ABGs (Part II)

So, now that you’ve read all the basics about ABGs in the first post, here is a little more about interpreting lab values and how to determine compensated conditions. This is where it gets fun!

First off, let’s take a look at what NORMAL lab values probably look like:

  • pH: 7.35-7.45
  • Partial pressure of oxygen (PaO2): 75 to 100 mmHg
  • Partial pressure of carbon dioxide (PaCO2): 35-45 mmHg
  • Bicarbonate (HCO3-): 22-26 mEq/L
  • Oxygen saturation (O2 Sat): 94-100%

There is some wiggle room (like, VERY little) on some of these and most EMR systems are going to flag them if they are out of normal range for the given population. We discuss some of these in greater detail in this post.

Partial pressure of carbon dioxide becomes particularly important when you are working with patients who have chronic obstructive lung conditions. If you recall from this post, a PaCO2 greater than 45mmHg indicates that someone is likely a CO2 Retainer.

Next, let’s talk about compensated conditions. The body is smart and will know when someone is having difficulty with acidosis or alkalosis conditions of either the metabolic or respiratory variety. If one system isn’t working properly, it will recruit the other system to compensate and attempt to normalize. However, remember that some conditions can only temporarily compensate and will continue to pose serious health risks unless they are medically treated. When the opposite system gets pulled in, the lab values that represent this system will be out of normal range. Because the body has compensated, pH will return to normal. These will be the key factors in identifying compensation on ABGs.

Photo by Karolina Grabowska on Pexels.com

In metabolic conditions, the lungs have to compensate so the lung values (PaCO2) will be out of normal range but the metabolic (kidney) values will be normal or only slightly elevated (HCO3-). pH is normal.

In respiratory conditions, the kidneys have to compensate so the kidney values (HCO3-) will be out of normal range but the pulmonary values (PaCO2) will be normal or only slightly elevated. pH is normal.

If pH and either the lung or kidney value are outside of normal range (depending on the dx), the condition is only partially compensated. Anion gap comes in to play here, but that is outside the scope of this post. You can also use the PaCO2 to guess. In metabolic conditions, PaCO2 and pH will move in the same direction (i.e. if pH goes up, so will PaCO2). In respiratory conditions, pH and PaCO2 move in opposite directions (i.e. if pH goes down, PaCO2 will go up).

Similar to how we read and interpret EKGs, there is a fixed process by which you should evaluate ABGs. If you follow the steps properly, you should arrive at the right answer. Here’s what to do:

  1. pH:
    • Out of normal range —> decompensated condition
    • Normal range —> compensated condition
      (or no condition but all other values need to also be normal for this to be the case)
  2. If the condition is decompensated:
    • What is out of range?
      1. PaCO2 —> Respiratory condition
      2. HCO3- —> Metabolic condition
  3. If the condition is compensated:
    • What is out of range?
      1. if PaCO2, respiratory compensation —> Metabolic condition
      2. if HCO3-, metabolic compensation —> Respiratory condition
  4. Acidosis or Alkalosis?
    1. pH > 7.45 —> alkalosis
    2. pH <7.35 —> acidosis

Let’s do some application here and practice your new skills. I’ll give you a few sets of ABGs and we will talk through them. Remember the process and follow the steps and you will get your answer. Big deep breath…… here we go!

Practice 1:

  • pH 7.49
  • HCO3- 34
  • PaCO2 46
  • PaO2 81
  • O2 saturation 95%

So let’s take this step by step. Obviously, if we were getting a full ABG, we would have more values to sort through, but I’m attempting to keep this easy.

Step 1: is pH out of range?
YES, so our condition is decompensated.

Step 2: what else is out of range?
HCO3-, so we have a metabolic condition

Step 3: is pH high or low?
HIGH so we have metabolic alkalosis.

This one was nice and straightforward. It will not typically be this simple. Let’s try something a bit harder…

Practice 2:

  • pH 7.35
  • HCO3- 32
  • PaCO2 64
  • PaO2 86
  • O2 saturation 93% on 3L cont via NC

Step 1: is pH out of range?
NO. So if there is a condition present, it is compensated.

Step 2: What is out of range?
HCO3-, but because this is compensated, a metabolic value indicates a respiratory condition.

Step 3: is pH high or low?
Neither, because the body has compensated. So let’s look at other indicators that might answer this question for us.

Look at the PaCO2 and correlate clinically. This patient has a VERY high PaCO2 indicating that they are a CO2 retainer. They are on 3L of supplemental O2 and their O2 saturation still isn’t great. This person is likely in respiratory acidosis because they are holding on to way too much CO2 (in the form of carbonic acid) and can’t get enough O2 in to hit that sweet spot on the curve. This is where the anion gap comes in. The anion gap is the factor that covers all the other missing acids (either digested or produced) in the body. You would need the anion gap measure to confirm acidosis, but from clinical correlation, the odds are good.

Ready for one more?

Practice 3:

  • pH 7.31
  • HCO3- 18
  • PaCO2 32
  • PaO2 73
  • O2 saturation 98%

Step 1: Is the pH abnormal?
Yes, the pH is abnormal so something is going on here that is decompensated.

Step 2: What is out of range?
Here is where we run in to a problem. Everything is out of range! HCO3- and PaO2 are both out of range. So, what the heck does that tell us? That means that this condition is in the process of compensating. When you have both factors presenting somewhat abnormal but nothing is crazy out of range, this signifies the process of compensation and is the most likely presentation you will see on ABGs. That’s why these people need to come to the hospital, right? To get help form medical staff to compensate and correct.

Step 3: Is the pH high or low?
The pH is slightly low so we can consider some type of acidosis. Determining the type would, again, require the anion gap. However, because both the pH AND PaCO2 are low, it is likely metabolic acidosis.

Characteristics of acid-base disturbances

DisorderpHPrimary problemCompensation
Metabolic acidosis↓ in HCO3↓ in PaCO2
Metabolic alkalosis↑ in HCO3↑ in PaCO2
Respiratory acidosis↑ in PaCO2↑ in [HCO3-]
Respiratory alkalosis↓ in PaCO2↓ in [HCO3-]
Kaufman, D. (2020)

Here is a nice table that lays it all out for you! Things can get really crazy when patients have more than one of these conditions present at the same time. It is actually possible to have both an alkalotic and acidotic presentation simultaneously. This can happen with COPD, sepsis, heart failure, or when someone has renal failure with pneumonia (like patients who have COVID-19). In that case, compensation becomes very tricky.

I hope you found these three steps for basic ABG interpretation an easy way to figure out what your patient is experiencing! And I hope you check out the first section of this post on basic ABG interpretation to learn more about what YOU as the PT can do to help someone in a critical condition so they can get the help they need!

How often do you utilize ABG outcomes in your treatment? Let me know in the comments!

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Kaufman, D. (2020). Clinical Education: Interpretation of Arterial Blood Gases (ABGs). American Thoracic Society. Retrieved from http://www.thoracic.org/professionals/clinical-resources/critical-care/clinical-education/abgs.php

Lewis, J. (2020). Acid-Base Disorders. Merck Manual Professional Version. Retrieved from https://www.merckmanuals.com/professional/endocrine-and-metabolic-disorders/acid-base-regulation-and-disorders/acid-base-disorders

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