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Do We Understand Oxygenation?



The Oxyhemoglobin Dissociation Curve can be confusing... However, it's an essential aspect to understanding oxygen physiology in critical care. Think of it as a "Manual for Dating". In a healthy relationship you have a good date, really like that person, but don't mind dropping them off after an evening out because you know you'll see them again. Hemoglobin picks up and drops off O2 when needed. (Normal Curve)

In another scenario, you pick up your date, but don't really like them too much, so you can't wait to get the date over. You drop them off fast! In a Right shift, hemoglobin has less of an affinity for O2. This means that the O2 can't attach and be stored before the hemoglobin dumps it off to the tissues. You have a resulting lower than normal SaO2, but excellent tissue oxygenation (PaO2). In this situation, think everything that's raised.

Raised acid

Raised CO2

Raised Temp

Raised 2-3 DPG 

Raised PO2

In contrast, when looking at the Left shift; you pick up your date, like them a lot, but you're insecure and don't want to drop them off. Jealousy!! Left shifts have higher affinity for O2 and won't let the O2 be dumped off to the tissues. This gives you a falsely high SaO2, and poor tissue oxygenation (PaO2). Remember, your SaO2 is a reflection of the amount of O2 bound to Hgb. If your Hgb is holding onto all the O2 stores and not releasing them, you'll have a highly saturated Hgb. But poor tissue oxygenation. Jealousy is BAD! In this situation, think everything Low.

Low acid

Low CO2

Low Temp

Low 2-3 DPG

Low PO2

My question is this! Should we rely on our SaO2? Does it really tell the story of oxygenation status? What's the best monitoring tool we have?

Article on massive transfusion of PRBC and affects on 2-3 DPG and oxygenation.

Pohlman. Damage control resuscitation. Blood Rev 2015;epublished January 17th

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