"Prolonged standing, especially in a warm environment, leads to occasional fainting, even in healthy people," says Christoph Schroeder, M.D., of Humboldt University in Berlin, Germany, first author on one report. "Our results show that drinking water right beforehand improves the ability to stand and may help prevent fainting."
Both groups studied healthy people without a history of fainting to follow up earlier research at Vanderbilt University. That research had shown that drinking water reduced the likelihood of fainting in people with malfunctioning autonomic nervous systems.
Such people are prone to pass out when they stand up. The autonomic nervous system regulates body functions that are not consciously controlled, including heart rate and blood pressure.
"As we considered that finding, it occurred to us that there is a big problem with people passing out after giving blood," says David Robertson, M.D., senior author of the other report. "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 donate blood," adds Robertson, a professor of medicine, pharmacology, and neurology at Vanderbilt University School of Medicine in Nashville, Tenn.
Both teams conducted their research in a similar manner, which involved using tilt-table testing. In that test, a person lies on a bed-like platform parallel to the floor. The bed is then titled upward so the head is above the feet. Both teams titled the tables so the person was at a 60 degree angle, two-thirds of the way between lying flat and standing upright.
"You might think you could lie at that angle all day," Robertson says. "But most people can't stay at that angle beyond 45 minutes without passing out."
Participants in each study were randomized to two groups and tested twice in what is called a crossover study.
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 two tests were done on separate days. Both teams monitored participants for physiological signs – such as falling blood pressure and heart rate – until they were nearing the point of fainting. The test was stopped before anyone passed out.
The Vanderbilt group provided 16 oz. of water five minutes prior to tilting. In this study, 10 of the 22 participants had abnormal heartbeats and/or a significant drop in blood pressure when tilt-tested for up to 45 minutes without drinking 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 about 16 oz. of mineral water 15 minutes before titling. 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 drinking 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 tilt protocol in the German study included lower-body negative pressure, which may explain why their subjects lasted less time on the tilt table and why the time difference after drinking water was not as much as in the Vanderbilt study, says Schroeder. Negative pressure is created with a chamber that causes a vacuum around the lower body.
Co-authors with Dr. Robertson were Chih-Cherng Lu, Jens Jordan, Andrè Diedrich, Sachin Paranjape, Paul Harris and Daniel Byrne.
Co-authors with Dr. Schroeder were Victoria E. Bush, Jens Jordan, Lucy J. Norcliffe, Friedrich C. Luft, Jens Tank and Roger Hainsworth.
Editor's Note: For more information on fainting and heart disease, see the Circulation Patient Update at http://circ.ahajournals.org/cgi/content/full/106/9/1048.