Clinton in good spirits as he prepares for open heart surgery this week

Former president Bill Clinton is telling friends he is in good spirits despite facing the challenge of a multiple bypass operation for blocked blood vessels to the heart. The procedure is likely to be undertaken at the beginning of this week, perhaps as early as tomorrow.

Speaking to CNN's Larry King show on Friday evening from his bed at the Columbia Presbyterian hospital in Manhattan, Mr Clinton, who turned 58 last month and had no previous history of heart trouble, joked: "Let me just say this - Republicans aren't the only people who want four more years here."

Asked if he was nervous to undergo open heart surgery, Mr Clinton replied that he was "a little scared, but not much. I'm looking forward to it. I want to get back. I want to see what it's like to run five miles again".

The former president's illness appeared to come out of the blue. Friends said he began to experience mild chest pains and shortness of breath in the middle of last week. He underwent initial tests at a hospital near the home he shares with his wife, Senator Hillary Rodham Clinton, in Westchester County, New York, on Thursday morning and was sent home.

However, he returned to the hospital on Friday morning and X-rays showed he had significant blockages of the bloodflow to his heart. He was then taken to the Presbyterian hospital on the Upper West Side of Manhattan, world-renowned for its heart surgery team.

Sources close to Mr Clinton said he is likely to undergo either triple or quadruple bypass surgery where pieces of blood vessel are cut from elsewhere in the body and used to detour around the diseased arteries. If all goes well, he will stay in hospital for several days.

The setback is almost certain, however, to prohibit the former president from fulfilling a pledge to the Democratic presidential candidate, John Kerry, to speak for him in the last 60-day stretch of the American election campaign. In recent weeks, he had been increasingly involving himself in the battle against President Bush.

But medical experts said that Mr Clinton could look forward to getting back to normal in the longer term. They pointed to Vice-President Dick Cheney, who suffered heart attacks in 1978, 1984, 1988 and 2000. Mr Cheney underwent bypass surgery in 1988.

The good news for Mr Clinton is that the chest pains gave him an early warning before he suffered any kind of heart attack. The angiogram X-ray tests on Friday revealed "multi-vessel coronary artery disease, normal heart function and no heart attack," said Dr Anthony Pucillo, who performed them.

That Mr Clinton was facing heart surgery came as a shock to friends and neighbours in Westchester who noted that he had lost a considerable amount of weight in recent months, in part by committing himself to the rigours of the low-carb South Beach diet.

As President, he enjoyed mostly good health, marred only by slight hearing loss and a cancerous lesion on his nose. He was famous, however, for enjoying fast and fatty foods, regularly tucking into cheeseburgers and fries.