The investigators found that people within the lowest and middle range of cardiac index had significantly lower brain volumes than people who had the highest cardiac index.
Jefferson said the researchers were somewhat surprised by this finding, as they had expected to see more of a linear relationship, meaning that as the cardiac index improved, they expected to see the brain volume improve as well, but that wasn't the case with the middle cardiac index group.
"We didn't expect the middle group, who have what's considered normal cardiac index values, to have decreased brain volume," said Jefferson.
The study authors also found that the association between cardiac index and brain volume was strongest in people under 60 years old. Jefferson suggested that one reason for this finding may be that as people get older, they have more competing brain health factors, such as the development of dementia.
The study's findings need to be replicated, she said, and researchers need to investigate whether changes in cardiac index over time can have an impact on brain health.
"We may have identified another mechanism for identifying abnormal brain changes, but we need further studies to see how cardiac index relates to brain changes over time," Jefferson said. "Is cardiac index a risk factor for Alzheimer's and other brain disease?"
One expert said it's too early to say whether there's a cause-and-effect relationship at work here.
"This is an interesting, strong association. Just as we measure someone's weight, blood pressure and cholesterol to assess their heart disease risk, we now have numbers for cardiac index and brain volume. But, we're still at the association stage, and can't tell if one factor is caused by the other," said Dr. Ralph Sacco, president of the American Heart Association and co-author of an accompanying editorial in the same issue of the journal.
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