Venous thromboembolism: diagnosis and management of pulmonary embolism

To the Editor: I read with interest the article by Lee et al regarding the investigation and treatment of pulmonary embolism (PE).1 The investigation of patients presenting with PE as a diagnostic possibility is of great interest to emergency physicians, and such presentations are a daily occurrence in emergency departments around the country.

Unfortunately, only a small amount of text is devoted to describing the relative merits of ventilation perfusion (V/Q) scanning and computed tomography pulmonary angiography (CTPA), and no guidance is provided as to which is the test of choice when both are available. The British Thoracic Society has recommended CTPA as the lung imaging modality of first choice for patients presenting with non-massive PE.2 There is a large and increasing body of evidence that CTPA provides superior specificity to V/Q scanning in the detection of PE. CTPA also provides the opportunity of establishing diagnoses other than PE and, in addition, a negative multi-slice CTPA is of sufficient sensitivity to enable the withholding of anticoagulation.3 It is also my experience that CTPA is easier to obtain out of hours, compared with V/Q scanning.

The authors state that V/Q scanning “reliably establishes the diagnosis of PE if the V/Q scan features suggest a high probability of PE . . .”.1 Unfortunately, this statement is incorrect. It is essential that V/Q scan results be interpreted in the light of the patient’s clinical probability for PE. In the PIOPED study, only 56% of patients with high probability V/Q scan reports had pulmonary embolism if the pretest probability was low.4

No mention is made of the special situation of pregnant women presenting with pleuritic pain, or which lung imaging test is considered “safest” for both mother and baby. Although the risks of PE are generally agreed to be increased in pregnancy, it is my experience that pregnant women are extremely reluctant to undergo any form of diagnostic investigation that exposes the fetus to radiation.

The Wells criteria have been validated for the assessment of PE in emergency department patients only, and provide a means for clinicians with little experience to make an accurate assessment of an individual patient’s clinical probability of PE.5 Once initiated, clinical assessment of the patient with possible PE is straightforward. The key question facing emergency physicians is this: is there a group of patients that have such low probability for PE that no investigation at all is required?