A new meta-analysis of existing research finds that patients who have immobilized lower legs due to a plaster cast or brace can greatly reduce their risk of a dangerous blood clot if they take a common anticoagulant drug.

In a new Cochrane Library review, the researchers suggest that adult patients whose upper or lower legs are in casts or braces should have routine treatment with the drug known as low-molecular-weight heparin. The risk of bleeding, a side effect of anticoagulant drugs, is low in such patients, the researchers report.

Although familiarly known as "blood thinners," anticoagulant drugs work by preventing the formation of blood clots. Low-molecular-weight heparin is a newer form of heparin.

At issue is the risk that a kind of blood clot known as a deep vein thrombosis will form in an immobilized leg and travel to the lungs, where it could clog vessels that bring oxygen to the heart.

Clots in the lungs, known as pulmonary emboli, kill about 300,000 people a year in the United States, said Dr. Victor Tapson, director of the Center for Pulmonary Vascular Disease at Duke University Medical Center. Other estimates vary, starting at 50,000 such deaths a year.

"That may not be as many as heart attacks, but it often is more sudden," Tapson said. "Someone with an embolism can be a young, apparently healthy patient, although more often it's someone with clearly defined risk factors."

People with hip fractures, hip replacements and any kind of reduced mobility are at special risk, Tapson said. Airplane passengers are in danger of pulmonary emboli because their legs are often immobile and cramped for long periods.

"The less mobile the legs are, the more likely you are to get a clot," Tapson said. "Blood isn't much like water. If it can't flow and move, it will be more susceptible to thickening and clotting."

A noted recent American victim of a pulmonary embolism is NBC reporter David Bloom, who died in 2003 after cramming his body in a military vehicle while covering the Iraq War.

In the new review, researchers in the Netherlands examined six randomized controlled studies in which 1,490 patients with immobilized lower legs received either low-molecular-weight heparin or a placebo or no preventive treatment. In all cases, plaster casts or braces immobilized the legs of the patients.

The review appears in the latest issue of The Cochrane Library, a publication of The Cochrane Collaboration, an international organization that evaluates medical research. Systematic reviews draw evidence-based conclusions about medical practice after considering both the content and quality of existing medical trials on a topic.

The researchers conducted a meta-analysis, pooling the findings from the studies.

The rate of blood clots for patients who were in a cast or brace for at least one week ranged between 4.3 percent and 40 percent among those who received no anti-clotting treatment. However, the rate fell by about one-half in those who received daily injections of low-molecular-weight heparin.

The drug can cause excessive bleeding because it is more difficult for blood to coagulate and block holes in vessels. Still, "major bleeding events" were very rare in the patients who took heparin, striking about one in every 300, the reviewers found. Up to 8 percent of the patients who took heparin had "minor" bleeding problems.

The researchers recommend low-molecular-weight heparin as a routine treatment for patients with upper- or lower-leg casts or braces, but point out that blood clots are still a risk, even when the drug is used.

When patients have leg injuries, "immobilization should be avoided as much as possible, and treatment requiring less immobilization should always be considered," the researchers write.

Tapson was impressed with the review and said he hopes it provide new insight for doctors.

Currently, doctors routinely do not give anticoagulants to everyone in a leg cast, Tapson said. "There aren't really clear recommendations," he said. "If you've got someone 70 years old in a cast, you're much more likely to consider [an anticoagulant]. A 25 year old may not be as high risk."

Even in light of the findings of the review, he said, doctors are not likely to give anticoagulants to all patients in casts, but instead consider it depending on their individual circumstances, he said.

Another pulmonary specialist said the review appears to be "sound." Dr. Kenneth. Leeper Jr., associate professor of medicine at Emory University, added that immobilized patients "probably" should receive low-molecular-weight heparin routinely, unless there is a good reason not to administer the medication.

The Cochrane Collaboration is an international nonprofit, independent organization that produces and disseminates systematic reviews of health care interventions and promotes the search for evidence in the form of clinical trials and other studies of interventions. Visit cochrane for more information.

Testroote M, et al. Low molecular weight heparin for prevention of venous thromboembolism in patients with lower-leg immobilization. Cochrane Database of Systematic Reviews 2008, Issue 4.

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