Write Better Bleeding 3/?: All About Transfusions
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This was quite a hot topic on the blog a little while back, and I’ve been feeling a little remiss in that I didn’t make a masterpost about this a while ago. So let’s continue the awesome Write Better Bleeding series!
Part 1 focused on the actual effects of blood loss, and what the dying process looks like due to hemorrhagic shock. Part 2 looked at various ways of stopping blood loss, from simple bandages to tourniquets and vascular surgery.
But today on Part 3 we’re going to look at when someone might need a blood transfusion, how doctors replace blood loss, how quickly the body does it, and what some of the risks are. But first we need to understand a couple of basic lab values. In particular there are 3 we’ll talk about: RBC count, Hemoglobin, and Hematorcit. All of these are part of a package blood value order called a CBC, or Complete Blood Count. While there are other components to the CBC, including platelets and white blood cells, we’re going to look at these values in particular.
The numbers are easy to remember in that they’re triples of each other: 5, 15, and 45. It’s worth noting that males and females have slightly different values, with women tending towards slightly lower numbers than men, but humans typically fall within specific ranges.
Blood Values: Red Blood Cells
This is an actual count of the number of red blood cells in an mL of blood; measured as “ # x 10^6″, but commonly just expressed as a number. A typical RBC value would be 4.5 - 5.5 in males, and 4.0 - 4.9 in females. (Mnemonic: 5)
There is no critical value for RBC count, but it’s definitely something to keep an eye on when interpreting blood results.
Blood Values: Hemoglobin (Hgb)
Hemoglobin is a measurement of the actual oxygen-carrying component of blood, and is measured as grams/deciliter. While the typical value is 13-16, the number to remember is 15. But when the Hgb drops to 7, that’s a critically low value, to the point that something has to be done about it. This seems to be the safe threshold for initiating transfusion. (Mnemonic: 15)
Blood Values: Hematocrit (Hct)
Hematocrit is a percentage. Specifically it’s the percentage of formed elements (think cells) in the blood. (Whatever isn’t formed elements is plasma, which is mostly water but also contains proteins and other substances.)
Typically hematocrit is 35-45, and it should actually be about 3x the hemoglobin value. In massive blood loss that ratio should hold, at least in the beginning, because RBCs and plasma are lost at an equal level. But that can be affected by well-meaning providers who try to raise blood pressure by administering fluids, which adds water–which doesn’t carry oxygen, the most critical task blood has–and thus drops the ratio of RBCs to plasma. (Mnemonic: 45)
Also, hematocrit will fall in the few hours after a massive hemorrhage, because water and other elements of plasma will shift into the bloodstream from the interstitial fluid. So a person with a Hgb of 8–just above the transfusion threshold–should have a Hct of 24, but may have one under 20 because of fluid administration or fluid shifting.
Typical Healthy People Values: RBCs 4.8, Hgb 14.9, Hct 44
Typical values of someone who needs a transfusion: RBCs 2.4, Hgb 6.5, Hct 16.8
Okay, The Blood Values Are Crazy Low. They’ll Just Make More Blood, Right?
Yes and no! The body certainly ramps up production of red blood cells after blood loss, but making mature red blood cells takes more time than your character might have. For reference, donating a unit of blood–about 500mL–it takes about 56 days before a donor has made enough blood to donate again. So it takes quite a while to get new blood cells up and running.
One thing that has to be evaluated is whether the person is continuing to bleed, especially internally (where it’s very hard to control). In a trauma patient, this can be evaluated in a number of ways. One that’s worth mentioning is a FAST exam, which is a Focused Assessment with Sonography in Trauma. Basically, an ultrasound probe is used to look at various places in the abdomen and the chest (including the heart), looking for “free fluid” which is assumed to be blood.
Okay, So Billy Badbones is Dying. What Now?
Well, there is going to be a flurry of activity around Billy’s bed. Nurses will be starting lines (if the medics didn’t do it already), drawing labs, administering oxygen; doctors will be doing assessments, ordering things. But in the context of this article, what we really want to talk about is: Massive Transfusion Protocol.
Just read those words. Massive Transfusion Protocol. That’s some sexy words, right? That sounds doctory. That will go well in stories! And it will, and it does, but let’s talk about what that means.
A Massive Transfusion Protocol is a specific protocol for administering blood in severe hemorrhage patients. A doctor, like grizzled attending Rusty Krust, calls down to the transfusion lab and activates the MTP. That immediately gets a cooler of blood products on its way up to the ER. Specific protocols vary, but one that’s easy to remember is a 1:1:1 combination of red blood cells (RBCs), fresh-frozen plasma (FFP), and platelets; a typical pack contains 4 to 6 units of each type. They’ll be infused together, rapidly, into Billy.
But What About Cross-Matching? Isn’t Blood Typing a Thing™?
It sure is! However, in someone who is actively dying of exsanguination, the risk of transfusion reaction is less than the risk of them bleeding to death. So the first pack will actually be an unmatched transfusion. That is, the lab will send a pack of blood products–usually O+, for reasons we’ll delve into–and basically hope that Billy won’t have a transfusion reaction.
Now, yes, blood typing is definitely a thing. For the basics, there are 4 blood types: A, B, AB, and O; and these can all be matched with a Rhesus factor, which is either + or -. So a full blood type might be A+ (the most common type in the US, and Aunt Scripty’s type, if she ever shows up bleeding in your ER). Since Rh+ is autosomally dominant over Rh-, most people (about 70%) are Rh+.
I could explain compatibility, but honestly, this chart will do more than I can with just words:
(Note: The “Blood Type” in this chart actually looks at the genetics, which has 2 chromosomes; someone who is BO is, functionally, the same as someone who is BB, because O is autosomally recessive; O is a very rare blood. Those with AB have 1 A allele and one B allele. Too confusing? Don’t worry. It’s trivia.)
So let’s take me as an example. I’m A+. (I don’t know if I’m AA or AO, and it’s functionally irrelevant.) That means I can accept blood from another type A donor, or from a Type O donor. Meanwhile, I can donate blood to a sister type A, or a type AB person.
But that chart doesn’t deal with Rh factor. People who are Rh+ can accept Rh- or Rh+ blood, but someone who is Rh- can only accept blood from another Rh- donor. So I can accept blood from someone who is either Rh+ or Rh-, which opens my options up to O+, O-, A+, or A-. But I can only give blood to Rh+ patients: A+ or AB+. Bummer.
Now, there is an exception to this. It takes exposure to a blood type to develop antibodies to attack that blood type. So the first time I–or Billy–get a transfusion of, say, type B blood, I won’t have antibodies to it by default. So a single transfusion of one to a few units of Type B blood, given quickly enough, will be the dose that sensitizes the immune system, but won’t cause severe transfusion reactions.
The same effect is true of Rh factor. The majority of the population is Rh positive, which means the majority of available blood is Rh+. And remember Billy? Dying up there, while we dither about blood types? Yeah. He’s still dying.
So Billy can get a single transfusion (even of multiple units) of unmatched blood. Typically hospitals will reach for O+ blood for males and women over 50, and O- for women under 50. (This has to do with preventing mother-fetal Rh incompatibility should that female choose to bear children). {There could be a WHOLE OTHER SERIES of posts about sexism and gender binary biases in medicine, but that’s waaaayyyy outside the scope of this post.}
Okay, So What’s a Cross-Match?
Excellent question! A cross-match is a test that the transfusion lab will do where samples of Billy’s blood and a potential RBC unit are mixed and watched to see if a bad reaction occurs. A “Type and Cross”, or blood typing and cross-matching, screens Billy’s blood for antibodies to various blood types and actually puts units of blood on hold specifically for Billy. This basically tells the lab “Hey, this patient is going to need X amount of blood”. By contrast, a “Type and Screen” is just analyzing Billy’s blood for type and antibodies without comparing it to a potential donor unit.
Okay. So How’s Billy Doing?
Last we left Billy, he was getting a batch of O+ rushed into the resus bay. Now the blood is warmed, and hung on an IV pole, and infused rapidly through big IVs (or even a central line). He’s getting a 1:1:1 ratio of RBCs to FFP to platelets. But Billy is still bleeding. They’re scheduling him an OR, but in the meantime, he needs more blood!
But now the type-and-cross comes back… oh no! Billy is Type O-…. and he’s obviously gotten some kind of transfusion in the past, because of course he has, he’s Billy Goddamn Badbones. And all of a sudden he’s starting to not look well…
Next Time, on Write Better Bleeding…
We’re going to talk about transfusion reactions, or what happens when Billy Badbones gets blood and his body attacks it.
Thanks so much for reading!
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