NEW YORK -- Doctors in New York say former President Bill Clinton is awake after undergoing a "relatively routine" quadruple heart bypass operation Monday, lasting about four hours.

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However, doctors at New York Presbyterian Hospital/Columbia said Clinton isn't quite alert yet and is still sedated enough to require a breathing tube.

Clinton, 58, has been hospitalized since Friday after suffering chest pains and shortness of breath. His tests showed no heart attack, but doctors said there was "extensive blockage" of major arteries.

A majority of his heart vessels were more than 90 percent blocked, and Clinton was in grave danger of a serious heart attack before the operation, the doctors said.

The former president may be ready to leave the hospital in as soon as four or five days, said Dr. Craig Smith, chief of cardiothoracic surgery at the hospital, who said Clinton's recovery seemed "straightforward."

Clinton is generally extremely healthy, and he should be able to continue his active schedule -- including campaigning -- in a matter of weeks, said Dr. Allan Schwartz, the hospital's chief of cardiology.

The doctors spoke at a news conference Monday afternoon after reading a statement from Sen. Hillary Rodham Clinton, who said she and daughter Chelsea are "deeply thankful" to get good news after going through what she termed an "emotional roller coaster."

Although doctors determined the need for surgery Friday, they waited a few days to proceed because Clinton was on blood-thinning medication. They wanted the medication to work its way out of his system before surgery to reduce the risk of bleeding.

"It was obvious he needed an operation, and it was equally obvious it would be safer to wait a few days," Smith said.

President George W. Bush sent "very best wishes" to his predecessor.

At a rally Monday in southeast Missouri, the president urged backers to join him in praying that Clinton has a "speedy recovery."

Bush reported that Clinton's surgeons think the procedure went well, which he said "is good news."

More than 45,000 get-well messages had been posted on the Clinton Foundation's Web site as of Monday evening.

A well-wisher from Chicago hoped Clinton has a speedy recovery. An admirer who had just read his autobiography said it was like having a friend in the hospital. A fan in Omaha warned about staying away from junk food.

The former president and his family issued a statement on the Clinton Foundation's Web site on Sunday, saying they felt "blessed and grateful for the thousands of prayers and messages of good will we have received these past few days." Read Full Statement

"While bypass surgery certainly isn't something to look forward to, we are very lucky that the condition was detected in time to have this procedure before something more serious occurred," the statement said.

Cardiologists said Clinton would likely spend less than a week in the hospital, and should fully recover in a month or two.

Columbia-Presbyterian Hospital has the highest death rate for coronary bypass surgeries in New York state -- just under 4 percent in 2001, according to a report by the state health department.

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