Imaging Options

Although CTPA is generally considered the gold standard for diagnosing PE, it is far from perfect, and ventilation/perfusion single-photon emission CT (V/Q SPECT) may have better test characteristics. CTPA is prone to over-diagnosing clinically irrelevant emboli in low-risk patients.8 Furthermore, although its sensitivity approaches 100 percent for clinically relevant PEs, those patients at high risk for PE, based on a Wells’ score >6, who have a negative CTPA should be counseled that up to 5 percent of high-risk patients may develop a PE within a few months of a negative CTPA.^9,10

Clot burden and clot location on CTPA have not been shown to accurately predict outcome or even symptoms. The clinical context is much more important, and markers such as hypotension and hypoxia are far better outcome predictors.¹¹

V/Q SPECT has been shown to have superior accuracy compared to traditional V/Q and has similar sensitivity compared to CTPA for pulmonary embolism.^12,13 V/Q SPECT eliminates intermediate probability scans and is reported dichotomously as positive or negative for PE. This avoids the ambiguity of results seen with traditional V/Q scan interpretation. Robust data are pending regarding its diagnostic utility compared to CTPA. However, the current evidence suggests that it may be superior with a lower dose of radiation to breast tissue.¹³ Consider V/Q SPECT not only in otherwise healthy young patients with a normal chest X-ray and those with CT contrast allergy but also in patients with existing lung disease.

Treatment Decisions

Is there any evidence for clinical benefit in treating subsegmental PE found on CTPA? An observational study by Goy et al in 2015 reviewed 2,213 patients with a diagnosis of subsegmental PE and showed that whether or not anticoagulation was given, there were no recurrent PEs. However, 5 percent of anticoagulated patients developed life-threatening bleeding.¹⁴ Other studies have yielded similar results.¹⁵

Shared decision making plays an important role for patients suspected of PE. Consider the patient’s bleeding risk (HAS-BLED score) and discuss potential treatment options. The 2018 ACEP Clinical Policy on Acute Venous Thromboembolic Disease gives withholding anticoagulation in patients with subsegmental PE a Level C recommendation and states: “Given the lack of evidence, anticoagulation treatment decisions for patients with subsegmental PE without associated [deep vein thrombosis] should be guided by individual patient risk profiles and preferences [Consensus recommendation].”¹⁶

Nonetheless, I recommend starting anticoagulants for subsegmental PE in the emergency department with a clear explanation that anticoagulants may (and probably should) be stopped in follow-up. While the risk of major bleeding with a full course of anticoagulation is significant, the risk of bleeding from a few doses of anticoagulant is very low. Thus, starting treatment for subsegmental PE in the emergency department and referring the patient for timely (within a few days) follow-up in a thrombosis or internal medicine clinic is a reasonable option. Consultants may risk-stratify low-risk patients with serial lower-extremity Doppler ultrasound to direct ongoing therapy.