According to the American Heart Association, pulmonary embolism affects about 600,000 Americans every year, and approximately 60,000 of them die from it each year, making it the third leading cause of cardiovascular mortality in the US.

The condition is difficult to diagnose, yet timely and accurate diagnosis is essential to patient survival. In the past decade, computed tomographic (CT) pulmonary angiography has become an increasingly popular diagnostic test, despite concerns about its sensitivity and about the limited comparative evaluations with ventilation-perfusion lung scanning, a well-established technique that has been used for more than 30 years.

Dr. David R. Anderson from Dalhousie University in Halifax, Nova Scotia, and his colleagues from McGill University in Montreal, from the University of Western Ontario in London and from Ottawa University in Ontario, all in Canada, and from the University of California, San Diego, decided to evaluate whether CT pulmonary angiography is as safe and reliable for evaluating the diagnosis of pulmonary embolism as is the standard ventilation-perfusion lung scanning procedure. One goal was to ensure that the procedure does not miss any clinically important pulmonary blockages.

As reported in the Dec. 19 issue of the Journal of the American Medical Association, the researchers tested 1417 patients who had signs or symptoms of acute pulmonary embolism in a clinical trial conducted at one US and four Canadian medical centers between 2001 and 2005.

Seven hundred and one patients were randomly assigned to CT pulmonary angiography and 716 to the ventilation-perfusion group. Those undergoing the CT procedure received either single- or multidetector scans after the injection of a contrast agent into a vein in the elbow joint. Single-detector scans were started 12 s after the injection, and 3-mm images were obtained at 3-mm intervals from the bottom of the aortic arch to 2 cm below the inferior pulmonary veins during a single breath-hold over 15 to 25 s. For multidetector scans, the contrast agent was injected at 4 mm/s, and 1.25-mm images were obtained at 1.2-mm intervals.

CT results were considered positive for pulmonary embolism if a specific defect was seen within the pulmonary artery vessel. Ventilation-perfusion scanning results were considered probable for pulmonary embolism if there were perfusion defects present.

For patients who received the CT test, 133 (19.2 percent of evaluable patients) were diagnosed with pulmonary embolism or deep vein thrombosis (which can lead to pulmonary embolism if the clot travels to the lungs) during the initial evaluation period. Of those in the ventilation-perfusion group, 101 (14.2 percent of evaluable patients) had a similar diagnosis. All of these patients were treated with anticoagulant therapy.

For the patients who were considered not to have pulmonary embolism, only two from the CT pulmonary angiography group (0.4 percent) and six from the ventilation-perfusion scanning group (1 percent) developed venous thromboembolism in follow-up evaluations.

The researchers believe that their results indicate that CT pulmonary angiography is as safe and reliable as ventilation-perfusion scanning in testing for pulmonary embolism and that a negative result in combination with a normal ultrasound safely excludes the diagnosis of pulmonary embolism.