Washington ? Vice President Dick Cheney will undergo surgery next weekend to repair an aneurysm, a ballooning in an artery, behind his right knee, his office announced Friday.

The vice president’s aide, Steve Schmidt, said the aneurysm was discovered during an annual checkup in July.

Cheney, 64, has had four heart attacks. He underwent three hours of tests in November at the George Washington University Medical Center after suffering shortness of breath, but went home when doctors found no abnormalities.

It is unknown why the knee region is prone to aneurysms. One theory is that the continual bending and flexing of the knee weakens the artery wall.

Unlike aneurysms of the aorta, aneurysms behind the knee do not tend to burst. But they are dangerous because they can form blood clots that can block blood flow at the knee or travel to the foot and cut off circulation there. Such clots often occur with no advance warning.

“If they clot off, there’s probably a 50 percent chance of losing your leg, whereas if you fix them electively it becomes a couple of percent,” said Dr. Steven Katz, professor of clinical surgery at the University of Southern California’s Keck School of Medicine.

Surgery to repair the aneurysm takes two to three hours and can be done several ways. Commonly, the aneurysm is tied off and a vein from the calf or thigh is used to construct a bypass from above the site of the aneurysm to below it.

There are a variety of risk factors for aneurysms behind the knee, which are known as popliteal aneurysms. They include smoking, aging, existing heart disease and family history.

People with popliteal aneurysms are more prone to aneurysms generally. About 60 percent to 70 percent of people with a popliteal aneurysm will go on to develop one at some other site in the body, Lawrence said. Cheney’s history of heart trouble began at least three decades ago. He had his first heart attack in 1978, when he was 37. In 1988, he underwent quadruple bypass surgery after his third heart attack. He had a fourth heart attack in November 2000 and underwent angioplasty to open a clogged artery.