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:

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 RR and the V_T 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 RR or V_T in order to adjust CO₂ is to think about a 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 RR or V_T. Conversely, if the CO₂ level is too low, the reverse is true: you can decrease RR or set a smaller V_T.

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 0.21) 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 RR, 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 last question…