With tests that track the brain's activity in response to specific tasks, researchers determined that people with high blood pressure get less blood to the brain than people with normal blood pressure. "The reduced blood supply reduces the brain's ability to perform needed tasks, such as remembering an unfamiliar phone number," said J. Richard Jennings, Ph.D., study author and professor of psychiatry and psychology at the University of Pittsburgh in Pennsylvania.
Since memory does "lose its edge" as a person ages, Jennings said that one way to look at high blood pressure is to think of it as adding a few years to mental age.
"For several years, researchers have observed that people with high blood pressure seemed to perform a little differently on mental tasks. These were subtle differences, almost the sort of thing that would suggest the test subject was not paying good attention," Jennings said. "But those reports were anecdotal observations with no physiological data to back them up."
When brain imaging technology advanced to the point where scans could detect the metabolic activity of the brain during certain tasks, Jennings' team decided to look for proof that high blood pressure did impair memory.
High blood pressure is defined as a systolic pressure (the top number in a blood pressure reading) of 140 millimeters of mercury (mmHg) or higher and/or a diastolic pressure (bottom number) of 90 mmHg or higher.
They recruited 59 volunteers, average age 60, with blood pressure readings below 140/90 mmHg, and 37 volunteers, average age 61, with hypertension, defined as systolic pressure of 140 mmHg or greater or diastolic pressure of 90 mmHg or greater. The volunteers with high blood pressure had an average pressure of 144/84 mmHg.
All volunteers underwent a number of neuropsychology tests to determine their baseline memory and cognitive abilities. They also had ultrasound imaging of their carotid (neck) arteries to measure blood supply to the brain.
The volunteers then took a series of computer-based memory tests that were performed while positron emission tomography (PET), a brain imaging technique, recorded the response in different brain regions.
The computer exercises tested the type of memory skills involved in daily life, such as looking up a phone number then walking to another room to pick up the phone and dial the number. This type of "running memory" function often begins to fail as people age, said Jennings. While the volunteers participated in the computer exercises such as remembering the location of flashing squares while tapping either right or left hand fingers, the functional scan tracked activity in the brain regions involved in memory tasks -- anterior and posterior watershed areas, the posterior parietal, prefrontal thalamic, and amygdala/hippocampal areas.
The scans showed that "blood flow wasn't as rapidly or fully available among people with high blood pressure as it was in the non-hypertensive volunteers," said Jennings. He said the most notable differences in blood flow occurred in the posterior regions of the brain. The diminished blood flow correlated to slightly worse scores on the memory tests. But he cautions that the differences were subtle -- "the hypertensive patients only missed a percentage point or two."
When a person has high blood pressure, the brain protects itself by remodeling its blood vessels to compensate for the higher pressure. This vascular remodeling, said Jennings, probably explains the blood flow differences observed with the brain scans. For that reason, blood pressure medicines that specifically address vascular changes -- such as angiotensin-converting enzyme (ACE) inhibitors or angiotensin receptor blockers (ARBs) -- are most likely to increase brain blood flow.
The lifetime risk of developing hypertension is about 90 percent for men and women at age 55. About one in five Americans has high blood pressure and about a third of them don't know they have it. High blood pressure contributed to about 118,000 deaths in 2000.
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