Former president Bill Clinton was hospitalized yesterday and preparing for heart surgery early next week after he complained of shortness of breath and chest pain, prompting tests that revealed he is suffering from dangerous blockage of his coronary arteries.

Physicians were alarmed enough to insist that Clinton, 58, undergo bypass surgery with minimal delay at Manhattan's New York-Presbyterian Hospital.

In a call from the hospital to CNN's "Larry King Live," Clinton said: "I guess I'm 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.

"My blockage is so substantial, I think if I don't do this, there's virtually a 100 percent chance that I'll have a heart attack," he said.

Clinton's health scare -- a surprise for a man who had been declared in good health after years of intensive annual physicals in his White House years and afterward -- may have political implications. He has been relishing the prospect of hitting the campaign trail on behalf of Democratic nominee John F. Kerry and other candidates this fall.

He told his close friend and Democratic National Committee Chairman Terence R. McAuliffe that he still plans to do that. "He told me he's raring to go," said the party chairman, who added that he began his phone conversation by observing that Clinton seemed to be taking extreme measures to cut into publicity over this week's Republican National Convention. "He's laughing and in great spirits."

Sen. Hillary Rodham Clinton (D-N.Y.), who heard the news about his diagnosis while in Syracuse, N.Y., said her husband will have the surgery "early next week." She said no information will be released until the surgery is completed and advised reporters to "enjoy your Labor Day weekend."

The procedure is invasive, requiring several days of hospitalization afterward, but in the vast majority of cases, it allows a return to full activity within a month, experts said.

The former president on Thursday went on his own initiative to Westchester Medical Center, near their Chappaqua home. He returned to his house Thursday night, then went back to the same facility yesterday morning for an angiogram, in which dye is inserted into the bloodstream, allowing physicians to capture an image of the blockage. After specialists saw the extent of Clinton's blockage, they sent him to New York-Presbyterian.

Before his diagnosis, Clinton had been planning to fly to Syracuse to join his wife at the New York State Fair. Instead, she canceled her appearances and flew to New York, where the Clintons' daughter, Chelsea, joined her parents at the hospital.

"He's in excellent hands at one of the great hospitals in the world," Hillary Clinton said.

The former president, though he has labored against a tendency toward chubbiness, has been particularly fit over the past year. He has lifted weights and uses an elliptical trainer, friends and former aides said, and he has lost weight using a modified version of the South Beach Diet. But he told CNN last night that he had regained 10 pounds during a tour promoting his memoir, "My Life."

"Some of this is genetic, and I may have done some damage in those years when I was too careless about what I ate," the former president said.

Clinton passed a stress test at a recent physical designed to detect heart problems, McAuliffe said. An angiogram, which might have revealed the blockage earlier, would not ordinarily be performed, even in a presidential checkup, without symptoms suggesting it was warranted, heart experts said.

In 60 to 70 percent of patients with blockage, less invasive treatments such as angioplasty, in which a clogged artery is opened by inserting a balloon, are indicated, according to Valentin Fuster, director of the cardiovascular institute at Mount Sinai School of Medicine in New York.

Clinton's office supplied few details about his medical care.

Heart experts not involved in his treatment said he would be an excellent candidate for either of the two widely used types of heart bypass surgeries. Because he is a relatively young patient, the risks of complications are low, and patients typically recover in about a month. Both techniques have now become so routine that one surgeon described the operations as "fancy plumbing."

"If your water supply is blocked between the water main and your kitchen tap, when you open your faucet you may get a reduced water flow," said Jeremy Ruskin, director of the cardiac arrhythmia service at Massachusetts General Hospital. "One way to deal with that is to bypass the blockage -- you put a pipe near the main and run it into the point where the old pipe is blocked."

The surgery shunts blood around blocked arteries that feed the heart muscle with blood and oxygen. In the traditional technique, surgeons stop the heart from beating and pump blood through the body with a heart-lung machine. The new technique is called beating-heart surgery -- in which arterial bypasses are constructed even as the heart continues to beat.

The benefits of beating-heart surgery appear to be most pronounced in older patients because it has a lower risk of stroke and fewer cognitive problems associated with it. Subtler benefits may exist among patients in Clinton's age group as well, said Paul Corso, chief of cardiac surgery at Washington Hospital Center, but the issue was still being studied.

Heart bypass surgeries are performed when one or more of the arteries feeding the heart are blocked or choked off by buildups of fatty plaque -- high cholesterol levels are the leading culprit. The chest pains that brought Clinton into the hospital appear to have developed abruptly, but the root causes of the problem are likely to be several years old: During his last physical before he left the White House, Clinton had a total cholesterol count of 233 and a "bad" cholesterol count of 177.

Corso said that figure could indicate danger for someone of Clinton's age and body profile. Doctors now recommend that patients aim for a total cholesterol of under 180 and bad cholesterol lower than 70.

Last night, Clinton acknowledged he needed "to keep my cholesterol down, keep my blood pressure down."

For someone in fairly good health who is not diabetic, the risk of death as a result of open-heart surgery is 1 to 2 percent, said Ramin Oskoui, a cardiologist at Washington Hospital Center.

Jim Kennedy, a spokesman for the former president, said Clinton took calls offering best wishes from President Bush and former president Gerald R. Ford.

Campaigning yesterday in West Allis, Wis., the president told supporters that Clinton "is in our thoughts and prayers" and that he wished the former president a "swift and speedy recovery."

Kerry, campaigning in Newark, Ohio, told a rally: "I want you to let a cheer out that he can hear all the way to New York."

Clinton invoked politics when discussing his health last night on television: "Let me just say this, Republicans are not the only people who want four more years here."

Clinton said that without surgery, a heart attack is virtually certain.Media representatives gather at New York-Presbyterian Hospital in Manhattan, where Clinton is to have surgery.