TUESDAY, July 7, 2015 (HealthDay News) -- People who suffer a blood clot in the lungs with no obvious cause can ward off a new clot with extended use of blood-thinning medications, a new study shows.
However, the length of time those blood thinners should be given is unclear, since their benefits wear off soon after use is discontinued, the French researchers found.
The clots are called pulmonary emboli, explained one U.S. expert, Dr. Richard Hayes.
"Pulmonary emboli are clots in the blood vessels to the lungs that arise from the veins of the leg or thigh," said Hayes, a cardiologist at Lenox Health in New York City. One type of clot is deep vein thrombosis (DVT), often nicknamed "economy-class syndrome" because of cases occurring after long-haul flights.
In many cases, Hayes said, there's a trigger -- extended bedrest, obesity, recent surgery -- for the clot, but in other cases the clots seem to arise without a specific cause.
"In these patients, there is a higher likelihood of recurrence," said Hayes, who was not involved in the new study.
So, how long should these patients take a blood thinner to ward off a second clot?
To help find out, a team led by Dr. Francis Couturaud of the Universite de Bretagne Occidentale, in Brest, France, tracked outcomes for 371 adults who had experienced an "unprovoked" blood clot in the lung. All of the patients received six months of treatment with a type of anti-clotting drug known as a vitamin K antagonist, which includes the standard blood thinner warfarin.
At the six-month point, patients then received either warfarin for another 18 months, or a "dummy" placebo pill.
The extended use of warfarin did seem to help patients: Further blood clots or major bleeding occurred in only 3 percent of those taking the drug, compared to 13.5 percent of those taking the placebo. That means that taking the warfarin reduced the risk by 78 percent, Couturaud's team reported.
However, that benefit disappeared soon after treatment with warfarin ended, according to the study published in the July 7 issue of the Journal of the American Medical Association.
The findings suggest that this group of patients may require long-term treatment to prevent recurrence of pulmonary embolism, the researchers said.
"Whether these should include systematic treatment with vitamin K antagonists, new anticoagulants or aspirin, or be tailored according to patient risk factors, needs further investigation," the French team concluded.
According to Hayes, the study suggests that patients may need blood thinners over the very long term.
"The take-home message: in those patients who have a blood clot in the lung with no obvious cause, there is an approximately 20 percent recurrence rate," he said. However, "we don't know how long the risk is elevated."
In Hayes' opinion, the French study "provides further support to lifetime anticoagulation with warfarin" or newer blood thinners.
Dr. Joseph Mathew is medical director of respiratory care at Winthrop-University Hospital in Mineola, N.Y. He agreed with Hayes that the new study "leads one to believe that patients with an unprovoked [clot] need lifelong anticoagulation, and it calls for a detailed risk-benefit discussion between the physician and the patient."
The U.S. National Heart, Lung, and Blood Institute has more about pulmonary embolism.