Former president Bill Clinton successfully underwent emergency heart bypass surgery yesterday and began a safe recovery from the four-hour operation, according to his surgeon and other doctors.

Four major blood vessels that supply oxygen to Clinton's heart were found blocked, some by as much as 90 percent, said doctors at New York-Presbyterian Hospital/Columbia University Medical Center. Without the surgery, they added, he probably would have had a major heart attack in the near future.

Surgeons wove four bypasses around Clinton's clogged arteries in an operation they described as routine. The surgery began around 8 a.m., and Clinton started to emerge from sedation by 4 p.m. Doctors said he was able to gesture and respond to commands.

At a news briefing, doctors said that Clinton was still breathing through a tube and that it was too soon to tell whether he might have any cognitive problems as a result of the operation. Whether bypass surgery causes such problems, how long they last and how severe they might be have been much debated in recent years. Clinton's doctors played down such fears, however, saying the former president, who is 58, is likely to soon be able to return to a packed schedule of work and play.

"It would be common for him to leave the hospital in four to five days," said Craig R. Smith, the chief of cardiothoracic surgery at the hospital, who performed the operation. "After that, people are usually 70 percent or so back to baseline in six weeks. To be back to 100 percent takes two to three months in many patients."

Responding to queries about whether Clinton could go on the presidential campaign trail for fellow Democrat John F. Kerry before the November election, Smith acknowledged that Clinton was an extraordinary case and said doctors would make special allowances for him.

Many patients report feeling fitter and stronger after the operation. Doctors say that is not surprising, given that their hearts are being properly replenished with blood and oxygen.

Clinton's surgery was scheduled on short notice for Labor Day after tests last week found that his coronary arteries were clogged. High cholesterol, genetics and a love of fast food are likely culprits.

The patient's wife and daughter, Sen. Hillary Rodham Clinton (D-N.Y.) and Chelsea Clinton, were with him at the hospital. Describing the past few days as an "emotional roller coaster," they said in a statement that they were grateful for the excellent care he has received. "The former president's optimism and faith will carry him through the difficult weeks and months ahead," they said.

Doctors painted a picture of a relatively young patient in good general health who had the good fortune to have a diagnosis and treatment before he had what could well have been a catastrophic heart attack. For several months, hospital cardiologist Allan Schwartz said, Clinton was experiencing shortness of breath and chest constrictions after activity. Over time, it appeared to take less and less activity to trigger the chest problems. Clinton had initially confused the problem with acid reflux, but he had a heart checkup last week.

In a measure of the severity of the problem, he was hospitalized immediately. Doctors said they would have performed the bypass surgery even earlier but waited until yesterday to allow Clinton's blood-thinning medication to wear off.

Coronary bypass surgery, performed on more than 300,000 patients a year, is done to relieve blockages in the coronary arteries. Those arteries nourish the heart, providing this most important muscle with vital oxygen. During exercise or activity, the heart pumps harder and needs more oxygen. When the arteries are constricted, blood and oxygen to the heart muscle are choked off. Patients then experience chest pain, fatigue or angina, as Clinton did.

In some cases, doctors expand a balloonlike device inside the arteries to relieve the blockages. If that does not work, or if the blockages are more numerous, more serious or inaccessible, bypass surgery is used to build shunts around the constrictions. Doctors usually take a healthy artery or a vein from elsewhere in the body to create a diversion around the blockage.

In Clinton's case, Smith said, a coronary artery that ran down the front of his heart, as well as a sub-branch of the artery, were bypassed using a healthy artery that was supplying tissues in the president's left chest wall. Another clogged branch was bypassed using a second artery that was supplying the president's right chest wall. A clogged artery at the back of Clinton's heart was bypassed using a vein from his left leg.

The location of the blockages and their complexity prompted doctors not to attempt "beating-heart" surgery, during which the heart is kept functioning during the procedure. Instead, they stopped Clinton's heart and had a heart-lung machine supply his body and brain with oxygenated blood as surgeons created the bypasses.

Using a heart-lung machine has been associated with some cognitive problems later on, such as memory loss and a lack of concentration, but the exact nature of the association is the subject of debate. In patients as young as Clinton, beating-heart surgery has also not been convincingly shown to cause fewer cognitive problems than the heart-lung machine.

As many as 30 percent of patients on the heart-lung machine have subtle cognitive problems after three months, but those problems often disappear within a year, Smith said.

Bypass surgery is considered routine, but it is still demanding and complex, and it involves a lengthy recuperation. There is a 3 to 4 percent risk of death or stroke, doctors familiar with the procedure said, but Clinton's prospects of recovery are probably better than that, given his relative youth and good health.

Doctors said they would work with the former president to develop a low-salt, low-fat diet and predicted that he would have to take a range of medications. They said Clinton's level of LDL, or "bad," cholesterol had been measured at 114 last week. During his last physical in the White House, his bad cholesterol was at 177. Doctors now recommend that number be less than 70. Clinton had been placed on cholesterol-lowering statin drugs, but the medications were discontinued after the former president lost weight and appeared to become fitter.

"Was that a mistake?" Schwartz asked. "No, it was a judgment. Now the levels of cholesterol we are going to target are going to be quite low and will certainly require medication."

Craig R. Smith performed heart bypass surgery on former president Bill Clinton at

New York-Presbyterian Hospital/Columbia University Medical Center.