9.10: Putting it All Together- A Case Study — Read

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

\[\begin{eqnarray*}RR&=&20\text{ bpm}\\\text{PC}&=&16\text{ cmH}_2\text{O}\\I_T&=&1.0\text{ sec}\\\text{PEEP}&=&10\text{ cmH}_2\text{O}\\\text{FiO}_2&=&0.4\end{eqnarray*}\]

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 $Two overlapping speech bubbles, one in black and the other in gray, symbolizing conversation or dialogue.$ . Let’s work though this one together.

A healthcare worker in protective gear tending to a patient in a hospital setting, focused on medical equipment. — Photo by Official U.S. Navy Imagery, CC BY 4.0

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 $Black silhouette of a cat sitting gracefully with its tail curled around its body.$ and $Black silhouette of a cat sitting upright with a curled tail.$ 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, $A black silhouette of a cat sitting gracefully with its tail curled around its body.$ . 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 $Black silhouette of a cat sitting gracefully with its tail curled around its body.$ 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. $A stylized silhouette of an owl perched on a branch against a plain background.$ when all other settings remain the same.

So, to fix the ventilation issues (pH correction by adjusting pCO₂), you could decrease the $Black silhouette of a cat sitting gracefully with its tail curled around its body.$ or decrease the pressure control.

Now, let’s look at the oxygenation issues. Your patient has a pO₂ of $A simple black outline of a house with a triangular roof and a chimney.$ . Remember that normal is $A simple black silhouette of a light bulb on a transparent background.$ . 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:

\[\begin{eqnarray*}RR&=&20\text{ bpm}\\\text{PC}&=&16\text{ cmH}_2\text{O}\\I_T&=&1.0\text{ sec}\\\text{PEEP}&=&10\text{ cmH}_2\text{O}\\\text{FiO}_2&=&0.4\end{eqnarray*}\]

What information are you missing here to assess whether to change the $Black silhouette of a cat sitting gracefully with its tail curled around its body.$ 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 $A silhouette of a human head with a brain highlighted in blue, representing thoughts or mental processes.$ calculations. You know your safe $Black silhouette of a cat sitting upright with a curled tail.$ levels are as follows:

$Stylized image of a cat's face with large ears and expressive eyes, outlined in black.$ is $A simple, stylized illustration of a light bulb, representing ideas or creativity.$
$A black silhouette of a person with a suitcase, standing beside a building with a roof.$ is $A minimalist black silhouette of a cat sitting, with a curled tail and upright ears.$
$Stylized text of the letters "S" and "T" in black on a white background.$ is $A simple black silhouette of a bicycle icon.$

You look on your ventilator screen and see that your tidal volumes are around $A simple black silhouette of a cat sitting upright.$ . The patient is within the safe $Black silhouette of a cat sitting gracefully with its tail curled around its body.$ range of $A stylized letter "M" in a bold, abstract design, suitable for a logo or artistic representation.$ . Both $Black silhouette of a cat sitting gracefully with its tail curled around its body.$ and $Black silhouette of a cat sitting upright with a curled tail.$ 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 $Black silhouette of a cat sitting upright with a curled tail.$ from where it is currently (approximately $Stylized text of the letters "S" and "T" in black on a white background.$ ). To do this, you would decrease your pressure control.

\[0.5\]

There is one additional option here. If your patient was triggering breaths above the set $Black silhouette of a cat sitting gracefully with its tail curled around its body.$ 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 $A simple black silhouette of a camera facing right.$ 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 $Black silhouette of a cat sitting gracefully with its tail curled around its body.$ and $Black silhouette of a cat sitting upright with a curled tail.$ 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 $Black silhouette of a cat sitting upright with a curled tail.$ by decreasing PC, and
decrease PEEP.

Tidal Volume Decrease

Your patient is currently at approximately $Stylized text of the letters "S" and "T" in black on a white background.$ . You want to move down one step to target a tidal volume of $A black silhouette of a person with a suitcase, standing beside a building with a roof.$ . Try decreasing pressure control by $A simple black silhouette of a dog with a curved tail, facing left.$ and assess the tidal volumes your patient is getting. If needed, you can adjust the pressure control up or down to hit a $Black silhouette of a cat sitting upright with a curled tail.$ of $A black silhouette of a person with a suitcase, standing beside a building with a roof.$ .

PEEP Decrease

Most likely, a clinician would decrease the PEEP by $A simple black silhouette of a dog with a curved tail, facing left.$ 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.

“Putting it All Together: A Case Study” from Basic Principles of Mechanical Ventilation by Melody Bishop, © Sault College is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

This page titled 9.10: Putting it All Together- A Case Study was last modified on Fri, 16 Oct 2026 18:25:49 GMT and is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Yvonne Drasovean via source content that was edited to the style and standards of the LibreTexts platform.

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