9.5 What are the problems with the blood gas?
When you interpret ABG results, you determine whether the pH is balanced and if you have abnormal CO2, O2 and HCO3 levels. The most important thing to remember here is that, as the ultimate goal, you want to leave the body with a normal pH. We cannot cause problems by fixing CO2 levels when that action will shift the pH further away from normal. Sometimes we need to allow for abnormal CO2 levels as long as we maintain a correct pH.
Let’s look at an example ABG and identify the problems in the gas, if there are any. Then, we’ll connect these findings to what we can fix with mechanical ventilation:
Patient | 7.31/57/68/24
Remember, the ABG for this patient is expressed as: pH/pCO2/pO2/HCO3. Take a second and interpret this ABG. Remember, you need to comment on:
- the compensation,
- whether it is a respiratory or metabolic issue,
- whether it is acidosis or alkalosis, and
- the oxygenation status.
Apply Your Learning
How would you interpret this ABG? Do not keep reading until you have made a guess at the answer.
Solution
If you got the answer uncompensated respiratory acidosis with mild hypoxemia, you are right!
So, what are the problems in this gas?
- The pH is low and the body is in an acidosis state.
- The pCO2 is too high and causing the acidosis.
- The pO2 is low and the patient is hypoxic.
The bicarbonate is normal, so there is no problem here.
Remember, with mechanical ventilation, we can change the O2 and CO2 directly, and indirectly affect the pH by changing the CO2 levels. We cannot affect the HCO3; that change is done by the body over time.
This is a perfect ABG to affect with mechanical ventilation because we can improve the patient’s oxygenation, change how much CO2 is being exhaled over time and then normalize the pH by that change in CO2.
Which settings could I change to fix the identified issue?
The next step to ask yourself is what changes to which settings will improve the gas. You have just identified whatever issue is in the ABG, the next step is to ask yourself what possible changes to each setting could you make?
We have talked about how ventilation and oxygenation both have two distinct settings that can impact CO2 and O2 levels, respectively. Let’s review:
|
Settings that affect Oxygenation |
Settings that affect Ventilation |
|---|---|
| [latex]\text{FiO}_2[/latex] |
[latex]RR[/latex] |
| [latex]\text{PEEP}[/latex] | [latex]V_T[/latex] |
Based on the issues you have identified in the ABG, what direction would you want to move these settings?
Let’s look at the settings that affect ventilation first. We know the only change that we can do on a ventilator to impact the overall pH of the patient is their CO2 level. We have talked about the [latex]RR[/latex] and the [latex]V_T[/latex] as options to change the pCO2 level and therefore the pH of the patient.
Object Lesson
The best analogy to keep in mind to understand what changes you would make to your [latex]RR[/latex] or [latex]V_T[/latex] in order to adjust CO2 is to think about being in a boat with a hole in the bottom in the middle of a lake. The water would start rushing into the boat. You would need to start bailing the water out of the boat to decrease the water level and stop the boat from sinking. In this analogy, think of the lungs as the boat and the water level as the CO2 level in the body. How fast and to what degree you are bailing refers to the removal of CO2 with every exhalation.
If the water was rising in the boat, you can impact the level of water directly by two options:
- bailing faster, or
- bailing at the same speed but with a bigger bucket.
Let’s think back to the lungs here. If you increased your bailing speed, that is the same as breathing at a faster rate. You are exhaling more CO2 (removing more water) by breathing faster. Conversely, if you maintained the same breathing rate (speed of bailing), you could get rid of more CO2 by getting rid of more air per breath (a bigger bucket). This would refer to taking a bigger tidal volume with every breath.
Keep this analogy in mind when we start going through examples.
As you learned in the object lesson above, you can decrease the CO2 by increasing [latex]RR[/latex] or [latex]V_T[/latex]. Conversely, if the CO2 level is too low, the reverse is true: you can decrease [latex]RR[/latex] or set a smaller [latex]V_T[/latex].
Now let’s look at oxygenation. This one is pretty intuitive: If the patient’s oxygen level is too low, giving them a higher FiO2 of oxygen or a higher PEEP to push the oxygen across the alveolar-capillary membrane will improve their oxygen level. Conversely, if their oxygen level is too high, decreasing the oxygen level supplied (never below [latex]0.21[/latex]) and decreasing the PEEP can decrease their oxygen level in their blood.
Let’s continue to look at our example patient…
Patient | 7.31/57/68/24
We have identified the problems in the ABG as follows:
- pH is acidotic—this is the primary goal to fix
- pCO2 is too high—we will change this to fix the pH
- pO2 is too low
In this blood gas, we have a ventilation problem (high CO2 leading to an acidotic pH) as well as an oxygenation problem (low pO2). So, which changes could be made to fix the issues?
The body is not clearing enough CO2 with the patient’s current breathing. Think back to the analogy of bailing out the sinking boat. In this situation, the water level in the boat is too high. What can you do to your bailing? Increase the frequency of bailing, or get a bigger bucket. Therefore, our possible ventilation changes are as follows:
- you could increase the [latex]RR[/latex], or
- you could increase the tidal volume to clear more CO2.
Now, let’s look at the pO2 issue with this patient’s ABG results. The oxygen level is too low, and the patient has a mild hypoxia. You need to get more oxygen into the blood either by delivering a higher amount of oxygen or by pushing it harder across the alveolar-capillary membrane to get more into the blood per breath. Therefore, your possible oxygenation changes are:
- you could increase the FiO2 delivered, or
- you could increase the PEEP to help push the oxygen across the membrane and recruit other alveoli.
Now that you have identified all of your options, you are able to narrow your focus to the best choices for your specific patient. Time to move on to your next question…
“What are the problems with the blood gas?” and “Which settings could I change to fix the identified issue?” from Basic Principles of Mechanical Ventilation by Melody Bishop, © Sault College are licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.
