Vasovagal syncope

How it works

The vagus nerve is the main brake on the heart³. In a vasovagal faint, this brake is pulled far too hard. The heart slows. At the same time the nerves that keep blood vessels narrow go quiet, so the vessels relax and widen. Blood pressure falls. Not enough blood reaches the brain, and the brain switches off for a moment¹.

Lying flat ends the faint. Once you are flat, blood no longer has to climb against gravity. The brain refills and you usually wake within 30 seconds⁵. You may feel sick, pale and sweaty for minutes or hours afterwards¹.

Common triggers are the sight of blood or needles, sharp pain, fear, heat, standing still for a long time, dehydration, coughing, and straining on the toilet¹². This reflex is the most common cause of fainting¹.

A faint usually gives warning²:

• light-headedness,
• vision that narrows or goes grey,
• ringing in the ears¹,
• feeling sick,
• pale, sweaty skin.

At the first sign, lie down or sit with your head low, so you do not fall. This often stops the faint.

How common it is

The “roughly 1 in 3” figure for fainting at least once comes from lifetime recall surveys, which depend on people remembering and reporting an event that may have happened decades earlier, often in the teenage years²⁴. Because a single, classic faint with clear warning signs is rarely investigated afterwards, most figures describe self-reported fainting in general rather than confirmed vasovagal syncope specifically. It is known to be more common in teenagers and young adults, but exact rates by age are not well pinned down¹².

Is it a problem?

By itself, no. The reflex does not damage the heart or the brain. The main risk is the fall. If you feel a faint coming, lie down and raise your legs, or sit with your head between your knees.

Call emergency services if someone faints and does not wake within a minute, or is not breathing⁵.

See a doctor if any of these apply²⁵:

• you fainted during exercise, or while lying down,
• there was no warning at all,
• you had chest pain, a racing heart, or a pounding heartbeat before the faint,
• you were unconscious for more than a minute, or your body jerked for a long time,
• someone in your family died suddenly at a young age from a heart problem,
• you faint often without a clear trigger, or you are an older adult²,
• you were injured in the fall.