WASHINGTON - Richard C. Holbrooke, the U.S. special envoy for Pakistan and Afghanistan, spent seven more hours in surgery Sunday, a day after undergoing 20 hours of surgery to repair a tear in the aorta, the largest artery leading from the heart, family and friends said.

The followup was needed after the first operation was hampered by failure of his blood to clot, according to friends, who spoke on condition of anonymity.

Holbrooke, 69, was admitted to George Washington University Hospital in Washington Friday after he collapsed during a meeting with Secretary of State Hillary Rodham Clinton.

About 2,000 cases of torn aortas occur yearly in the United States, and 90 percent of untreated patients die within two days, said Duke Cameron, chief of cardiac surgery at Johns Hopkins Hospital in Baltimore.

Without surgery, Holbrooke might have had a heart attack, stroke, organ damage, or paralysis as the tear prevented oxygen-carrying blood from being pumped to other organs, Cameron said.

Surgery usually takes four to five hours, he said.

The length of the surgery suggests the damage was extensive, said James Willerson, chief medical officer of the Texas Heart Institute in Houston.

"The object of surgery is to repair the site of the tear with a background graft" to ease pressure that can cause further ruptures in other parts of the heart, Willerson said in a phone interview. "They may have needed to reattach some critical blood vessels that were blocked by the tear."

Research suggests that tears may result from a combination of high blood pressure and weak tissue, Cameron said in a telephone interview.

Holbrooke has spent the last two years traveling to Afghanistan and Pakistan, seeking support from allies to help promote economic development in and stabilize the two neighboring countries plagued by terrorism.

President Obama said in a statement that he and Michelle Obama were praying for Holbrooke's recovery.