Low and intermediate risk patients with a positive D-dimer and high risk patients require further imaging. Imaging techniques include the following:
CTPA is the investigation of choice due to its greater sensitivity and specificity for PE than V/Q scanning and its ability to identify alternate diagnoses. The sensitivity of CTPA is between 85-90% but its negative predictive value is high (96-97%) and therefore it can reliably exclude PE even in high risk groups18,19. Studies that have followed up patients sent home following negative CTPAs found favourable outcomes so it would seem that even if some PEs are missed these are likely to be small and unlikely to reoccur. CTPA can identify very small PEs in peripheral vessels below the 5th division of the pulmonary artery. There is some debate as to whether this size of thrombus may represent the normal turn-over of clot in the body which have been filtered out by the lung. At the moment however evidence that these small clots do not require treatment is lacking and for ethical reasons such evidence may not be forthcoming in the future.
In patients with a high pre-test probability for PE, CTPA may be falsely negative. Thus in patients with a high clinical suspicion of PE and a negative CTPA, additional imaging may be considered (e.g. D-Dimer, leg ultrasound, MRI, invasive pulmonary angiography)
The image above is a CTPA showing massive filling defect (clot) in main pulmonary vessels. Click on the CT to enlarge.
V/Q scanning can be used for patients with a normal chest radiograph and no chronic cardiopulmonary disease. It risk stratifies patients into low, intermediate and high probability of pulmonary embolism:
Unfortunately, isotope lung scanning rarely confirms the diagnosis of PE reliably and many patients with scans where the probabilities are intermediate and the risks uncertain leaving the clinician with difficult diagnostic decisions or the requirement for further tests. For this reason isotope lung scanning has been largely replaced by CTPA.
Echocardiography can be useful in assisting with the diagnosis of PE especially in seriously ill patients that are too ill to be taken for a CT scan. With large PEs an echocardiogram may provide indirect evidence of the diagnosis revealing right ventricular strain and high PA pressures but only rarely will clot in the right side of the heart or pulmonary artery be seen20. The most common role for echocardiography in PE patients is to risk stratify patients with large PEs who are not haemodynamically unstable in order to help guide management.
Leg vein ultrasound is useful for patients with clinical evidence of a DVT and identification of clot will preclude the need for advanced chest imaging (e.g. in pregnancy). However, a negative scan does not exclude sub clinical DVT and in one study a third of patients with negative leg ultrasound were subsequently found have a pulmonary embolus on angiography21.
Leg ultrasound alone cannot reliably exclude venous thromboembolism in patients presenting with symptoms suggestive of pulmonary embolism5.
Patients with a negative CTPA, in whom there is ALSO a clinical suspicion of DVT should go on to have imaging of their leg veins.
Note: CTPA and V/Q scanning involve ionising radiation and the injection of contrast media exposing the patient to a level of risk.