Many of the estimated 50,000 to 100,000 deaths annually from blood clots in the lungs could be prevented if doctors routinely prescribed measures to reduce clot formation in high-risk patients, a panel of medical experts concluded yesterday.

Clots that form in leg veins far below the skin and sometimes travel to lungs are "a major health problem in the United States," causing as many as 600,000 hospitalizations each year, and "preventive measures . . . are widely underused," said Dr. Harold R. Roberts, the panel's chairman.

Roberts, from the University of North Carolina, spoke at the end of a three-day conference sponsored by the National Institutes of Health.

Such clots also may develop during hospitalization for other problems, occurring in about one-fourth of general surgery patients over age 40 and in more than half of patients undergoing hip surgery, Roberts said. They are also common in those with heart attacks, heart failure or pneumonia.

Although why clots form is not understood, the risk is greater in older patients, following most operations, and when the legs are immobilized by bed rest.

The panel found that, despite the risk, many doctors fail to employ low-risk treatments to prevent the condition, known medically as deep venous thrombosis.

Such treatments include low doses of heparin, a medication that prevents clotting; dextran, an intravenous drug that makes clots dissolve easily, and special boots or stockings that compress leg veins and prevent blood from pooling.

If such treatments are used routinely in high-risk patients, "we think that out of 200 patients, it will prevent 20 episodes of deep venous thrombosis . . . and one fatal pulmonary embolism," the medical name for a clot that travels to the lung, he said.

Roberts suggested that doctors have been slow to adopt the treatments because they find studies of their efficacy contradictory, because they fear that heparin or dextran might promote bleeding or because they fail to appreciate the likelihood of blood clots forming after surgery.

In patients undergoing general surgery, the panel said, low doses of heparin decrease the chance of deep venous thrombosis by 60 percent. Low-dose heparin and dextran slightly increase the chance of bleeding but rarely cause major hemorrhages. Boots and stockings carry no risks other than minor discomfort.

The most effective treatment varies for different patients, but Roberts emphasized that all work only if started before a clot begins to form.

For example, in patients undergoing general surgery and considered high-risk -- those over 40, obese or with certain medical conditions -- the panel recommended starting low-dose heparin two hours before surgery and injecting it every 8 to 12 hours until the patient is walking.

For women with gynecologic operations, it recommended stopping birth-control pills or estrogen medications, which promote clotting, four to six weeks before surgery, and treating high-risk patients to reduce clot formation while in the hospital.

The panel also suggested low-dose heparin during pregnancy in women at high risk, since pregnancy increases clot formation fivefold.

"The most dangerous type of surgery appears to be orthopedic surgery," particularly on the hip, Roberts said. Low-dose heparin appears less effective here, but intermediate doses of heparin and other drugs have been found to prevent clots.

The panel recommended using one of several anticlotting drugs for at least seven days in those undergoing major hip or knee surgery and for elderly patients with broken hips.

It endorsed routine administration of low-dose heparin injections in patients with heart attacks, heart failure and lung infections. It also issued recommendations for those undergoing neurosurgical and urologic operations and for patients recovering from strokes or spinal-cord injuries.