Feeling Faint? You Should Have Drunk Some Water!

Summarized by Robert W. Griffith, MD
July 2, 2004

Introduction

A single episode of fainting is reported by over 1 in 5 people, and as many as 1 in 10 have repeated attacks. Mostly, these events happen at times of prolonged standing or emotional stress. They may also occur when someone makes a blood donation. They are sometimes called vasovagal reactions, indicating that the autonomic (or 'beyond our control') nervous system is responsible; the medical name is 'syncope'.

The cause of syncope is lack of oxygen reaching the brain, and the blood pressure and heart rate are low in such victims. (Low blood pressure on standing is known as orthostatic hypotension.)

A study has now been done that shows that a simple measure - drinking 16 ounces of water - can prevent the occurrence of syncope in susceptible individuals by raising their blood pressure. Here's a summary of the report published in the medical journal Circulation.

What was done

Twenty-two healthy volunteers - 11 men and 11 women aged 18 to 42 - were entered into a study done at Vanderbilt University, Tennessee. They had no history of syncope.

A tilting table was used to put the volunteers in a 60 degree head-up position for 45 minutes, or until they experienced presyncope or syncope. (Presyncope was defined as a fall in systolic blood pressure (the top number) of more than 30 mm Hg together with a fall in heart rate of more than 10 beats/minute, while syncope was a systolic pressure below 70 mm Hg and a heart rate below 50 beats/minute.)

On the first occasion, half the participants were given 16 oz (473 mL) of water 5 minutes before the tilting began, and on the second occasion the other half were given water. Selection for water-first or -second was done randomly.

What happened

During the first 30 minutes of tilting, 8 of 22 volunteers without water developed presyncope, while it occurred in only one of 22 who had taken water before tilting. The 'protection' provided by drinking water was associated with an increase in the resistance offered by the blood vessels to stretching, effectively increasing the blood pressure.

The average times that volunteers tolerated head-up tilting were 41 minutes if they had taken water beforehand, and 33 minutes when they were without water. This is a 26% increase in the tolerance time. Moreover, in all 3 subjects who had presyncope both with and without water beforehand, water increased the tolerance times (11 vs. 7, 35 vs. 25, and 34 vs.12 minutes).

What this means

Drinking 16 oz of water before undergoing a model situation for producing fainting significantly improves the chances of avoiding the signs of syncope. In other words, a pint (2 glasses) of water before encountering a 'risky' situation, such as blood donation, may prevent fainting.

Apparently the mechanism of this protective action - reversal of decreased blood volume by increased peripheral resistance of the blood vessels - is also effective in older people; indeed, water intake may represent a major unrecognized reason for individual blood pressure variability from one doctor visit to another in older patients.

Source

• Water ingestion as prophylaxis against syncope. C-C. Lu, A. Diedrich, C-S. Tung,  et al., Circulation, 2003, vol. 108, pp. 2660--2665

Related Links
National Library of Medicine: Fainting
I get dizzy!
Alternative Medicine: Syncope

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