Mr Cheney had a pacemaker fitted after his fourth heart attack

Jennifer Mayfield said he was "awake, alert and comfortable" after six hours of surgery under local anaesthetic.

He will stay in hospital for 48 hours to monitor his recovery, she added.

Mr Cheney, 64, has a history of heart attacks. He had his fourth heart attack at the end of 2000 and in June 2001 he was fitted with a pacemaker.

Arterial aneurysms were diagnosed in Mr Cheney's right and left knee during his annual medical check-up in July.

An aneurysm is when a section of blood vessel becomes bloated and stretched, forming a bulge which can rupture.

Daily exercise

Mr Cheney had been due to have an operation only on his right knee, with surgery on his left knee at a later date.

But doctors decided during Saturday's operation to repair both at once, his office said.

CHENEY'S MEDICAL HISTORY 24 Sept 2005: Operation to repair aneurysm behind right knee 29 June 2001: Announces he will be tested for a pacemaker 5 March 2001: Hospitalised for cardiac catheterisation 22 November 2000: Undergoes operation to unclog arteries after a mild heart attack 1988: Undergoes quadruple bypass surgery 1984: Suffers heart attack 1978: Experiences his first heart attack

Ms Mayfield said Mr Cheney had undergone "successful, elective, minimally invasive" surgery at the George Washington Hospital.

"Presently, the vice-president is awake, alert and comfortable," she said.

"He will remain in the hospital for up to 48 hours to monitor his recovery and is expected to resume his regular schedule when he is released."

After the July tests, doctors said Mr Cheney was in good overall health and his pacemaker had never been needed to assist his heart, AP said.