It was the fall of 1996, and Boris N. Yeltsin was running for re-election as Russia’s first president in the post-Soviet era. But he faced a crisis far more threatening than any opponent: he was desperately ill.

Mr. Yeltsin had had a heart attack. He was experiencing chest pain from angina. He needed a coronary bypass operation. But his Russian doctors said he could not survive such surgery.

For independent advice, Mr. Yeltsin reached out to an American doctor as renowned in Russia as he was in the United States: Dr. Michael E. DeBakey, the pioneering Houston heart surgeon.

Dr. DeBakey agreed to go to Moscow, and after examining Mr. Yeltsin he determined that the Russian leader could indeed survive a bypass operation. It was not widely noted in the obituaries for Mr. Yeltsin, who died last week at 76, but that consultation very likely saved his presidency, if not his life.

In doing so, it changed the course of history. Among other things, if Mr. Yeltsin had not been re-elected, he would never have had the opportunity to reach deep into the Russian bureaucracy to select Vladimir V. Putin, then an obscure functionary, as his successor.

“All the doctors agreed Yeltsin would have died if he did not have the bypass,” Dr. DeBakey said in an interview yesterday. “No question about that, because he was deteriorating and going into early heart failure.”

Dr. DeBakey, now 98, had often visited the Soviet Union during the cold war to lecture on — and occasionally perform — the operations and procedures that he had developed. But in those days, Kremlin officials would hardly have asked for a consultation with an American surgeon, no matter how eminent.

“Calling in Dr. DeBakey was very important, a signal that he was in very serious condition, and consulting with a world leader in surgery this way was almost unthinkable in the Soviet period,” said Marshall I. Goldman, a Russian expert and senior scholar at Harvard. “It was a measure of Dr. DeBakey’s stature in Russia.”

Thomas R. Pickering, who was the United States ambassador to Russia at the time, said it was not clear who would have been elected if Mr. Yeltsin had dropped out of the race. Both he and Mr. Goldman said in separate interviews that it could have been a member of the Communist Party.

Dr. DeBakey has said in interviews, in 1996 and since, that he told Mr. Yeltsin he could withstand a coronary bypass operation after treatment, including transfusions, to correct anemia from internal bleeding and to restore his low thyroid function. Despite Mr. Yeltsin’s reputation for excessive drinking, his liver function was normal.

Mr. Yeltsin immediately asked about his chances of being able to get back to work. “Excellent, possibly in two or so weeks,” Dr. DeBakey said he replied. He then carried out Mr. Yeltsin’s request to give the news to journalists.

“The next morning, Mr. Yeltsin’s doctors were kidding me that the Communist Party did not like your report,” Dr. DeBakey said. Until then, he said, he did not appreciate his recommendation’s political significance.

Dr. DeBakey said his confidence in Mr. Yeltsin’s ability to undergo bypass surgery was based on an examination far more thorough than the ones the Russian doctors had conducted. One of those doctors confirmed that assertion in an interview with me in 1996, when I went to Moscow to cover the operation for The New York Times.

Dr. DeBakey told Mr. Yeltsin that he had confidence in the Russian heart surgeon who would eventually perform the quintuple bypass. That surgeon, Dr. Renat S. Akchurin, had briefly trained under Dr. DeBakey in Houston. Dr. Akchurin initially specialized in microsurgery, wearing jeweler-type loupes and using an operating microscope to magnify the surgical field when repairing human nerves injured in accidents. His medical bosses recognized his talent and asked him to become a heart surgeon.

Mr. Yeltsin insisted on having the operation at Dr. Akchurin’s hospital, even though it did not have all the emergency equipment Dr. DeBakey thought might be needed. (Dr. DeBakey knew the hospital because he had consulted on its design.)

So Dr. DeBakey also promised Mr. Yeltsin that he would watch the bypass operation and bring a team that included his surgical partner, Dr. George P. Noon, along with the equipment in case it was needed. It was not.

Mr. Yeltsin’s medical team followed Dr. DeBakey’s advice to correct the thyroid and other problems to prepare him for the bypass operation. A key monitor of heart function is the ejection fraction, a test that measures the proportion of blood ejected from the main chamber of the heart in each beat. After the medical tuneup, Mr. Yeltsin’s ejection fraction rose significantly, to 40 from the high 20s. After the operation, the fraction improved to 50, good but still a bit less than normal.