Bypass surgery generally isn't done unless 75 percent of an artery is blocked, said Dr. George Sopko, a cardiologist at the National Heart Lung and Blood Institute in Bethesda, Md.

The fact that the surgery is scheduled and not done on an emergency basis is a good sign, said Dr. Luca Vricella, a cardiac surgeon and professor at Johns Hopkins University in Baltimore.

However, Vricella said the fact that Clinton is getting bypass surgery instead of less invasive procedures — angioplasty or stents such as Vice President Dick Cheney — means the blockage is too extensive or too complicated to be fixed with stents or a balloon inserted through arteries.

"Quadruple bypass means you have a multiple vessel disease, a pretty advanced disease," Sopko told Knight Ridder.

A number of factors, including weight, blood pressure, cholesterol and genetics, cause arterial blockages, the doctors said.

Clinton in January 2001 was put on a cholesterol-lowering prescription because of elevated "bad" cholesterol of 177, up from 134, Dr. Connie Mariano, the president's personal physician, told reporters in a briefing 3 1/2 years ago. Recent studies indicate ideally that number should be in the 60-to-70 range, Sopko said Friday.

Mariano said she talked to Clinton about his cholesterol levels and said the president "acknowledges that it's a combination of not the right type of diet, food that's on the road and long hours, and also not enough exercise."

Clinton, who's lost weight since he left the White House, often talked about being on the trendy South Beach Diet. But on Wednesday, the former president was seen in New Orleans eating gumbo, catfish, black-eyed peas and fried beignets — fried sugared doughnuts — during his book tour.

Chitwood said he doubted that weight was an issue for Clinton's artery disease.

"I don't think the president is really obese; he's kind of chunky," Chitwood said.

Clinton left office weighing 214 pounds, which is considered overweight for someone 6-foot, 2-inches tall, according to the federal government.

Both President Bush and his Democratic opponent, Sen. John Kerry of Massachusetts, wished the former president well during campaign appearances. Some in Bush's audience booed when he wished Clinton well, while those in Kerry's cheered.