finding considering the risks of radiation to the fetus. 

Another key point of this study is that it is not always appropriate to first do compression ultrasonography in pregnant patients who might have a VTE. 

According to the YEARS algorithm, the patient in the above scenario would require a CT angiogram because she has at least one YEARS criterion (high index of suspicion for PE), and a D-dimer >500 ng/mL. 

The study found that the yield is too low in patients, who - key point – DO NOT have clinical signs of a deep vein thrombosis. In this surprisingly large subgroup, only 1% had positive compression ultrasound findings (this includes looking for proximal vein thrombosis anywhere above the knee). In other words, 99% of pregnant patients in this study without signs of DVT moved on to CT angiogram or V/Q scanning after their ultrasound compression study. 

Based on the results of this large study, CT angiogram or V/Q scan are indeed the recommended first line studies for pregnant patients without signs of DVT. For patients WITH clinical signs of DVT, a compression ultrasound remains the first study. 

Clinically speaking, nobody would fault a physician for getting a compression ultrasound; it would just be a wasted study that does not alter outcome in 99% of this rather large subcategory of patients (pregnant without signs of DVT).

Pearls:

• In the YEARS study, only 7% of pregnant patients WITH signs and symptoms of DVT had a positive compression ultrasound study, so even in this group, 93% go on to CT angiogram or V/Q scanning.
  
  
• In another recent study (2018), the risk of early breast cancer was found to be similarly low after ventilation–perfusion scanning and CT pulmonary angiography, which supports the notion that both imaging methods are valid options in patients without cardiopulmonary disease.
  
  
• The YEARS study found that their algorithm prevented unnecessary radiation studies in 46% of patients during the second trimester, and 32% of patients during the third trimester. This decreasing specificity of the algorithm is due to the physiological rise in the d-dimer level that commonly accompanies the later part of pregnancy.
  
  
• The YEARS algorithm leads to a reduced detection of potentially clinically irrelevant subsegmental pulmonary embolism, leading both to a shorter visit time and reduced costs in the emergency department.