CT Procedure Effective for Detecting Pulmonary Embolism


By Judith Groch
HALIFAX, Nova Scotia, Dec. 18 -- In a head-to-head comparison of CT pulmonary angiography versus ventilation-perfusion lung scanning for ruling out pulmonary embolism, the newer CT procedure proved noninferior.
However, significantly more patients were diagnosed with venous thromboembolism (DVT and pulmonary embolism) with the CT procedure, raising concern about whether some of the emboli may be unimportant, David R. Anderson, M.D., of Dalhousie University here, and colleagues reported in the Dec. 18 issue of the Journal of the American Medical Association.
As Jeffrey Glassroth, M.D., of Northwestern in Chicago, put it in an accompanying editorial, this study raises the issue of whether CT pulmonary angiography, if not inferior to ventilation-perfusion lung scanning, is "actually superior or even too good as a first-line imaging modality for patients suspected of having pulmonary embolism."
For 30 years, ventilation-perfusion lung scanning has been the procedure of choice for suspected pulmonary embolism, although diagnostic uncertainty has been a concern. In the past decade, CT pulmonary angiography has been adopted rapidly, despite concerns about the sensitivity of the test, the researchers wrote.
To determine whether the CT method is a safe alternative that does not miss clinically important pulmonary arterial blockages, the researchers undertook a randomized single-blind noninferiority trial at one U.S and four Canadian tertiary care centers from May 2001 through April 2005.
The study included 1,417 patients considered likely to have acute pulmonary embolism on the basis of a Wells clinical model score of 4.5 or greater or a positive D-dimer assay. By excluding patients with a negative D-dimer test, the researchers said, these individuals were spared further testing.
Of the patients, 701 were randomized to CT pulmonary angiography and 716 to ventilation-perfusion scanning. Patients in whom pulmonary embolism was considered to be excluded did not receive antithrombotic therapy and were followed for three months.
The primary outcome compared the rates of symptomatic pulmonary embolism or proximal DVT occurring during the three-month follow-up of patients in whom venous thrombosis was initially excluded.
Two of 561 patients (0.4%) randomized to CT pulmonary angiography versus six of 611 patients (1%) undergoing ventilation-perfusion scanning developed venous thromboembolism in follow-up (difference: −0.6%, 95% CI: −1.6% to 0.3%, P=0.29), including one patient with fatal pulmonary embolism in the ventilation-perfusion scanning group.
With either strategy, the rates of patients returning with confirmed venous thromboembolism during three months of follow-up in whom the diagnosis of embolism had been excluded were low and not clinically or statistically significant, they said.
However, the overall rate for venous thromboembolism (composite of DVT and pulmonary embolism) was 5% greater for patients randomized to the CT strategy, an unanticipated finding, the researchers wrote.
Of patients in the study, 133 (19.2%) in the CT group versus 101 (14.2%) in the ventilation-perfusion scanning group were diagnosed as having pulmonary embolism in the initial evaluation period (difference: 5%; 95% CI: 1.1% to 8.9%) and were treated with anticoagulant therapy.
Study limitations included the fact that clinicians appeared less comfortable in excluding the diagnosis of pulmonary embolism in patients with nondiagnostic ventilation-perfusion scanning scans than with negative CT studies. Another limitation was that the majority of the patients enrolled were outpatients, while only a few were hospitalized, precluding comparisons.
Overall, Dr. Anderson and his colleagues said, these findings were robust. Further research is required to confirm whether some pulmonary emboli detected by CT pulmonary angiography may be clinically unimportant, the equivalent of DVT isolated to the calf veins and do not require anticoagulant therapy.
In his editorial, Dr. Glassroth said that the study is notable because it confirms the noninferiority for CT pulmonary angiography, given that the CT method has already largely supplanted conventional lung scans.
However, he noted that if CT identifies clots that are not likely to be clinically significant, this might be a problem exposing patients to risk of anticoagulation.
Discussing the study's clinical implications, Dr. Glassroth wrote that first clinicians should consider the likelihood of pulmonary embolism in a structured manner based on patients' presenting histories and physical examinations much the way Dr. Anderson and colleagues did. After these assessments, they should proceed, as necessary, to D-dimer testing.
These two steps may substantially reduce the need for additional study. Where significant concern remains, especially for co-morbidities, additional testing should be pursued. If available, a reasonable next step is lower extremity ultrasound studies to search for DVT to treat those patients found to have such clots.
If DVT is excluded or if ultrasound is not immediately available, then chest imaging is indicated. At the current state-of-the-art, the CT technique appears to be an excellent imaging choice unless there is a contraindication to dye administration or, perhaps, in pregnancy, because of the higher dose of radiation with CT.
For patients who cannot be studied by this method, ventilation perfusion scanning would still be available. Of note, he said, there is no evidence from the Canadian study that the CT technique is superior.
The study was funded by an operating grant from the Canadian Institutes of Health Research. The researchers and Dr. Glassroth, the editorial writer, reported no financial conflicts.
Additional source: Journal of the American Medical Association

Source reference: Anderson D, et al "Computer tomographic pulmonary angiography vs ventilation-perfusion lung scanning in patients with suspected pulmonary embolism: a randomized controlled trial" JAMA 2007; 298: 2743-2753. Additional source: Journal of the American Medical AssociationSource reference: Glassroth J, "Imaging of pulmonary embolism: too much of a good thing?" JAMA 2007; 298: 2788-2789.