Two enlarged and weakened arteries behind Vice President Cheney's knees were repaired yesterday in a six-hour procedure at George Washington University Hospital.

The procedure, which consisted of snaking metal-reinforced fabric sleeves into the damaged vessels, was performed under local anesthesia, according to statement released by Cheney's office yesterday afternoon. The vice president, who has a history of heart problems, may spend as much as 48 hours in the hospital. He was described as "awake, alert and comfortable" after the procedure. He planned to be briefed on the impact of Hurricane Rita on the Gulf Coast.

The condition Cheney had, called a popliteal aneurysm, is not life-threatening. The chief risk it poses is the loss of a leg, which can occur if a clot forms in the damaged stretch of artery, stopping blood flow or, in some cases, breaking off and plugging vessels downstream.

Most popliteal aneurysms are treated with surgery. The nonsurgical procedure Cheney underwent is somewhat similar to angioplasty and stent placement, which many people with coronary heart disease get as an alternative to bypass surgery.

The device, a self-expanding metal mesh over a synthetic sleeve, was inserted through a catheter put into an artery above the knee somewhere.

"What he got is relatively new. I suspect the decision was based on his operative risk," said David G. Neschis, a vascular surgeon at the University of Maryland Medical Center.

"The beauty is that you avoid an operation, you avoid general anesthesia, you avoid surgical incisions. It is less stressful to the heart, which is a significant issue, given the vice president's cardiac history," said Bruce A. Perler, chief of vascular surgery at Johns Hopkins Hospital.

The disadvantage, Perler said, "is that you really don't know the long-term performance of these devices."

Neither surgeon knows any details of the vice president's case. They spoke only generally about the condition and the possible reasons for the treatment he received.

Cheney's procedure was done by a team headed by Anthony Venbrux, director of interventional radiology, Jonathan Reiner, director of the cardiac catheterization laboratory, and Joseph Giordano, chairman of surgery at George Washington. Joining the team were Barry T. Katzen, director of the Baptist Heart and Vascular Institute at Baptist Hospital in Miami, and Peter Gloviczki, chair of vascular surgery at Mayo Clinic, Cheney's office said. The hospital released no information.

The popliteal artery passes down the leg behind the knee joint. The main unanswered question about the stent grafts is how they will hold up under repeated bending of the knee.

Similar devices are used to treat aneurysms of other arteries, including ones of the aorta, the body's largest blood vessel. However, those vessels do not cross highly mobile joints such as the knee.

The statement from Cheney's office said "two overlapping Viabahn stent grafts" were put in the artery behind his right knee. The procedure "went exceedingly smoothly," and was then repeated on the left knee. Originally, the procedure on that leg was to have been done at another time.

The aneurysms were found during the vice president's annual physical examination in July. Cheney has had four heart attacks, bypass surgery and two angioplasties for coronary artery disease. He has an implantable defibrillator in his chest.

Perler said popliteal aneurysms can send clots into one, and occasionally two, of the three arteries below the knee without causing symptoms. However, if the third vessel is blocked or the popliteal artery clots off, "then you have a true emergency," he said. "When patients are operated on in that situation, the amputation rate is anywhere from 20 to 50 percent."

The condition is most commonly seen in men 65 and older, and especially in those with a history of untreated hypertension. In half to two-thirds of the cases, the aneurysms are in both knees, Neschis said.

When the aneurysms are in both knees, there is a 70 percent chance the person also has an abdominal aortic aneurysm -- a far more dangerous condition that can cause sudden death if it ruptures.