A heart attack is generally a massive medical event, when blood stops flowing to the heart. It's often a terrifying experience — the rushed trip to the hospital, the risk to one's life — that can sometimes lead to post-traumatic distress disorder.

This is what makes new new research from a team of Harvard professors so surprising: it shows that a wide swath of patients misremember whether they even had a heart attack in the first place.

patients' memory of their health care doesn't always align with reality

About a quarter of Medicare patients over 67 who say they've had a heart attack have no Medicare record of having any heart-related hospital admission. On the flip side, about one-third of Medicare beneficiaries who did experience a heart attack said they hadn't.

Perhaps the most surprising finding is about the people who incorrectly reported heart attacks. Even though they hadn't experienced the acute event, they were still twice as likely to die within the next year as average Medicare patients. Their report of a heart attack seemed significant — it indicated a higher risk of death — even if a heart attack hadn't occurred.

"In medical school, they teach students that the history is one of the most important things you can do, but people aren't that great at it," says Laura Yasaitis, a postdoctoral fellow at Harvard School of Public Health and lead author of the study, which was published in the journal Circulation this week.

One-quarter of people who said they've had a heart attack ... actually haven't

Yasaitis and her co-authors were able to cross-reference two databases — one of Medicare claims and the other a national survey of Americans' health status — to compare what types of medical care patients thought they had undergone with what had actually happened.

The Health and Retirement Survey is a national, longitudinal study that since 1996 has tracked a group of Americans over age 50. One of the questions it asks is whether the respondents have had "a heart attack or myocardial infarction."

why do patients mis-report heart attacks? "It's a mystery."

Yasaitis compared those self-reported heart attacks with each patient's actual Medicare claims, which showed which patients had doctors who had billed for treating a heart attack. And that allowed her to find discrepancies.

She found that only one-third of those who said they'd had a heart attack had indeed undergone a myocardial infraction (the medical term) during an inpatient admission that their doctor billed for. Another 41 percent had a hospital admission for another, heart-related condition that was not a heart attack.

But that still left about a quarter of those who reported a heart attack who hadn't had a hospital admission for any cardiac condition at all. Among those, 17 percent hadn't even had an inpatient admission to a hospital in the past 2.5 years.

"They're somewhat of a mystery," says Yasaitis. It's possible that more information would turn up if they looked at outpatient data; this study relied on hospital inpatient information.

One-third of heart attack patients don't remember having a heart attack

When Yasaitis looked at who did report a heart attack, she turned up an equally mysterious finding: one-third of them didn't report it in the Health and Retirement Survey. They said they hadn't had a heart attack when their Medicare billing records showed that they had.

The lack of reporting might indicate other issues. Those who didn't report their heart attack tended to be older and more likely to have only completed a high school education than those who did report. Those who had disabilities were also less likely to report it.

"If you had a heart attack, you're supposed to take medication," says Yasaitis. "There are a lot of studies showing that adherence to those medications is really low. One reason might be people don't understand their diagnosis."

People who falsely report a heart attack are actually more likely to die

This was another surprise in the study: even among those who reported a heart attack that wasn't in their medical records, that information still seemed to be important.

Those people turned out to be twice as likely to die in the next year as the average Medicare patient. So there is something different about this group, even if it's not the fact that they've had a heart attack recently.

This is why Yasaitis doesn't see her findings as a reason for physicians to write off patient-provided information as useless. Just the report of a heart attack, even if it's incorrect, suggests that a patient is at increased risk of death.

"Physicians should definitely still be listening to their patients," she says. "The self-report does have importance, but you also have to look at what it means, whether it is in fact a heart attack or a sign that there might be other, more complex illnesses in their history."