Putting it All Together: A Case Study

You receive your patient at the start of your shift. They are intubated and ventilated on Pressure Control Ventilation (PCV). Their ventilator settings are

• RR 20 bpm
• PC 16 cmH₂0
• I_T 1.0 sec
• PEEP 10 cmH₂0
• FiO₂ 0.4.

When giving you her report, the nurse tells you she just received the results from an ABG that she took 10 minutes ago, and the results are 7.47/38/115/30. Let’s work though this one together.

Question 1: What can be fixed on an ABG with mechanical ventilation?

Your primary acid base goal is to restore a normal pH. We can only affect the CO₂ levels with ventilation. The HCO₃ is done by the kidneys “behind the scenes.”

Question 2: Which ventilation setting affects which ABG value?

You already know FiO₂ and PEEP affect pO₂, and that RR and V_T affects CO₂ directly and pH indirectly.

Question 3: What are the problems with the ABG, if any?

Looking at the ABG, you can see that your patient has an abnormal pH, normal pCO₂ and abnormal HCO₃. The oxygen is also abnormal. You can’t do anything about the HCO₃ levels, but you can focus on normalizing the pH by altering the CO₂ levels and also correct the oxygen levels back to normal.

Question 4: What settings could you change to fix the problem?

Let’s look at the ventilation issues first. The pH is alkalotic due to elevated bicarbonate. You need to normalize the pH, and the only way to do so with a ventilator is to adjust how much CO₂ you are removing to move the pH from alkaline to normal. You don’t want more base; you need to add more acid. Think of the analogy of the water in the boat, and remember, CO₂ = acid. You need to let the CO₂ level (acid level) rise to move the pH back to normal. How would you let the water level rise in the boat? Bail slower or use a smaller bucket. So, you can decrease the RR or decrease the tidal volume on the ventilator.

Remember, though, you are currently ventilating in Pressure Control! How do you adjust the tidal volume when in Pressure Control? How does pressure relate to volume? If you are thinking you would decrease the pressure applied to result in a lower tidal volume, you are correct! Remember, that if you blow up a balloon with a lower pressure of air, it will be inflated to a lower volume. Pressure = Volume when all other settings remain the same.

So, to fix the ventilation issues (pH correction by adjusting pCO₂), you could decrease the RR or decrease the pressure control.

Now, let’s look at the oxygenation issues. Your patient has a pO₂ of 115 cmH₂0. Remember that normal is 80-100 mmHg. Your patient has too much oxygen in the blood. The two settings that affect oxygen levels are FiO₂ and PEEP, so you could decrease either one to decrease oxygen levels.

Question 5: Which change(s) would be most correct?

Let’s look at the options you identified above, and decide which change would be the most correct to make for the ABGs.

The ventilator settings are:

• RR 20 bpm
• PC 16 cmH₂0
• I_T 1.0 sec
• PEEP 10 cmH20
• FiO₂ 0.4

What information are you missing here to assess whether to change the RR or the PC? If you guessed the tidal volume and patient information, you are correct!

Let’s say you have already done your IBW and safe V_T (6-8 ml/kg) calculations. You know your safe V_T levels are as follows:

• 6 ml/kg is 420 mls,
• 7 ml/kg is 490 mls
• 8 ml/kg 560 mls

You look on your ventilator screen and see that your tidal volumes are around 550 ml. The patient is within the safe RR range of 10-24 bpm. Both RR and VT could be decreased safely here. But remember, you are only going to pick one change out of the two options.

When keeping in mind the dangers of VILI and the goals of decreasing barotrauma and volutrauma, the most correct change would be to decrease the V_T from where it is currently (approximately 8ml/kg). To do this, you would decrease your pressure control.

What about the high oxygen levels? With the current vent settings, the patient has a PEEP of 10 and an FiO₂ of 0.4. The patient is below the threshold of FiO₂ 0.5, but at the higher levels of PEEP. Although either setting could be adjusted here, because your patient is at the higher level of PEEP, and below the threshold of FiO₂ of 0.5, a clinician would most likely opt to decrease PEEP.

There is one additional option here. If your patient was triggering breaths above the set RR and vitally stable and not requiring large amounts of medication or infusions, they might be stable enough to attempt a change to PSV (spontaneous) instead of staying in PCV (control) and just decreasing settings. Remember, the ABG is over ventilated. If their disease process is resolving and they appear to have a drive to breath, consider decreasing sedation enough that they would be consistently triggering breaths, and consider attempting a change to PSV. Stay at the bedside for approximately 10 minutes and ensure the patient is breathing at minimum 14-20 bpm with tidal volumes that are appropriate for their ideal body weight. If they are not, change back to the previous control mode and make your changes there. Consider decreasing sedation a little bit and trying again later. If they appear to be breathing with a RR and V_T that are adequate for their needs, recheck an ABG in 30 minutes and confirm it is still normal or over vented—never let it go underventilated or allow CO₂ levels start to rise after a change like that.

Question 6: How much should I change my settings by?

The two changes that you have decided are the most correct to make are to

• decrease V_T by decreasing PC, and
• decrease PEEP.

Tidal volume decrease. Your patient is currently at approximately 8 ml/kg. You want to move down one step to target a tidal volume of 7 mL/kg. Try decreasing pressure control by 2 cmH₂0 and assess the tidal volumes your patient is getting. If needed, you can adjust the pressure control up or down to hit a V_T of 7mL/kg.

PEEP decrease. Most likely, a clinician would decrease the PEEP by 2 cmH₂0 at this point, and then potentially decrease FiO₂ as well, if SpO₂ or pO₂ remains higher than targeted ranges. PEEP changes are usually done only once a day because it takes so long for them to work (and then reassess). This is why FiO₂ adjustment will be your second move if needed—FiO₂ changes can be made frequently as needed.