Can blood transfusions increase an O2 sat from 80 to 87%?

YES! When the patient is on ECMO😆! (Or has a shunt…)

Buckle up for a hopefully understandable deep dive into some ECMO (/shunt) physiology with (⚠️warning) math and bad pictures.


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First, to address the elephant and 44% who correctly say this is generally impossible: Normally, you *cannot* increase O2 sats by adding Hg, bc sats refer to the saturation of Hg. Adding Hg doesn’t enable the lungs to saturate the new Hg any more than all the Hg it had before.
But unlike "real"/"native" lungs (let’s assume they are completely non-functional here), which all the blood must pass thru, when a pt is on ECMO (an artificial or “membrane” lung), some blood goes through the ECMO circuit, but some blood doesn’t, and that’s where the fun begins.
Let’s draw this, using a (hopefully forgivable) oversimplified model of a veno-venous (VV) ECMO circuit.
The 🫀 pumps purple (imperfectly oxygenated) blood to the body. The body extracts the O2 it needs, and now the blood is blue/venous. *Some* blue/venous blood diverts to ECMO and gets fully oxygenated/red (100%!), then mixes w/the non-diverted blue blood, creating our purple blood
Note: Basically, we're describing a R➡️L shunt. Some blood goes thru ECMO (i.e, "lungs") to get O2, and some skips, or shunts past, ECMO. This tweetorial will stick to ECMO, but you may see how similar rules will apply to a patient with a physiologic shunt. (h/t @AartikSarma)
Ok, let’s define parameters for our pt at Time 0, when (say) the pt is satting 87% (on the way to 80😬).
🩸Hg = 8
🩸Venous O2 sat = 60%
🫀 Cardiac output (CO) = 6L/min
🏭Flow thru ECMO = 4L/min... and the remaining 2L/min is thus *not* going thru ECMO

We've updated the image!
🤷‍♂️Why is the sat 87%? The patient is on ECMO! It should be 100%!

Because, as you can see in the model, even when ECMO fully saturates the blood going through it (red), it still mixes with the blood that does not (becoming purple). This drives down the overall arterial sat.
2 factors determine the arterial sat:

1⃣ The sat of the venous/blue blood (the blood coming from ECMO will be 100%)
2⃣ The relative amount of flow going through ECMO

The arterial sat is a weighted average of these values. Let’s plug it in with our patient's data to get 87%⤵️
Ok, moving on.

New concept: For this critically ill pt, we must know how much O2 they consume per minute, to ensure ECMO will supply the O2 needed. This is called VO2, and we'll say this patient’s VO2 = 250ccO2/min, which is what we might expect for the decompensating ICU pt.
Is ECMO providing a VO2 of 250?

To figure this out, we must know the amount or *blood content* (not sat!) of O2 entering (inlet O2) and leaving (outlet O2) ECMO, to know the net O2 supplied.

O2 content = 1.34*Hg*sat, ignoring the often small role of pO2

Let’s plug it in!⤵️
You see the problem?

Our pt needs 250ccO2/min and ECMO is only supplying 172! Where will the pt get this extra O2 from?

Our pt will keep extracting from poorly saturated available blood, driving the venous sat down even further. Our pt is using more O2 than we're giving😳⤵️
⭐️Per equation #1: this will also drive down the arterial sat down, because, on ECMO, overall sat depends on how low the venous sat is.

⭐️Per equation #2, this desat will continue until an equilibrium is reached when ECMO is fully supplying the deficit.

Let's walk thru this!
Our pt will continue extracting oxygen, and desatting, until the venous O2 gets as low as *42%*!! Why stop there?

Because at that point, the inlet O2 is much lower, i.e., ECMO has much more room to add O2, and it will supply the full 250 ccO2/min that we need. Take a look⤵️
Looking at the equation, note that when Hg is constant (foreshadowing!), ECMO’s ability to supply oxygen *content* is limited by the *sat* of the inlet O2.

The constant 1.34 does not change, the Hg does not change, so the *sat* is the only determinant of inlet O2 *content*
In words instead of equations, using an imperfect analogy:

Think of 'inlet blood flow' as a cup; 'inlet O2 sat' as the amount of water in the cup after a gulp; 'VO2' as the size of the gulp; and 'ECMO' as an enormous, infinite water pitcher refilling the cup after each gulp.
If the pt takes bigger gulps (VO2⬆️), the cup gets emptier. Sats drop.

But the ECMO water pitcher will still completely fill the cup after each gulp, supplying more water per cycle than before.

So sats ⬇️, but O2 supplied by ECMO ⬆️

(disclaimer: no, ECMO is not infinite)
So back to our desatting patient: When equilibrium is reached, what is our arterial sat going to be?

Well, you guessed it. Based on equation 1, it will be… *80%*. Just like the overnight team told you. Let’s see the math, and update our ECMO model⤵️
Ok! So now we have our pt, satting 80%, with a Hg of 8. The overnight team considers their options and decides to transfuse! A lot. To a Hg of 12. Without any other changes to the circuit, cardiac output, meds, etc. (suspend some disbelief, plz)

What were they thinking??
Well, now, ECMO has room to supply more O2! Our outlet O2 will be higher.

This means that ECMO can give us 250 ccO2/min *even if the inlet blood content was higher than before*.

Let me show you⤵️
See? With more Hg, ECMO has room to supply 250 ccO2/min even when the inlet blood venous sat increases from 42% to 62%!

Our cup got taller. ECMO fills the cup. Now, when the pt takes the same size gulp, more water remains in the cup after. Inlet O2 sat, and overall sat, rises.
Let’s finish this. When the venous sat is 62%, what’s our peripheral arterial sat? You guessed it. 87%!

The overnight team was telling the truth. Transfusing from 8 to 12 increased the pt sats from 80 to 87.

We've all been the overnight team - have some faith in them/us!
That’s a wrap! Thank you for reading, I hope it was easy to follow! This isn't theoretical - transfusion for desats on ECMO is a real option in some settings for this reason!

Would note a few disclaimers that are entire additional discussions...
Ok, at least 4 disclaimers:

❗️If the body is getting the O2 it needs from ECMO, it may not matter if the sat is 80%!

❗️ECMO cannot supply unlimited O2; limits depend on each device.
❗️While pO2 is often negligible, it may be up to 10% of O2 content in ECMO when it can reach the 500s!

❗️Adding blood will probably change the cardiac output.

Please feel free to add any I missed! /end
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