What Is the Vasovagal Reflex?

An automatic response responsible for feeling faint

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If you've ever become sweaty and felt like you're about to pass out while pooping or gotten dizzy at the sight of blood, you've experienced a vasovagal response.

The vagus nerve is the largest nerve in the body. It runs from the brainstem to the chest and abdomen and is tasked with regulating critical body functions such as heart rate, blood pressure, breathing, and digestion.

When triggered, the vasovagal reflex causes a sudden drop in heart rate and blood pressure. As a result, you feel lightheaded, nauseous, and sweaty and may even faint (vasovagal syncope). Straining during a bowel movement, stress, seeing blood, and standing for a long time are common triggers.

This article will explain your body's vasovagal reflex, what triggers it, and steps you can take to prevent or minimize these episodes.

Overview

The vasovagal reflex is an automatic response that stimulates your vagus nerve. It can affect your central and peripheral nervous systems and cardiovascular system.

When triggered, the vagus nerve sends a message to the brain that may cause a sudden drop in your blood pressure and heart rate. This can make you feel faint.

The word vasovagal describes the two parts of your body that cause the response: "vaso" refers to your blood vessels and "vagal" refers to your vagus nerve.

Vasovagal Reflex Symptoms

fainting warning signs

Verywell / Jessica Olah

If the vagus nerve is suddenly stimulated, the reflex response is automatic. Several body changes may occur. These can be early warning signs of a fainting spell, which causes you to temporarily lose consciousness.

  • Dizziness or feeling faint or lightheaded
  • Suddenly feeling warm and sweaty or as if you have cold and clammy skin
  • Blurry vision or tunnel vision (blackness in the periphery of your vision)
  • Feeling nauseous
  • Ringing in your ears
  • Turning pale

Vasovagal symptoms can sometimes be caused by another condition. Tell your healthcare provider if you experience them so you can get a proper evaluation.

Causes

The vasovagal reflex is not necessarily abnormal and the nerve pathways involved are most likely present in everyone. Some scientists believe this response developed as humans evolved and began to stand upright. The difference now is that some people are more susceptible to involuntarily triggering the reflex.

One study notes that 42% of women and 32% of men experience at least one vasovagal syncope event by the time they reach age 60. According to that study, some people tend to faint more often than others.

In some cases, there may be an underlying medical condition, such as a neurological condition (one that affects the brain or nerves), that can cause a person to experience these symptoms.

According to the National Institute of Neurological Disorders and Stroke, a fainting spell that does not have a triggering event may also be a sign of an underlying heart problem. However, it is not uncommon to find no specific cause for the episode.

What Triggers the Vasovagal Reflex?

Possible triggers for the vasovagal reflex include:

  • Becoming overheated
  • Coughing
  • Dehydration
  • Emotional stress
  • Intense emotions, like fear
  • Not getting enough sleep
  • Pain
  • Seeing blood
  • Skipping meals
  • Standing for too long
  • Standing up quickly

Why You May Feel Faint When You Poop

Straining while having a bowel movement and symptoms related to irritable bowel syndrome may also provoke the vasovagal reflex, causing you to sweat and feel sick while you poop.

Treatment

Vasovagal responses usually resolve without treatment. If you feel faint or dizzy, sit down and put your head between your knees or lie down and raise your feet.

Some people only have one episode in a lifetime, while others may have several, especially if you're exposed to triggers again.

If your healthcare provider identifies a specific condition that is causing your fainting spells, treatment will focus on addressing that issue. For example, medications or procedures might be needed to correct a heart rhythm problem.

If your healthcare provider does not find any medical diagnosis to explain your symptoms, you may benefit from following some basic self-care recommendations. For syncope, it's advised to do your best to avoid things that may trigger the vasovagal reflex.

Tips for Controlling Triggers

There are some things you can do when you begin experiencing the signs of syncope. Try crossing your legs and tightening your hand, arm, and leg muscles. You can also slowly lower your head toward the floor to help stabilize your blood pressure.

If stress is a trigger, keep yourself as calm as possible. Although stress episodes can be scary, you can use calming self-talk and mind-over-body practices to help get yourself through a stressful period and avoid panic. Remind yourself, too, that your symptoms will pass.

You can also find reassurance in the fact that your healthcare provider has checked you and ruled out any more serious causes of your symptoms.

When to See a Healthcare Practitioner

Fainting usually isn't an emergency. However, if you faint frequently, have a family history of syncope, faint during exercise, or have heart palpitations or chest pain when you faint, you should get checked out by a healthcare practitioner.

Summary

The vasovagal reflex is a common cause of fainting. The vagus nerve is connected to the heart and blood vessels. If it is triggered, it can lower your blood pressure and heart rate and make you feel faint. Regardless of whether you lose consciousness or not, it is important to talk to your healthcare provider about your vasovagal symptoms and what may trigger your episodes. In most cases, the response is normal, but they may recommend tests to see if there is a medical reason behind them.

7 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  2. Alboni P, Alboni M. Vasovagal syncope as a manifestation of an evolutionary selected trait. Journal of Atrial Fibrillation. 2014 Aug 31;7(2):1035. doi:10.4022/jafib.1035

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Barbara Bolen, PhD

By Barbara Bolen, PhD
Barbara Bolen, PhD, is a licensed clinical psychologist and health coach. She has written multiple books focused on living with irritable bowel syndrome.