Still, it is unclear when Mr. Sanders will return to the campaign trail or whether he will be able to participate in the next Democratic debate, on Oct. 15 in Westerville, Ohio, near Columbus.

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Mr. Sanders’s heart issue could prove to be a political problem for him, and it has the potential to change the dynamic of a still-fluid Democratic race. Mr. Sanders has largely avoided scrutiny of his age and his health. But he and his rivals will now be under increasing pressure to release detailed medical records as Democratic voters try to settle on the best candidate to take on President Trump, who is himself 73.

Mr. Sanders, Mr. Biden and Senator Elizabeth Warren, who is 70, have all said they would release their records before the first voting starts in February.

When Mr. Sanders returns to the campaign, he may find what was already his biggest challenge — finding new converts to his mission — to be even more difficult.

“I do think this makes it harder for him to persuade new supporters to come into his column because this will at least be in the back of people’s minds,” said Erik Smith, a longtime Democratic strategist.

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The setback with his health also comes amid something of a political slump for Mr. Sanders in his second run for the presidency. He has continued to raise substantial amounts of money from his dedicated supporters — on Tuesday, his campaign celebrated an impressive third-quarter fund-raising haul of $25.3 million — and has remained among the top three contenders in the primary. But he has been unable to expand his base beyond those enthusiasts. In recent weeks, he shook up his staff in Iowa and New Hampshire, the first two nominating states, in an effort to jump-start his candidacy as Ms. Warren passed him in some polls.