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Vasovagal Response and Vasovagal Syncope

Last Updated: Mar 1, 2019


Have you ever felt nauseous or fainted while having blood drawn, experiencing intense anger, or standing in a hot room? Your reaction was probably due to activation (overstimulation) of the vagus nerves. They’re two large nerves that control your heartbeat, breathing, blood pressure, and other involuntary processes in your body, and they’re very sensitive to your emotions and body sensations.

Vagus nerve descending from the medulla oblongata

When the vagus nerves are activated due to a stressor, your heart rate slows, your blood pressure drops suddenly, and the blood vessels in your legs dilate (widen). This causes blood to pool in the lower part of your body rather than flowing freely to your brain. This sequence of events is called the vasovagal response, and it can cause you to feel weak, nauseous and even faint. In fact, it’s the most common cause of fainting. When vasovagal response causes fainting, it is called vasovagal syncope.

Vasovagal response is usually harmless. It almost always goes away on its own, and even people who faint wake up almost immediately. However, some people have been injured after fainting suddenly and falling. Vasovagal syncope can also be mistaken for more serious conditions like heart arrhythmia and epilepsy.

Just about everyone experiences vasovagal response now and then. Vasovagal syncope is also very common, affecting up to 25 percent of the population. It’s most often seen in children and teens but can happen at any age.


Vasovagal response is usually triggered by physical or emotional distress:

  • Prolonged standing
  • Sudden changes in body position (standing quickly after sitting for a long time)
  • Being in a hot, crowded or poorly ventilated place
  • Sight of blood, having blood drawn
  • Intense emotions, including shock, anger, pain, or anxiety
  • Straining, such as when lifting something heavy or having a difficult bowel movement.

Recent studies suggest that the tendency toward vasovagal syncope may be inherited. Further research is needed to confirm this finding and pinpoint the genes involved. Interestingly, even in cases where the condition appears to run in families, members have different triggers.


During a vasovagal response, a person might experience:

  • Pale skin
  • Lightheadedness
  • Tunnel vision, blurry vision, or “graying out”
  • Nausea
  • Feeling too hot
  • Cold, clammy sweat
  • Yawning.

People who faint due to vasovagal response might give a few jerky muscle movements. (More continuous movements suggest a seizure, which is treated differently.) The pulse is typically slow and weak. Upon waking, the pupils may appear dilated.

Diagnosis and Treatment

First Aid

A person experiencing vasovagal response can often avoid fainting by lying on his or her back with the legs elevated. Another option is to sit down and lean the head between the knees. Once the discomfort passes, the person should slowly return to a normal posture.

People who faint due to vasovagal syncope usually wake up quickly. Have the person roll onto the back, elevate the feet above the heart, and encourage the person to stay lying down for 15 to 30 minutes. This allows sufficient time for blood flow to return to normal and helps prevent another fainting episode.

Call 911 if a person who has fainted doesn’t wake within a minute or has difficulty breathing.


A person who has fainted should see a doctor if:

  • This is a first fainting episode, and there wasn’t an obvious trigger involved.
  • Fainting caused a significant injury, such as a fall on the head.
  • Additional symptoms are present like shortness of breath, irregular heartbeat or seizures.

After a fainting episode, the doctor’s first goal is usually to rule out any heart and brain conditions that could be causing the problem. Evaluation usually includes:

  • History and physical exam. The doctor will want to know what happened, including possible triggers.
  • Electrocardiogram (EKG). This test records the heart’s electrical activity through the skin. It’s used to detect fast or irregular heart rhythms that can cause fainting. An EKG can be performed in the doctor’s office or over a longer time period using a wearable device called a Holter monitor.
  • Echocardiogram. Ultra-high-pitched sound waves are passed through the chest and reflected to visualize the heart in motion. This test can detect issues with valves and blood flow that might cause fainting.
  • Exercise stress test. This measures heart activity during exercise, usually with the patient on a treadmill.
  • Blood tests. The lab can detect conditions like low blood sugar and anemia (a shortage of healthy red blood cells) that might cause a person to feel weak or faint.
  • Tilt table test. The patient lies on a table that leans at various angles while blood pressure and heart rate are monitored. People prone to vasovagal syncope often faint during this test.


Vasovagal response and syncope rarely require medical treatment. However, about one percent of syncope patients have frequent attacks that interfere with their daily activities. For them, options include:

  • Medication. Drugs used to treat low blood pressure can help reduce episodes.
  • Physical therapy. Leg exercises and aids like compression stockings may help to prevent blood from pooling in the legs.
  • Diet changes. Increasing sodium (salt) and fluid intake can raise blood pressure and prevent attacks.
  • Pacemaker. This device is surgically implanted in the heart and uses electrical signals to prevent the drop in heart rate.


People with vasovagal response almost always recover completely and quickly and have no after-effects.


The best way to prevent vasovagal response is to avoid or manage common triggers:

  • Sit or lie down during blood draws.
  • Avoid hot, crowded, stuffy areas.
  • Drink plenty of fluids.
  • If standing or sitting for a long time, tense your leg muscles to help keep blood moving.


  • Fainting (2014). The Nemours Foundation. Retrieved Nov. 9, 2015, from
  • Syncope (Fainting)(2014). The American Heart Association. Retrieved Nov. 9, 2015, from
  • Vasovagal syncope (Feb. 19, 2013). Mayo Foundation for Medical Education and Research. Retrieved Nov. 9, 2015, from
  • Vasovagal Syndrome: More Evidence for a Genetic Basis (April 17, 2014). Medscape Medical News. Retrieved Nov. 9, 2015, from

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Tina Shahian, PhD

Tina is a writer for Innerbody Research, where she has written a large body of informative guides about health conditions.


A communication specialist in life science and biotech subjects, Tina’s successful career is rooted in her ability to convey complex scientific topics to diverse audiences. Tina earned her PhD in Biochemistry from the University of California, San Francisco and her BS degree in Cell Biology from U.C. Davis. Tina Shahian’s Linkedin profile.


In her spare time, Tina enjoys drawing science-related cartoons.