WASHINGTON - Doctors discovered a blood clot in Vice President Dick Cheney's left leg Monday, a condition that could be fatal if left untreated.

Vice President Dick Cheney delivers his remarks at the joint opening session of Veterans of Foreign Wars and Ladies Auxiliary National Community Service Conference national legislative conference, Monday, March 5, 2007. [AP]

The 66-year-old Cheney, who has a history of heart problems, will be treated with blood-thinning medication for several months, said spokeswoman Lea Anne McBride.

She said Cheney visited his doctor's office in Washington after feeling minor discomfort in his calf. An ultrasound showed the blood clot - called a deep venous thrombosis - in his left lower leg.

Blood clots that form deep in the legs can become killers if they break off and float into the lungs. This is called a pulmonary embolism. Deep vein thrombosis strikes an estimated 2 million Americans each year, killing 60,000.

Many people suffer DVT after spending long periods without moving, such as passengers on long-haul airline flights. Cheney spent about 65 hours on a plane on a nine-day, round-the-world trip that ended last week.

"When you're just sitting there with your legs hanging down for long periods of time, that's what predisposes you to the problem," said Sean O'Donnell, chief of vascular surgery of Washington Hospital Center.

"Treated properly, it poses a small threat," O'Donnell said. "Untreated and unrecognized, it's a very serious problem."

NBC correspondent David Bloom died in 2003 of a pulmonary embolism after spending days in a cramped military vehicle while covering the invasion of Iraq.

To fight DVT, airlines often encourage passengers to walk the aisles and wiggle their feet. But flying's not the only risk of DVT; it extends to anyone older than 60 or who has heart failure. Recent surgery, fractures, childbirth and taking birth control pills also can raise the risk.

Clots that form in the thigh are more likely to break off and migrate to the lungs than those that form elsewhere in the legs. They can also flow to the heart and cause a heart attack or, in rarer cases, move to the brain and cause a stroke.

Cheney returned to the White House after the medical exam and continued his work day.

"He'll maintain his regular schedule," McBride said. "He feels fine."

Dr. Jennifer Heller, director of the Johns Hopkins Vein Center in Baltimore, said it was good news for Cheney that he wasn't hospitalized.

"He certainly does have a very complex cardiac history, but I would suspect his cardiac function and reserve is good, otherwise they would have kept him," Heller said, adding later, "I am sure they would be more conservative with him."

The vice president's office declined to answer questions about which medicines and blood thinners Cheney uses.

Cheney's health has long been an issue.

In 2005, he underwent six hours of surgery on his legs to repair a kind of aneurysm, a ballooning weak spot in an artery that can burst if left untreated.

He has had four heart attacks, quadruple bypass surgery, two artery-clearing angioplasties and an operation to implant a special pacemaker in his chest.

Doctors initially treat DVT patients with an anticoagulant medicine called heparin. Heparin was long given intravenously but can sometimes now be injected in shot form. That can eliminate or shorten a hospital stay. DVT patients are then given the blood-thinning drug warfarin, which sometimes must be taken for months.