Medical Why do people faint when getting blood drawn?

  1. russ_watters

    Staff: Mentor

    So today, I got some blood drawn. A few minutes afterwards, while back sitting in the waiting room, I started to pass out. Over a period of what seemed like perhaps 5 minutes, my vision slowly greyed-out and I started to sweat. I called to a nurse and she helped me into a reclining chair, which made those symptoms go away immediately (I didn't actually pass out - I just got close). She measured my blood pressure at 105/68, when just a few days ago it was 130/75. Pulse was 52, which is about normal for me.

    So why did this happen? It has never happened before*. I looked at the wiki for it ( ). The cause of the fainting itself is low blood pressure due to dilated blood vessels, which causes a decrease in blood flow to the brain. Pretty obvious.

    But why? What causes it that? The wiki mentions a number of causes, and the nurses focused on the typical one we hear: fear of needles/blood/hospitals causing a nervous reaction. But that doesn't seem likely to me. I'm not squeamish about having blood drawn. Heck, I draw my own blood all the time when doing home improvement. And if I have a splinter or ingrown hair, I think nothing of taking a needle and digging it out. When I'm at the doctor's I'm completely calm - I never have elevated heart rate or blood pressure that would imply nervousness.

    Now the wiki mentions vasodilation and here's my real question: can vasodilation be completely reflexive? Here's why I ask: the pain when having blood drawn is a little unusual. It isn't like a bee sting or pin prick, which is sharp and localized. It is more like the feeling when you hit your "funny bone", kind of a numb/achey pain over a much larger area (in this case, perhaps a diameter of about 6 inches around the prick). Could this be caused by a direct nerve injury like bumping your "funny bone" that basically short-circuits that part of the nervous system?

    A similar example to what I mean would be the nerves running down the sides of your neck and along the top of your shoulders. There is a hand-to-hand combat technique I learned in the Navy where you stand behind someone and basically karate chop both sides at once. They feel something akin to an electric shock in their entire body and go limp for a split second - just enough time to yank them down to the ground from behind. (We tried this on each other - it was actually kind of fun!). Another example would be the knee-bump relfex test.

    *I did have one incident in high school when I gave blood. I didn't feel any symptoms, but the nurses told me I was white and had me lie down. I attributed that to having just given a pint of blood while being malnourished (It was wrestling season - I wasn't eating well and had almost no body fat).
  2. jcsd
  3. turbo

    turbo 7,063
    Gold Member

    I don't know, Russ, but I was in a program during college in which if you gave 12 units of blood, your immediate family would get blood free from that blood-bank if it was needed. I had given blood at least 6-8 times and pretty much jumped up and headed back to classes, etc, and then one time I stood up and my legs buckled and I hit my head on a filing cabinet. They kept me for 15-20 minutes and fed me cookies, juice, etc.
  4. does it have anything to do with your blood sugar level lowering?
  5. russ_watters

    Staff: Mentor

    Dunno. I forgot to mention the diet thing, though, so thanks for reminding me. I haven't been eating well lately due to the problem I was being checked-out for: I have a hernia, which hurts if I eat until full, so I haven't been eating full dinners, but instead doing just a little snacking thrroughout the evening. I had a breakfast cereal/yogurt/fruit bar (150 cal) and some water for breakfast.

    They always give orange juice in these situations, which is a blood sugar thing.
  6. mgb_phys

    mgb_phys 8,809
    Science Advisor
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    Was it just a blood sample? If they only took a few cc it is likely a psychological reaction. Your brain thinks you are bleeding so it does all the stuff it would do for a major injury, shutting down blood supply to extremeties, dropping blood pressure etc, it just overdoes it.
    If they took a blood donor unit then you do need to rest for a while to build up the blood volume/pressure again.
    I'm the same - I almost faint at the pin-prick test but have no problem giving blood.
  7. russ_watters

    Staff: Mentor

    Yes, it was just a sample. Why would there be such a reaction to such a tiny amount of blood?
  8. mgb_phys

    mgb_phys 8,809
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    Your brain is in charge of making the chemicals that cause your reaction - soemtimes it gets it wrong.
  9. Lisa!

    Lisa! 882
    Gold Member

    Interesting! It happened to me as well! The nurse(and also my mom) told me not to look while he's taking the sample but I didn't care and looked, so I was about to faint. So they gave me chocolate!
  10. vanesch

    vanesch 6,189
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    For the sample taking, did you have to go there without having breakfast ?
    That usually does it for me: I feel a bit light in the head each time I have to give a blood sample, simply because that's usually around 9 AM or so, while I usually get up around 5:30 AM, so that's a long wait without breakfast. Going to the nurse and so on keeps you busy, but when I'm in the waiting room I feel light in the head - not really close to passing out, but already half way there.
    A cup of coffee with sugar (after the sample) usually gets me up again.
    I guess it is a matter of low sugar levels...
  11. I had that happen once, the most recent time. When I was giving a sample for testing to enter college. I really don't think it's low sugar, at least not for me. But I was in really bad shape and seemed like it was just how the needle went in. I've had lots of injections, shots, withdrawals over the years, and this one time was my only bad experience. Just as you described, but it really, really made me feel awful. Nurse threw a cold rag on my head and helped me onto the bed. I never want to go through that again.
  12. DaveC426913

    DaveC426913 16,537
    Gold Member

    How do you feel about needles? Do they hurt? I don't have a problem with needles but I do tend to anticipate the pain when they're preparing to stab me. The anticipation can tense me up and raise my heart rate - all unconsiously mind you - I don't really realize I'm tense.

    Now, when I have stressful situations, for example if I get into an unanticipated argument, I get the same tensing and heart rate. If that last more than a minute, I occasionally have a sugar crash. My body was ticking along at just adequate sugar levels when I made a 'fight or flight' demand of it. Suddenly I need to eat something. If I were standing, I'd need to sit down.

    So the upshot is that the onset of a stressor can bring about a rapid sugar crash causing weakness.
  13. maybe your body notices the loss of blood and assumes there is a leak somewhere. reducing blood pressure could be a way to slow down blood loss. it also forces you to lay down which reduces your heart rate which would also reduce blood loss.
  14. Andy Resnick

    Andy Resnick 6,143
    Science Advisor
    Education Advisor

    There shouldn't have been. The change in blood pressure you report is really large- even though both numbers are in the normal range, the change is indicative of transient hypotension. Anything unusual happen that morning or previous evening?
  15. berkeman

    Staff: Mentor

    Psychogenic shock is caused by a sympathetic reaction to some stimuli -- it can be anything from something that scares you, to the unusual pain stimuli that Russ mentioned. The body dilates the core blood vessels, which causes the drop in blood pressure, resulting in hypoperfusion to the brain (lightheadedness and possibly loss of consciousness) and the associated shocky symptoms. Usually the standard treatment for shock helps the symptoms to go away pretty quickly (lie down, legs elevated, keep warm).

  16. I get blood work done every three months, I want to cry when I do. I'm really sensitive to needles. They say it might be an allergic reaction. The nurses always make sure I don't faint, I always feel like I might but never have. If you don't feel pain though you're probably just psychosomatic, schock several minutes latter to something you didn't even feel doesn't really make much sense................................Usually younger girls do the fainting???????????
  17. Moonbear

    Moonbear 11,955
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    First Hurkyl, now Russ.

    Though, in this case, Russ is mentioning a very sharp pain comparing it to hitting your funny bone, which sounds like they stabbed a nerve in the process. The nerves running down your arm to your forearm and hand pass beneath the vein they use to draw blood. In most people, they are deep enough that they aren't going to be hit when drawing blood except by the most inexperienced/incompetent of nurses/phlebotomists who is jabbing around really deep. However, there is a good deal of anatomical variation in this region of the body, with plenty of people having extra nerve branches, nerves that split higher or lower in the arm, or ones that run more superficially than the textbook tells you it should (some people also have an artery that runs superficially too...I think it's somewhere between 5 and 10% of the population IIRC...and that can really make a mess if someone tries to get blood from an artery instead of vein).

    That sort of vasovagal response to having a nerve directly injured with a needle stick certainly would be plausible, with your body responding as if you've just had a major trauma to the arm.

    Beyond that, the mechanisms for such reactions are not well understood. I have learned since the time of Hurkyl's thread that when people have a persistent problem with vasovagal syncope (frequently fainting for no particular reason) without any other underlying medical cause, it can be treated with beta blockers. Triggers can be hard to identify. Since my added knowledge has been through meeting someone with this condition, we figured out that in her case, it was simply her hands getting too warm of all the odd things. If she put on gloves and left them on too long, she'd pass out. If she changed them often (so not an allergy to the gloves) so her hands could cool off in between, she'd be okay.

    Of course, if you're being evaluated for a condition that is preventing you from eating well, it could have simply been coincidence that the episode occurred after a blood draw (it's usually a pretty quick response during or immediately after the blood draw, not after you've already walked back to the waiting room). The vagus nerve innervates, among other things, the stomach and intestines.

    And, as to your other question, reflexive isn't the correct term to use for vasodilation in this case, but autonomic's one of those body functions that occurs without voluntary control. You can't tell your blood vessels to dilate or contract any more than you can tell your heart rate to increase or decrease or your intestines to slow down or speed up motility. This is distinguished from somatic nerve functions such as flexing a muscle, which is done under voluntary control.
  18. russ_watters

    Staff: Mentor

    It's never been diagnosed, but my dad may have vasovagal syncope (it isn't frequent enough to be a real bother). It happens to him at the doctor's office when giving blood, but also occasionally if he just stands up too quickly from a resting position. Happened to him once on the subway when he was in his 20s. So this sort of thing apparently runs in my family.

    Thanks for the info, guys.
  19. A long time ago I also almost fainted when my blood was drawn. They could not draw blood from the vein they intended to use and took the vein next to it (they used the largest vein you can see if you look at the opposite side of the ellbow). I think that caused the problem. I guess my blood pressure had been lower than normal (I had been ill for a while and was feeling very weak). Perhaps that was why the attempt to draw blood from the usual vein did not work.
  20. It could be because of the drop in blood sugar or the blood pressure. Or the mere fact that you could have lost alot of blood in a short amount of time. I have fainted because my blood sugar got too low. But I have never actually fainted after gettin blood drawn, though I was close a few times. I think alot of the times, it's mind over matter.
  21. Moonbear

    Moonbear 11,955
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    In that sort of situation, it could have been pure coincidence. If you were already weak, possibly dehydrated or with low blood pressure (from your illness), you may have been about to pass out with or without the blood draw.
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