8 February 2000

Can’t remember the night before?

by Kate Melville

Ever had a few too many and passed out? It's a blunt question but one that far more people than we might expect would have to say, 'Yes' to.

Now joint research findings from the University of Iowa College of Medicine and the Mayo Clinic may explain why this happens.

"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," said Virend K. Somers, a cardiologist at the Mayo Clinic.

During the study researchers found that alcohol impairs the body's ability to constrict blood vessels, a normal phenomena that helps maintain blood pressure when a person moves from a seated to a standing position.

Upon standing gravitational forces lessen the volume of blood returning to the heart, meaning that the heart has less blood available to pump. In normal circumstances this is very brief as the body automatically compensates by narrowing the blood vessels, increasing blood pressure,

What the researchers looked at was whether or not the compensatory restricting mechanism occurring when someone stood up was affected by alcohol's dilation of blood vessels.

According to Krzysztof Narkiewicz, M.D., Ph.D., lead author of the study, "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."

There are already several well-known groups of people who already suffer from similar problems when they stand. These include the elderly, diabetics and people on medicines to dilate their blood vessels.

There were 14 young adults who took part in the study (1 woman and 13 men). Each subject was placed in a cylinder with a vacuum, which simulated the gravitational stress of standing up. Researchers then measured the blood flow in participants' forearms at four different levels of gravitational simulation to see how much their blood vessels tightened in reaction to decreased blood pressure. This was done twice, once when the participants had consumed alcohol and once when they had not.

Researchers found that when free of alcohol, the subjects' vessels constricted normally and blood pressure did not change significantly. However, with alcohol the blood vessels failed to constrict and all participants had low blood pressure at every level of gravitational stress (even at the lowest level, blood pressure dropped twice as much).

So it's now clear that alcohol seems to disrupt this reflex feedback system to raise blood pressure by constricting the vessels.

So this may be why you faint after drinking, but it might still just be the effect of those six martinis!