ABG Analysis and the Compensatory System

Recently I received a question from a coworker of mine about the Acid-Base Buffering system and how it applies to ABG analysis.  Below is our discussion. Hope this helps others.

Hey there Sean, I'm review a lecture covering acid-base balance and have a question.  This lecture is discussing partially compensated acidosis and alkalosis [using the example of a respiratory acidosis with low pH and high PCO2]. I'm having some trouble understanding how how bicarb "offset[s]" the acidosis and make it partially compensated? Wouldn't the ABG still be uncompensated if the pH and PCO2 are still acidotic?

Here's My Response.

Good question. I'll try to explain in a way that makes sense.

Remember that in the body everything is in a constant state of flux and the body is constantly trying to maintain homeostasis.

In the Acid/Base system there are two balancing components: Acids and Bases. We measure the relationship between Acids and Bases by the pH scale 1-14 w/ 1 being acidic and 14 being basic. 7 is neutral.

Our bodies are always slightly basic at a pH between 7.35-7.45. Anytime someone's pH is outside this range they are in an uncompensated state, either acidotic or alkalotic.

Once you've identified that a patient is either acidotic or alkalotic, the next step in the process of figuring out why is to look at their CO2 and HCO3.

If the CO2 is outside it's normal range of 35-45 than the problem is related to the respiratory system.

If the HCO3 is outside it's normal range of 22-28, than the problem is metabolic.

When CO2 goes up, pH goes down and when CO2 goes down, pH goes up. Remember CO2 and pH are inversely related.

Bicarbonate (HCO3) is the buffer to acid. If the HCO3 is low than there isn't enough buffer in the system to balance the hydrogen so pH is low (acidotic).

Likewise, if the HCO3 is elevated, then there is more bicarbonate in the system that can bind H+ and the pH rises (alkalosis).

Remember, Bicarb and pH are directly related.

Now that I've covered the basics, lets look at your questions a bit closer.

It sounds like you're question is related to a respiratory acidotic ABG.

Lets say a pH of 7.24 and a CO2 of 85. If the HCO3 was still within it's normal limit of 22-28 than this would indicate the kidneys have not yet started to respond to the acidosis by producing more bicarbonate and the patient would be in an Uncompensated Respiratory Acidosis.

However, if the HCO3 is elevated, meaning the renal system has recognized the imbalance and is producing more bicarbonate to bind with the excess CO2, than this would be considered "partially compensated" meaning the body is responding but has not yet caught up.

Once the pH is brought towithin a normal range it would then be considered "compensated".

I hope this simplified explanation has benefited you.  If you like this content, or have questions, please leave a comment below.

Thanks and we'll be seeing you.