What Is the Vasovagal Reflex and What Causes It?

Vasovagal response is common, with one study noting that 42% of females and 32% of males will experience at least one episode before age 60. If you've ever felt like you're about to pass out while pooping, or became sweaty and dizzy at the sight of blood, you've experienced a vasovagal response. Common causes also include dehydration, standing too long, or intense pain.

The vagus nerve is the largest nerve in the body. It runs from the brainstem to the chest and abdomen, and regulates key functions like heart rate, blood pressure, breathing, and digestion. If stimulated, it can cause a sudden drop in heart rate, blood pressure, and other symptoms. As a result, you feel lightheaded, nauseous, and sweaty and may even faint (vasovagal syncope).

Symptoms

The word "vasovagal" describes two parts of your body involved in the response: "vaso" refers to your blood vessels and "vagal" refers to your vagus nerve. This is sometimes called a neurocardiogenic response.

If the vagus nerve is suddenly stimulated, the reflex response is automatic. Immediate messages are sent to the brain, affecting central and peripheral nervous systems and cardiovascular system.

Several body changes may occur, often as warning signs before you faint:

• 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

Causes

The vasovagal response 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 compare the response with that found in fish or other mammals.

The difference now is that some people are more susceptible to involuntarily triggering the reflex, with vasovagal syncope affecting about a third of the population. In some cases, there may be an underlying medical condition, such as a heart problem.

However, it is not uncommon to find no specific cause for the episode.

What Stimulates the Vasovagal Response?

Possible exposures and factors in a vasovagal response 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

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. Most people who do faint find the episode lasts only a minute or two, though you may feel shaky for a little while.

Rarely, a long episode may trigger a seizure. Some people only have one episode in a lifetime, while others may have several. 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 stimulate the vasovagal reflex, like standing up too quickly or becoming dehydrated.

Prevention

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.

Fainting occurs more often in older adults, who may be more at risk of fall injuries when a vasovagal response occurs. Be sure to check for injuries and see a healthcare provider, if needed.

Summary

The vasovagal response 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.