DALLAS, Feb. 1 -- Researchers are closing in on why people who overindulge in alcohol sometimes pass out, according to a study in today's Circulation: Journal of the American Heart Association.
"From our own personal observations and existing scientific data it seemed that alcohol was linked to fainting spells," says Virend K. Somers, M.D., Ph.D., a co-author of the study, and a cardiologist at the Mayo Clinic in Rochester, Minn.
"We're all aware of people who, after drinking considerably, get up to leave, stagger and then pass out. But exactly why they pass out after drinking alcohol has never been fully understood."
In their study, which was conducted at both the University of Iowa College of Medicine in Iowa City and the Mayo Clinic in Rochester, Minn., researchers discovered that alcohol impairs the body's ability to tighten the blood vessels, a natural process that maintains blood pressure when a person moves from a sitting to a standing position.
When someone stands up, gravity reduces the amount of blood returning to the heart, so the heart has less blood available to pump. Normally, that situation is momentary because the body has a reflex feedback mechanism that narrows the blood vessels and increases the pressure, Somers says. He adds that while it was well known that alcohol dilates the blood vessels, scientists didn't know whether alcohol had any effect on the reflex feedback system.
"We believe this is the first study to demonstrate that alcohol consumption in social drinkers causes low blood pressure upon standing because of impaired constriction of blood vessels," says Krzysztof Narkiewicz, M.D., Ph.D., lead author of the study, and an associate professor of internal medicine in the department of hypertension and diabetology, Medical University of Gdansk, in Gdansk, Poland. He conducted the study as a National Institutes of Health fellow at the University of Iowa College of Medicine.
"From a patient care perspective, there are several groups of people who already have lightheadedness on standing because their blood pressures are not maintained adequately. That includes diabetics, the elderly and people on medicines that dilate their blood vessels, as well as those who suffer from fainting spells for other reasons," says Somers. "Drinking alcohol might magnify their risk of fainting and injury from falling."
The researchers studied 14 healthy young adults -- 13 men and one woman -- with an average age of 26. The subjects reclined in a metal cylinder while a vacuum was applied to the cylinder to reduce the pressure on the subjects' lower bodies. This simulates the gravitational stress of standing up.
Somers and his colleagues measured blood flow in the study participants' forearms at four different levels of gravitational simulation to determine how much their blood vessels tightened in response to the drop in blood pressure.
They had the subjects repeat the test on two different days, once after consuming an alcohol-free drink, and on another day after drinking the equivalent of two to three beers, enough to be mildly intoxicated, Somers says.
The researchers found that during the alcohol-free sessions, the subjects' vessels constricted normally and blood pressure did not change significantly, except at the most extreme negative pressure. However, the subjects' blood vessels failed to constrict after drinking alcohol and the subjects had a low blood pressure response at every level of gravitational stress. In fact, at the lowest level, blood pressure dropped twice as much as it had in the same subjects when they abstained from drinking alcohol.
The reflex feedback system that raises blood pressure by constricting the vessels is important to keep blood flowing to vital organs such as the brain, the researchers say. "Alcohol appears to disrupt this mechanism even in healthy young people, at fairly modest levels of alcohol intake," says Somers.
Co-author was Ryan L. Cooley, M.D.
Materials provided by American Heart Association. Note: Content may be edited for style and length.
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