Computed tomographic pulmonary angiography may be a safe alternative to ventilation-perfusion scans for excluding pulmonary embolism; however, because angiography may detect more clots, it may raise questions about clinical significance, according to an article in the December 19 issue of the Journal of the American Medical Association.

For 30 years, ventilation-perfusion (V/Q) lung scanning has been the noninvasive imaging procedure of choice in patients with suspected pulmonary embolism. In the last decade, computed tomographic pulmonary angiography was introduced as an alternative and has been adopted rapidly despite some concerns about the sensitivity of the method.

In the current study, David R. Anderson, MD, of Dalhousie University, Halifax, Nova Scotia, Canada, and colleagues conducted a comparison of techniques to determine if computed angiography is a safe, reliable alternative and does not miss detection of clinically important pulmonary clots.

The trial was conducted at four Canadian and one U.S. tertiary care centers between May 2001 and April 2005 and included 1,417 patients considered likely to have acute pulmonary embolism. Patients were randomized to undergo either standard scanning (716 patients) or computed angiography (701 patients). Patients in whom testing did not indicate pulmonary embolism did not receive anti-thrombotic therapy and were followed for three months.

Of the patients randomized to angiography, 133 (19.2 percent) were diagnosed with pulmonary embolism or deep vein thrombosis in the initial evaluation period; 101 (14.2 percent) of patients in standard scanning group had a similar diagnosis. Both groups of patients were treated with anti-coagulant therapy. The overall rate of venous thromboembolism (the composite of deep vein thrombosis and pulmonary embolism) found in the initial diagnostic period was significantly greater in patients randomized to angiography (difference, 5 percent).

Of those in whom pulmonary embolism was considered excluded, 2 of 561 patients (0.4 percent) randomized to angiography vs. 6 of 611 patients (1.0 percent) undergoing standard scanning developed venous thromboembolism in follow-up (difference, -0.6 percent).

"The results of our study are reassuring given previous reports of relatively low sensitivity of computed tomographic pulmonary angiography for the diagnosis of pulmonary embolism," the authors wrote.

"… an unanticipated finding in our study was that computed tomographic pulmonary angiography resulted in a significantly greater number of venous thromboembolism diagnoses than did ventilation-perfusion scanning," they added. "Further research is required to confirm whether some pulmonary emboli detected by angiography may be clinically unimportant, the equivalent of deep vein thrombosis isolated to the calf veins, and not require anti-coagulant therapy."

In an accompanying editorial, Jeffrey Glassroth, MD, of the Feinberg School of Medicine, Northwestern University, Chicago, wrote that the findings by Anderson and colleagues have clinical implications.

"First, clinicians should consider the likelihood of pulmonary embolism in a structured manner based on patients' presenting histories and physical examinations much the way Anderson and colleagues did, and based on those assessments, proceed, as necessary, to D-dimer testing. These two steps may substantially reduce the probability that pulmonary embolism, at least large clots, are present and obviate the need for additional study. Where significant concern remains, including some patients whose pulmonary embolism probability may not be very high but whose comorbidities put them at great risk were an embolism to occur, additional testing should be pursued. If readily available, lower extremity ultrasound studies to search for deep vein thrombosis to treat those patients found to have such clots is a reasonable next step."