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.
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:
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:
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.
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:
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:
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:
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: