November 07, 2006


One of my classmates donated blood for the first time last week. I thought it was very brave of him, considering how he was with needles. We practiced giving shots on each other intramuscularly (IM) and subcutaneously (subQ) with our clinical skills preceptor. He was very nervous about being stuck with a needle for our little blood draw...

"I can draw blood from other people just fine; I'm actually really good at it!" he said. He certainly was adept at taking someones blood. He did have some squeamishness about giving his own, though. He had some traumatic experiences with a gadolinium shot once and he passed out.

So this past Friday, I was waiting in line to donate and someone who was rejected came out of the bloodbank mobile. "One of your classmates just fainted in there!" he told me.

I knew right away who it was.

Some people have a vasovagal response to needles and blood. Emotional stress can trigger the hypothalamus in the brain to reduce their sympathetic "fight or flight" responses. As a result, their blood pressure drops precipitously from sudden slowing of the heart rate and their arteries dilate. Less blood flows to the brain and the decrease in oxgyen causes them to feel dizzy and in my classmate's case, pass out.

As blood flowed painlessly out of the venipuncture in my arm, Tom remarked about his current physiologic state. "My blood pressure was 110/80 before and it dropped to 80/70 when I passed out... now its back up, but now I'm tachycardic with a heart rate of 120!" Normally, the heart rate is between 60-100 beats/minute. "Explain that, pingpong," he asked me.

Ah... reflex tachycardia was likely induced as a way of bringing the blood
pressure back after vasovagal-induced vasodilation. After all, the blood
pressure is dependent on three factors: the heart rate, stroke volume and the
total peripheral resistance. In response to a drop in resistance (caused by the
peripheral blood vessels suddenly opening up and causing the heart to pump less
blood back to the heart,) the heart rate increased to compensate for the

If I were quick on my feet, I would have said something coherent and impressive like that. At the time, I just a bit flustered, hoping that I wouldn't be a victim to syncope (pronouced seen-coh-pee) as well.

I have my own little personal theory about this, which I shared with him as he was recovering:

"Back in the day, if you were attacked with a sword, you would've fallen to the ground and appeared dead, only to rise later on in the battlefield. It's a great survival mechanism..." I said cheerily.

"So, you're telling me that my grandparents were cowards?!?" he said in a somewhat horrified, but joking way.

"NO! I think they are survivors!"

It didn't occur to me at the time that what I said could've been construed as offensive. In retrospect, I realized that it certainly wasn't the most comforting thing to say to a good guy. He's got a great way of approaching his problems, though. Even though he was "black-balled from donating blood for ten years," he wanted to learn more about what was happening to himself. He wanted to turn this negative experience into a positive one.

I like that about my classmates. They are really good at that. Even if it would be his very first and last time donating blood.

Reference: PubMed
Clin Sci (Lond). 1991 Nov;81(5):575-86. The Vasovagal response.

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