Causes

Vasovagal syncope occurs when the part of your nervous system that regulates heart rate and blood pressure malfunctions in response to a trigger, such as the sight of blood.

Your heart rate slows, and the blood vessels in your legs widen (dilate). This allows blood to pool in your legs, which lowers your blood pressure. Combined, the drop in blood pressure and slowed heart rate quickly reduce blood flow to your brain, and you faint.

Sometimes there is no classical vasovagal syncope trigger, but common triggers include:

• Standing for long periods of time
• Heat exposure
• Seeing blood
• Having blood drawn
• Fear of bodily injury
• Straining, such as to have a bowel movement