NEW YORK — Bill Clinton had a successful quadruple heart bypass operation yesterday to relieve severely clogged arteries that doctors said had put the former president in grave danger of a major heart attack sometime soon.

Clinton is expected to make a full recovery, but doctors said he was fortunate to have checked himself into the hospital when he did. The heart disease they repaired was extensive, and blockage in several of Clinton’s arteries was “well over 90 percent,” said Dr. Craig R. Smith, the surgeon who led the operation.

“There was a substantial likelihood that he would have had a substantial heart attack,” said Dr. Allan Schwartz, chief of cardiology at New York Presbyterian Hospital/Columbia.

Smith said Clinton could leave the hospital in four or five days. Clinton was awake but sedated about four hours after the operation ended, Schwartz said.

The four-hour surgery came three days after Clinton arrived at the hospital complaining of chest pain and shortness of breath. But doctors said Clinton’s problems were not as sudden as had been portrayed. He had suffered shortness of breath and tightness in his chest for several months, blaming them on off-and-on exercising and acid reflux, his doctors said.

In addition, the former president had high blood pressure and may not have been adequately treated for high cholesterol. His doctors said he was put on a cholesterol-lowering drug a few days ago. Clinton was prescribed cholesterol medicine in 2001 as he was leaving office.

In bypass surgery, doctors remove one or more blood vessels from elsewhere in the body — in Clinton’s case, two arteries from the chest and a vein from the leg — and attach them to arteries serving the heart, detouring blood around blockages.

During the operation, Clinton’s heart was stopped and he was put on a heart-lung machine for 73 minutes. That process, used for more than 75 percent of bypass patients, carries a small risk of stroke and neurological complications.

As many as 30 percent of patients suffer “measurable but very subtle” problems in mental functioning after bypass, but those problems are gone within a year, Smith said.

Asked whether there were any troubling moments during the surgery, Smith said: “There are always a few minor anxious moments during heart surgery. There was nothing in this case that was outside the realm of routine.”

Schwartz said it would be possible for Clinton in the future to lead an “extraordinarily active lifestyle” — including hitting the campaign trail.

“He is recovering normally at this point. Right now everything looks straightforward,” Smith said.

Still, Dr. W. Randolph Chitwood, chief cardiovascular surgeon at East Carolina University and a spokesman for the American College of Cardiology, agreed with Clinton’s doctors that the president had been in a dangerous state leading up to the operation. “Within the next couple of weeks, something was going to happen,” he said.

Clinton was described as upbeat in the days before the surgery, resting with his wife and daughter. One New York Post photo showed the former president reaching for a Boggle game near his hospital-room window.

Clinton has blamed his heart problems in part on genetics — there is a history of heart disease in his mother’s family — but also said he “may have done some damage in those years when I was too careless about what I ate.”