Here's how to know if patient is a CO2 retainer

Question: How do you know how to tell someone is a PaCO2 retainer by looking at the blood gas?

My humble answer: It's easy. First, the CO2 will be chronically elevated. Usually, you'll find it in the 50s, but it can be as high as 80 on a good day. Second, look at the pH. If the pH is normal and the PaCO2 is elevated, you have a PaCO2 retainer.

Question: So, what if you have a patient who is laboring and has a pH of 7.20. Can you still tell if this patient is a retainer, when his PaCO2 is 80 or even 100?

My humble answer: Great question. Yes. What you do here is look at the BiCarb. If a patient is a CO2 retainer, his Bicarb will be chronically elevated. So, if you have a BiCarb greater than 30, chances are the patient is a retainer.

Question: Can you give a sample blood gas?

My humble answer: Sure. Consider ph 7.38, PaCO2 50, Po2 50, HCO3 35. That's a blood gas of a Co2 retainer on a good day. Note the CO2 is elevated and the HCO3 is elevated to compensate to keep the pH normal. This is a normal homeostatic procedure. So, if you had a patient in respiratory acidosis, his gases might look like this on room air: pH 7.20, PaCO2 80, Po2 36, HCO3 38.

Question: So what if the doctor sees that low Pao2 and high PaCO2 and thinks you got venous blood. How can you prove to him it was not venous?

My humble answer: Sometimes you can't. However, you know if you got the artery by how well it filled the syringe. Still, a doctor might not believe you. You can just tell him that the HCO3 is high, so chances are the PaCO2 is chronically elevated. If the patient is in acidosis, chances are the PaCO2 is elevated even more. It's worth trying anyway.  Another way to tell if this is arterial or venous is to place the patient on 100% non-rebreather. If the oxygen goes up, you know you had arterial. This is the best approach either way, considering a PaO2 that low is life threatening, and requires oxygen anyway.

Edited on July 5, 2016