This Is the Real Reason Why Seeing Blood Makes You Faint

Giving blood, getting vaccinated, or a stuffy, crowded train car—for some people, these brief moments of everyday discomfort can result in a loss of consciousness.

This dramatic reaction is known as vasovagal syncope or neurocardiogenic syncope. (Syncope, by the way, is a fancy term for “fainted.”)

And, says Larry A. Chinitz, MD, a cardiologist and the director of the NYU Langone Heart Rhythm Center, this syndrome is fairly common.

While fainting in response to unpleasant conditions can be deeply inconvenient and downright dangerous, vasovagal syncope is not itself a problematic medical condition. If you—or someone you’re close with—has experienced this syndrome, here’s what you need to know.

Why do people faint in unpleasant situations?

Our bodies react in response to certain stimuli by slowing down, says Dr. Chinitz. It’s known as the Bezold-Jarisch reflex, after the two doctors who discovered the phenomenon—and we all have this reflex, fainters and non-fainters alike.

Here’s how it plays out.

First, there’s a triggering event—common ones are giving (or seeing) blood, getting overheated, standing for a very long time (like at a crowded concert), or fearing bodily injury, according to the Mayo Clinic. That’s just a shortlist of causes—Dr. Chinitz adds that pain, stomach upset, or overeating can also be a trigger.

Blood can make you faint

Next up, the body responds to the discomfort by slowing down. Heart rate and blood pressure both drop. Again, we all have this reflexive response, but for some people, the response is exuberant, says Dr. Chinitz. “The blood pressure and/or heart rate lower so much that the blood pressure falls dramatically and patients pass out,” says Dr. Chinitz.

Vasovagal syncope is typically harmless—but you should still check in with your doctor

There’s no known reason why some people experience this reflex in such an extreme way while others don’t, says Dr. Chinitz.

But while the experience of vasovagal syncope is not pleasant, it’s not a harbinger of a serious illness or condition. “It’s a totally benign syndrome. It’s not associated with any other heart problems,“ says Dr. Chinitz.

Still, while fainting can be caused by something harmless, such as vasovagal syncope, it could also be a symptom of something potentially harmful, notes Dr. Chinitz. “We take passing out very seriously,” he says.

If you faint, especially if it occurs repeatedly, let your doctor know. A thorough history—with questions on the circumstances leading up to the event—helps doctors determine the cause. Your doctor may also do some tests, such as an electrocardiogram (EKG) to check if you have an irregular heart rate and blood tests to check for anemia, among other more serious potential causes of fainting.

Fainting is more than just inconvenient

Responding to minor events with an extreme slowdown is not exactly a practical, helpful reaction on the part of your body. Pass out in the wrong place, or at the wrong time, and you can seriously hurt yourself, says Dr. Chinitz.

Just think of the injuries that can occur if this response occurs on the stairs, for instance. Or, well-intentioned bystanders might call 911 after they see you faint, and the resulting ER visit could rack up thousands in unnecessary medical bills. Your fainting spell can be dangerous to others too—imagine the consequences if you pass out while driving.

Your best strategy is avoidance—stay away from triggering events. For instance, if you know you faint at the sight of blood, look away from the needle during your annual physical. Better yet: Tell the phlebotomist, so they’ll have you lie down during the blood draw. (Fainting rarely occurs when you’re lying down, says Dr. Chinitz.) Schedule your commute to avoid crowded train cars, and avoid that hot yoga class that leaves you light-headed.

Know the symptoms that precede fainting—feeling light-headed, tunnel vision, nausea, and sweaty or clammy skin. Once you feel those sensations, do your best to lie down promptly, says Dr. Chinitz. “When you’re standing up, you’re most vulnerable to that drop in blood pressure, which would lower [blood flow] to the brain and cause you to pass out,” he says.

If you lie down—and particularly if you put your feet up—you can potentially ward off the fainting episode, he says. Drinking lots of fluids to avoid dehydration is also recommended.

For the majority of people who experience vasovagal syncope, these common-sense tactics will do the trick. But a small percentage of people with vasovagal syncope will pass out very frequently. “There is a form of this that needs to be treated more aggressively,” Dr. Chinitz says. In those cases, he says, drugs or a pacemaker may be recommended.

That’s not very common however, and for most people with this syndrome, the two-part follow-up is far simpler: Avoid those triggers, and lie down when you sense a faint coming on.

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