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What is vasovagal syncope?
Vasovagal syncope occurs when your heart rate and blood pressure suddenly drop, causing you to faint. It’s the most common cause of fainting. It is rarely life-threatening. Vasovagal syncope lasts for less than a minute, and most people recover in 20–30 seconds.
Fainting occurs when your body overreacts to stressors happening to you (like severe pain) or around you (seeing blood). The trigger over-stimulates the vagus nerve, which controls many functions in the body—including heart rate and blood pressure. The stimulation temporarily reduces blood flow to the brain, which is what makes you faint. Blood flow returns to normal when you fall down or someone lays you down on the ground.
Below are some of the most common triggers of vasovagal syncope.
Emotions that most commonly lead to vasovagal syncope include stress, anxiety, and fear. Fear of blood, needles, and other medical tools (like a scalpel) are common triggers. Some people may faint when having blood drawn at a doctor’s office or if they donate blood. Other emotional causes include fear of heights, being exposed to a traumatic event, and fear of injury. Even feelings of disgust can be so overpowering that they can make you faint. For example, you may faint if you smell a particularly powerful and foul odor. In rare cases, laughter can trigger it.
Intense pain can overstimulate the vagus nerve and cause you to faint. The pain may be due to a medical condition, a very painful period, or a sudden trauma like being hit.
A severe, forceful cough can sometimes be so intense that it leads to vasovagal syncope. Sometimes called cough syncope, it is more common in middle-aged or older adults, particularly men who are overweight or very muscular. Intense coughing can increase pressure in the veins in the brain, briefly decreasing blood flow in the brain and leading to fainting.
Straining during a bowel movement
Intense straining when you have a bowel movement can make you faint. Called defecation syncope, it’s thought to be more common in older people, especially women. Older people may be more likely to experience straining during bowel movements because they are at increased risk of constipation.
Vasovagal syncope can sometimes occur when you urinate or immediately after you urinate. Men are more likely to be affected than women. It’s not clear why urinating could make you faint, but emptying a full bladder quickly may cause a sudden drop in blood pressure.
Hot temperatures can lead to vasovagal syncope, often in older people and those who are not used to the heat (for example, if you travel to an area that’s much hotter than where you live). Your blood vessels widen to release heat, which can temporarily limit blood flow to the brain, causing you to faint. It may be more likely if you exercise in the heat.
Moderate or severe dehydration may lead to fainting. When you’re not drinking enough fluid, the volume of blood in your body decreases. This causes your blood pressure to drop, potentially triggering vasovagal syncope. Dehydration can also be caused by diarrhea, taking medications like diuretics, and poorly controlled diabetes.
Fainting while exercising can sometimes be a sign of a more serious problem like an arrhythmia (irregular heartbeat) or hypertrophic cardiomyopathy (thickening of the heart muscle). If you faint during or after exercise, it's important to see a healthcare provider.
Standing after sitting, lying down, or bending
The gravitational pull that occurs when you stand up is powerful enough to cause blood to pool in your legs. When this happens, less blood goes back to the heart, which may cause a drop in blood pressure that can make you faint. Simply standing for a long period of time can also lead to vasovagal syncope (orthostatic vasovagal syncope).
Some people who faint when they stand up have a condition called orthostatic hypotension, a type of low blood pressure. Normally, the blood vessels in your legs get narrower (constrict) when you stand up, which encourages blood to flow to the heart and brain. This process is delayed in people with orthostatic hypotension and can lead to a quick drop in blood pressure that causes fainting.
The hormones released when you are pregnant can relax the blood vessels, lowering the amount of blood flow to the brain and causing fainting. While once considered harmless, vasovagal syncope during pregnancy—especially the first trimester—has been linked to certain pregnancy complications. Always tell your prenatal care provider if you faint.
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