October 4, 2002

More water, faint less — VUMC team encourages water consumption to reduce fainting

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Consuming 16 ounces of water before activities such as donating blood may reduce fainting episodes. (photo by Dana Johnson)

More water, faint less — VUMC team encourages water consumption to reduce fainting

A team of Vanderbilt University Medical Center researchers has found that people who are otherwise healthy, but sometimes experience episodes of fainting after excitement or exertion, may be able to prevent fainting episodes simply by consuming 16 ounces of water prior to activities that often precipitate fainting, such as donating blood.

Dr. David Robertson, Elton Yates Professor of Medicine, and director of the Vanderbilt Clinical Research Center, and Dr. James Chih-Cherng Lu, a visiting scientist from Taipei, presented VUMC’s findings last Thursday in Orlando at the American Heart Association’s 56th Annual High Blood Pressure Research Conference.

Robertson’s group at VUMC, and a team of German researchers led by Drs. Jens Jordan and Christoph Schroeder of Humboldt University in Berlin were each seeking to answer the same question. Each team conducted similar trials that led to the same results.

Both groups studied healthy people without a history of fainting to follow up earlier research performed at VUMC. That research found that drinking water reduced the likelihood of fainting in people with malfunctioning autonomic nervous systems.

“As we considered that finding, it occurred to us that there is a big problem with people passing out after giving blood,” said Robertson. “We believe that around 150,000 people a year faint or experience near-fainting after blood donations, and many of those people never give blood again.

“We wondered if water might help prevent fainting in healthy people who donated blood.”

Both teams conducted their studies in a similar fashion, which involved using tilt-table testing. In a tilt test, the participant lies on a bed-like table that can raise and lower at varying speeds, and can stop at varying degrees of tilt. Both teams tested subjects at a 60-degree angle, slightly more than halfway between lying flat and standing upright.

“You would think that you could rest at that angle all day,” said Robertson. “But many people can’t stay at that angle for more than 45 minutes before passing out.”

On the first round of testing, one of the randomized groups drank water and the other didn’t. On the second round, the two groups switched, with the water drinkers going without and the nondrinkers consuming water. The tests were performed on different days.

VUMC researchers provided their research subjects 16 ounces of water five minutes prior to the tilt test. Ten of the 22 subjects had slow heartbeats and/or a significant drop in blood pressure when tilt-tested for up to 45 minutes without consuming water. Only one person who drank water nearly fainted. Drinking water increased the time that test participants could tolerate remaining at a 60-degree angle to an average of 40.9 minutes, vs. 33 minutes without water. Those who drank water had a smaller decrease in heart rate associated with tilting than those who didn’t drink.

Schroeder’s group had nine women and five men drink 16 ounces of mineral water 15 minutes before being tilt-tested. The German team found that drinking water increased the time before participants nearly fainted by an average of five minutes, compared to when they were tested without consuming water. The researchers also found that drinking water tended to lower the heart rate while lying down, and improve the force and flow of blood in the body both when horizontal and upright.

The German team’s protocol included lower-body negative pressure, which may explain why their subjects lasted less time on the tilt table, and why the time difference after consuming water was not as much as in VUMC’s study, said Schroeder. Negative pressure is created with a chamber that causes a vacuum around the lower body.

Both teams monitored participants for physiological signs until they were near fainting. Testing was stopped before any of the participants passed out.

Robertson says data from the VUMC study shows that the mechanism of water’s effect is not simply a matter of increasing the volume of blood, but probably involves molecular mechanisms eliciting nervous system excitement.

“People who know that certain circumstances cause them to faint should drink water before periods of vulnerability,” said Robertson.