Brain and heart link studied
“Keeping your heart fit and strong can slow down the ageing of your brain,” says BBC News, reporting that a poor cardiac output may age the brain by an average of nearly two years.
The research behind this news looked at a measurement of heart blood flow called cardiac index, which indicates a person’s heart output rate for their size. In the examination of 1,500 volunteers, scientists found there was an association between a greater cardiac index and higher brain volume, irrespective of whether participants had clinical symptoms of cardiovascular disease. However, neither cardiac index nor brain volume appeared to be linked to brain functioning.
Overall, the study’s design prevents it from being able to assess whether heart output actually causes changes in brain volume or function. The researchers admit that the link between heart function and brain ageing is not yet clear. However, this is an important topic and further research to clarify the association between cardiac index and brain volume is required.
Where did the story come from?
The study was carried out by researchers from Boston School of Medicine and was funded by the US National Heart Lung Blood Institute as part of the larger, ongoing Framingham Heart Study.
The study was published in the peer-reviewed medical journal, Circulation.
This study was covered accurately by the BBC, which highlighted the preliminary nature of this research.
What kind of research was this?
This was a cross-sectional analysis looking at whether decreasing heart function with age was associated with accelerated brain ageing. Participants were taken from the ongoing Framingham Heart Study, a large cohort study of cardiovascular health that has followed several generations of American citizens since 1948.
The researchers note that cardiomyopathy, a heart problem caused by severe wasting of the heart muscles, has been associated with dementia in previous studies. However, it is not known how other heart diseases affect brain ageing. They suggest that the brain may be affected by problems in the heart as the brain needs adequate blood flow in order to stay healthy, and disrupted blood flow may result in damage to the brain.
As the researchers say, a cross-sectional analysis within a cohort study cannot confirm causation (i.e. that poor heart function causes accelerated brain ageing). It can only suggest whether there could be an association between heart function and brain ageing.
What did the research involve?
The participants in this study were recruited to the ongoing Framingham Offspring study (a prospective cohort study) between 1971 and 1975, and examined every four years subsequently. This sub-study looked at data from 1,504 patients who had attended their seventh examination cycle and who had agreed to undergo a brain MRI study and a MRI of their heart. The participants in the study were aged between 34 and 84, with an average age of 61.
In the examination the researchers recorded blood pressure, current smoking status and any medication use. They also looked at details from the participants’ medical history, such as whether they had diabetes, previous or current cardiovascular disease or heart problems. None of the included participants had dementia, nor had they had a stroke.
The researchers used various tests of brain function, including tests of verbal memory, visual-spatial memory, verbal learning, executive function/information processing and recognition of language/objects.
The researchers took a brain MRI and measured the size of various brain areas and a heart MRI to look at the volume of blood passing through the heart with each heartbeat.
What were the basic results?
The researchers found that cardiac index (rate of blood flow through the heart relative to the person’s size) was low (less than 2.5 litres per minute per m2 of body surface area) in 30% of the samples. They were interested in participants that may have subclinical heart problems (i.e. problems with no symptoms), and therefore repeated their analysis excluding 112 participants who had documented cardiovascular disease. They found that 30% of the remaining participants still had low cardiac indices.
They also found that a higher cardiac index was associated with a higher brain size and that a higher cardiac index was associated with a smaller volume of the heart ventricles (the lower chambers of the heart). Once they had excluded participants with cardiovascular disease from their analyses, the relationship between cardiac index and brain volume remained, although there was no association between cardiac index and heart ventricle size.
The researchers suggested that a higher cardiac index reflected a better functioning heart. They compared participants whose cardiac indices were in the top third to participants who were in the middle third and the bottom third. They found that participants in the middle third and the bottom third had smaller brain volume than the people in the top third.
Participants with low cardiac indices (less than 2.5 litres per minute per m2) had poorer performance on information processing speed, but there was no association between cardiac index and any of the other tests of brain function.
They found that association between cardiac index and brain volume was stronger in those under the age of 60 compared to older adults, and was also stronger in men than in women.
How did the researchers interpret the results?
The researchers say that, even in the absence of cardiovascular disease, cardiac index is related to brain volume. They suggest that reduced body blood flow may contribute to subclinical brain injury by the disruption to brain blood flow mechanisms. Additionally the researchers said that their study showed that around 30% of participants without cardiovascular disease still had low cardiac indices, and suggest that further investigations should examine why this proportion was so high.
Conclusion
This relatively large cross-sectional study found an association between a low cardiac index and smaller brain volume. However, a low cardiac index, and the associated reduction in brain volume, did not seem to have a strong effect on brain function. Also, as a cross-sectional analysis thatonly examines factors at a single point in time, it cannot demonstrate causation between cardiac function and the physiological signs of brain ageing assessed here.
There are some further limitations of this research that need to be considered, some of which the researchers have highlighted:
- The population of the Framingham Offspring study is predominantly white, of European descent and middle-aged or elderly, so the results from this study may not apply to the UK population as a whole.
- The study excluded participants with who had previously had a stroke and only included those who were willing to undergo MRI examination. This may have lead to the sample being a healthier group of people and not representative of the population as a whole.
- The researchers suggest that the study may not have included enough people in order to perform reliable statistical tests to detect changes in brain function.
- The researchers had attempted to adjust for confounding factors such as blood pressure and medication usage, but there may have been additional, unmeasured factors that influenced the results.
- The researchers highlighted that their research involved making multiple statistical comparisons, and that this increases the likelihood of false positive results.
The researchers say that their work is preliminary and their findings require replication in other samples. At this stage, the association between subtle subclinical changes in heart function and brain ageing remains unclear, but further investigation into the clinical significance between cardiac index and brain ageing is merited.
