Role of Lower Extremity Ultrasound in Diagnosing Pulmonary Embolism
Lower extremity duplex Doppler ultrasound is recommended as an alternative diagnostic approach for patients with suspected pulmonary embolism who cannot receive intravenous contrast, as it can detect deep vein thrombosis which is the source of most pulmonary emboli. 1
Diagnostic Algorithm for Suspected PE in Patients Who Cannot Receive IV Contrast
Initial Assessment
- Begin with clinical probability assessment and D-dimer testing; if low clinical probability and negative D-dimer, PE can be safely excluded without imaging 1, 2
- For intermediate/high probability or positive D-dimer, proceed to imaging options appropriate for patients who cannot receive IV contrast 2
Primary Imaging Options for Contrast-Contraindicated Patients
- Ventilation-perfusion (V/Q) scan is the first-line recommended imaging test for patients with suspected PE who cannot receive IV contrast 2
- Lower extremity ultrasound is a valuable alternative or complementary test when V/Q scanning is not available or inconclusive 1
Utility of Lower Extremity Ultrasound
Diagnostic Value
- A positive finding of DVT on lower extremity ultrasound in a patient with symptoms consistent with PE can be considered evidence for venous thromboembolic disease and may preclude the need for additional diagnostic imaging 1
- Complete lower limb ultrasound examining both proximal and distal veins has higher sensitivity (93%) compared to limited ultrasound (55%) that only examines popliteal and femoral veins 3
Specific Clinical Scenarios for Using Lower Extremity Ultrasound
- Patients with obvious signs of DVT for whom venous ultrasound is readily available 1
- Patients with contraindications to CT contrast (renal insufficiency, contrast allergy) 1
- Pregnant patients, particularly in the first trimester, to avoid radiation exposure 1
- Patients with a history of multiple CTs for PE to reduce cumulative radiation exposure 1
Integration with Other Diagnostic Methods
- For patients with intermediate pretest probability and negative CT angiogram, consider lower extremity venous ultrasound as an additional test to exclude VTE disease 1
- For patients with high pretest probability and negative CT angiogram, additional testing including lower extremity venous ultrasound is recommended 1
Limitations and Considerations
Technical Limitations
- Lower extremity ultrasound does not allow evaluation of abdominal and pelvic venous systems 1
- Ultrasound is less reliable for diagnosing asymptomatic, isolated distal, and recurrent deep venous thrombosis 4
- The quality and accuracy of ultrasound is operator-dependent 4
Efficiency Considerations
- Performing lower extremity ultrasound before more advanced imaging can reduce the need for CTPA in approximately 10% of patients 1
- A screening ultrasound protocol may predict the need for CTPA and can establish alternative diagnoses that correlate with CTPA findings 5
- Cost analysis suggests potential savings when using ultrasound as an initial screening tool before proceeding to more expensive imaging 6
Best Practice Approach
- For patients who cannot receive IV contrast, begin with clinical probability assessment and D-dimer testing 1, 2
- If D-dimer is positive or clinical probability is high, proceed with V/Q scan as first-line imaging 2
- Consider lower extremity ultrasound as an alternative or complementary test, especially when V/Q scanning is unavailable or inconclusive 1
- A positive finding of DVT on ultrasound is sufficient to diagnose venous thromboembolic disease and initiate treatment 1