For several decades, ventilation-perfusion (V/Q) scans have been used to detect pulmonary embolisms in patients. It is clear that a normal V/Q scan excludes the diagnosis of a pulmonary embolism, and a high probability V/Q scan essentially confirms the diagnosis. The challenge is what to do with patients who have a non-diagnostic (i.e., low to intermediate probability) V/Q scan.
Computed tomographic pulmonary angiography (CTPA) is an alternative test used to detect pulmonary embolisms that has several advantages over the V/Q scan. It provides a clear (yes or no) result, and it is also capable of detecting non-thrombotic causes for a patient's symptoms. The downsides of CTPA are that this technique exposes the patient to more radiation than V/Q scans and that it may cause dye-induced nephrotoxicity. In spite of these tradeoffs, CTPA has largely replaced V/Q testing for pulmonary embolic disease. Although CTPA has become widely accepted as a standard test for this disease, there is controversy over whether small-vessel disease can be imaged as well by CTPA as by V/Q scans.
In this single-blinded, noninferiority, multicenter clinical trial, 701 patients were randomized to undergo CTPA and 716 were randomized to V/Q scanning. The results showed that 19.2 percent of the CTPA group and 14.2 percent of the V/Q group were diagnosed with a pulmonary embolism and treated with anticoagulation. Of the patients who were not initially diagnosed with a pulmonary embolism, 0.4 percent of the patients in the CTPA group and 1.0 percent of the patients in the V/Q scan were subsequently diagnosed to have thromboembolic disease.
In Brief
This study by Anderson and colleagues convincingly demonstrates that CTPA is at least as effective as V/Q scans at diagnosing pulmonary embolisms. In fact, the incidence of detected pulmonary embolisms was 30 percent higher in the group randomized to be analyzed by CTPA ― a statistically significant difference. Some patients who had negative V/Q scans were subsequently diagnosed as having a pulmonary embolism once they had a CTPA; this suggests that CTPA might be an even more sensitive diagnostic test than V/Q scans.
These findings raise the interesting question about whether our technology is getting too good. Is CTPA capable of diagnosing emboli that are too small to be clinically significant? In the Anderson study, approximately 7 percent of patients diagnosed as having a thromboembolism by CTPA had emboli only in small subsegmental pulmonary vessels. Do we really know that these small clots require therapy? It should be noted that eight patients in the Anderson trial who had no obvious pulmonary embolism diagnosed by either CTPA or V/Q scans ultimately developed a pulmonary embolism (including one patient who died of a clot). This implies that these eight symptomatic patients probably had very small pulmonary emboli that were missed by their initial imaging study. We need to keep in mind that the true value of treating a pulmonary embolism with anticoagulation is to prevent the next one, rather than the thromboembolism that has already occurred. Using this rationale and the currently available data, we should be treating all pulmonary emboli regardless of their size, and perhaps even seeking out technologies that diagnose smaller and smaller clots.
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Competing Interests
Dr. Abrams indicated no relevant conflicts of interest.
