ABG Assistance please

by CandisRenee CandisRenee (New) New

We had our test over ABG's and I totally blew it. I thought I understood the concept, but apparently I didn't. I'm not sure how to determine if a value represents a metabolic or respiratory process, such as alkalosis or acidosis.

Additionally, I'm not sure when to make a determination that compensation has occurred or not.

For example (taken from I site I am using to quiz myself):

pH: 7.51

HCO3: 30 mEq/L

PaCO2: 26mmHg

Ok, I get that the pH is alkalotic. And I get that the HCO3 is high, which would indicate and excessive amount of bicarb. Bicarb means alkalosis. Is this respiratory or metabolic and why is it the one it is? How do you know tell?

The PaCO2 is low, which means less CO2, which means less hydrogen ions, so alkalosis. But what does that mean for my overall picture?

And in my lectures and on my exam, we had to understand whether or not the PaO2 was high/low. Why does that not seem to be a factor in the websites and android phone apps that I have found dealing with this?

What will I see and have to understand in the real world?

Any help would be appreciated



Specializes in NICU. Has 16 years experience. 1 Article; 6,677 Posts

Rest assured that this is not the most straightforward or the most common ABG.

It sounds like you've come to the conclusion that two things are contributing to the alkalosis (CO2 - a measure of ventilation/respiratory status, and also HCO3 - an indicator of metabolic status). Is it possible to have an alkalosis or acidosis that stems from both respiratory and metabolic issues?

It's good to look at your pO2, particularly in a respiratory acidosis, because it helps to assess your patient's oxygenation status. However, those examples may omit it since pO2 isn't central to classifying a gas as respiratory or metabolic; acidosis or alkalosis; compensated, partially compensated or uncompensated. And remember that if you draw a venous or capillary gas rather than an ABG, you can ignore the pO2.



Specializes in ER. Has 5 years experience. 234 Posts

It is tough to get a GOOD picture with just one set of ABG values, in the real world. Trends are really what you need to understand and recognize.



Specializes in Cardiac, Rehab. Has 2 years experience. 306 Posts

Just remember the nemonic, ROME: respiratory opposite, metabolic equal. If Ph is going up and PCo2 is going down or Ph going down and PCo2 going up its respiratory in nature. On the flip side, if Ph and HCO3 are going in the same direction, its metabolic (renal). And there is also the issue of whether its compensated or not. If for example you have a case of metabolic acidosis, the body can try to balance itself by increased respiratory effort which "blows off CO2", which can drop the PaCO2 levels and start to bring the Ph back up closer to 7.4. So you can have either Uncompensated, partially compensated or fully compensated values. But get the basics first before you worry about compensation.



Specializes in New Critical care NP, Critical care, Med-surg, LTC. Has 10 years experience. 4 Articles; 2,214 Posts

I didn't find the whole arrows thing helpful for me, I need a picture. The best way I found was to draw out three lines- one for pH, one for CO2 value and one for bicarb. Then, label them with the normal values (7.35-7.45, 45-35 and 22-26), the one thing to remember is that they increase left to right except CO2. Then, you draw an X with your values. There will be two on the same side, the pH and either the CO2 or HCO3. That tells you whether you're resp or metabolic. Now, compensation is easy with this system, because you just put the X on the value. If it falls within the normal ranges, it's uncompensated, because it hasn't started doing anything to pull the pH back to normal. If it's outside the normal range and the pH is still outside the normal range, it's partially compensated. If you're fully compensated, the pH will be back within the normal limits, but still to either the acidotic or alkalotic side. I can't go wrong with the picture.