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WASHINGTON — Democratic presidential hopeful Bernie Sanders has resisted calls to release additional medical information in the wake of a heart attack he suffered in October.

"We have released as much" medical information "as any other candidate," the independent senator from Vermont said on NBC News' "Meet the Press" this month.

When pressed on whether he would release more, Sanders responded that "you can start releasing medical records and it never ends."

But by releasing one simple indicator of his heart health, Sanders could address some lingering questions, Richard Kovacs, president of the American College of Cardiology, or ACC, told NBC News.

Following Sanders' heart attack, doctors inserted two stents in an artery and he was hospitalized for several days. After promising to release "comprehensive" records after the procedure, his campaign balked, saying no more information would be forthcoming.

"We released the full report of that heart attack," Sanders said at last week's NBC News/MSNBC debate in Las Vegas, pointing to letters from three doctors the campaign distributed in December, which aren't the same as medical records.

Kovacs, who reviewed the letters for NBC News, said they omit a "standard" measure of Sanders' heart health.

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The indicator, called the left ventricular ejection fraction, is provided to any patient after a heart attack, Kovacs said. It's a measure of how much blood volume the heart pushes out with an individual heartbeat, and it correlates with the risk for future cardiac events and mortality rate.

"Normally the heart will push out 60 percent," Kovacs said. "If you go down to 40 or 50 percent, we regard that as mild impairment of the left ventricle. Thirty to 40 percent would be moderate. If you get to 30 percent, that would be severe."

Sanders hasn't revealed what the number was at the time of his heart attack nor what it is today. Campaign spokesman Mike Casca, responding to questions from NBC News, declined to provide the ejection fraction number to NBC. The doctors' letters "are pretty comprehensive," he said.

The letters, Kovacs said, "imply with the heart attack that he had diminished heart muscle strength" or a lowered fraction. Still, the letters provided by his doctors also suggest that he has improved since then.

"Full transparency would be to release his ejection fraction," said Dr. Hadley Wilson, an ACC board trustee and a cardiologist at the Sanger Heart & Vascular Institute in Charlotte, North Carolina.

"A subsequent stress test in January and the other information about his clinical response were all very positive and encouraging," Wilson said. "I don't really know" why Sanders wouldn't release it, he added, offering that it could be over concern that it could be "misconstrued" in the broader clinical picture.

"It is an important baseline, but it does have to be taken into the whole clinical context," including the fact that Sanders hasn't had additional incidents, which is all positive, he said.

Sanders isn't a patient of Kovacs', and Kovacs said he wouldn't comment specifically on his condition. But he did note that presidents routinely release the results of similar health benchmarks, like cholesterol and colonoscopy readings.

"It's a patient's personal private information, but there's the importance of the number," Kovacs said.

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One letter released by the campaign, from University of Vermont cardiologist Martin LeWinter, called Sanders' heart function "stable and well preserved" but omits the number.

"I don't know what that means, but it's not the ejection fraction, which is just a simple number," Kovacs said. "They've chosen not to reveal the ejection fraction."

In another letter, Sanders' primary care doctor, Brian Monahan, the attending physician at the U.S. Capitol, said that Sanders is in "good health" and that he has stopped taking several medications that were required after the heart attack.

Yet the ejection fraction rate would be a good indicator of the progress of Sanders' recovery, Kovacs said.

"That is a number that we routinely obtain that is important for guiding therapy," Kovacs said. "I say here is your ejection fraction and here's what we will need to do down the line."