"We were very surprised" by this finding, Dr. Marc Carrier of the Ottawa Hospital Research Institute, in Canada, told Reuters Health by email.
"These clots are so small (1.5 to 6 mm only) that we initially thought that they were not clinically important and that patients with subsegemental pulmonary could be managed like patients without any clots," he said.
Given the new findings, "most patients should probably receive anticoagulation unless they are at high risk of bleeding from the blood thinners (or included in a clinical trial)," Dr. Carrier said.
The SubSegmental Pulmonary Embolism (SSPE) study enrolled 266 patients at 18 centers over a 10-year period to determine the rate of recurrent VTE in patients with newly diagnosed single and multiple isolated subsegmental pulmonary embolisms managed without anticoagulation.
Recurrent VTE during the 90-day follow-up period (primary outcome) occurred in eight patients, for a cumulative incidence of 3.1% (95% confidence interval, 1.6% to 6.1%) during follow-up, Dr. Carrier and his colleagues report in Annals of Internal Medicine.
The cumulative incidence of recurrent VTE was 2.1% (95% CI, 0.8% to 5.5%) and 5.7% (95% CI, 2.2% to 14.4%) in those with single and multiple isolated subsegmental pulmonary embolism, respectively (hazard ratio, 2.7; 95% CI, 0.7 to 11.0). No recurrent VTE events were fatal.
The SSPE study stopped recruitment early due to a recurrent VTE rate without anticoagulation that was higher than initially expected. However, follow-up of enrolled participants was continued without anticoagulation.
"During the study planning phase, there was uncertainty in the expected rate of recurrent venous thromboembolism and concerns about potential harms in managing these patients without anticoagulation; therefore, we adopted a conservative stopping rule," the study team explains in their report.
Dr. Carrier told Reuters Health there is a "wide variation" in how this patient population is managed (with or without anticoagulation).
"Some physicians would treat them whereas others would not. The American College of Chest Physician clinical practice guidelines suggests treating high-risk patients but to do clinical surveillance only in low-risk patients. To highlight this important clinical equipoise, the (New England Journal of Medicine) recently had a little debate on this topic," he noted (https://bit.ly/3nACDip).
The SSPE study demonstrates a "higher-than-expected" rate of recurrent VTE in patients with isolated subsegmental pulmonary embolism without proximal deep venous thrombosis on repeated bilateral ultrasonography who are managed without anticoagulation, the authors say.
Subgroup analyses suggested that the rate of recurrent VTE may differ in younger versus older patients or those with single versus multiple isolated subsegmental pulmonary embolisms. "A subgroup of patients with lower risk for recurrent events might be identifiable in future studies; however, our results support the use of anticoagulation in this patient population," the researchers write.
"We believe our results are generalizable to patients with isolated subsegmental pulmonary embolism not requiring oxygen supplementation. However, we did not include patients with high-risk features (such as cancer or previous venous thromboembolism), in whom the risk for recurrent events is likely to be even higher," they note.
The study was funded by the Heart and Stroke Foundation of Canada and the French Ministry of Health Programme Hospitalier de Recherche Clinique.
SOURCE: https://bit.ly/2ZbwWhK Annals of Internal Medicine, online November 22, 2021.
By Megan Brooks
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