9.6 Which change(s) would be most correct for my patient?

For oxygenation and ventilation issues, you usually have two different settings that could fix whatever problem you have identified in the ABG. Usually, when making changes on the ventilator, a clinician will choose to change just one of the two options and then recheck the ABG before making additional changes. So, here is the most important question to ask yourself: which setting is the most appropriate to change?

Are both settings an option? Rarely, either setting could be chosen, but in most cases, changing one setting makes more sense than adjusting the other. Through this book, you have learned all of these situations and it is time to put your knowledge together to learn how to make informed decisions for your patient.

Ventilation issues: [latex]RR[/latex] or [latex]V_T[/latex]

If the pH is abnormal and you are going to try to normalize it by increasing or decreasing the pCO₂ level, you know [latex]V_T[/latex] and [latex]RR[/latex] are both options to impact the amount of CO₂ being exhaled every breath—but which change is most correct?

The answer depends on where you are currently with your ventilator settings. Remember the safe ranges for [latex]RR[/latex]? Medical providers with basic ventilator knowledge are encouraged to start with an [latex]RR[/latex] between [latex]\text{14 - 18 bpm}[/latex], with a caution to not go above an [latex]RR[/latex] of [latex]24\text{ bpm}[/latex] without guidance from physicians or RRTs due to the impact on adequate time to exhale and the danger of causing patient asynchrony. If the [latex]RR[/latex] is above [latex]14\text{ bpm}[/latex] and below [latex]18\text{ bpm}[/latex], you have flexibility to go up or down safely within the range of [latex]\text{10 - 24 bpm}[/latex] without causing issues.

Now think about your tidal volume ranges. This parameter is much more definitive with its allowances. The safe tidal volume ranges for medical providers with basic ventilator knowledge is [latex]\text{6 - 8 mL/Kg}[/latex]. Remember, we do not go above [latex]8\text{ mL/Kg}[/latex]—that is an absolute maximum. Using the knowledge you have of ideal body weight (IBW) and calculated safe tidal volume ranges, you would compare the tidal volume your patient is getting to your calculated ranges. Do you have room to move to correct the problem? If you do, this would be an option to take. However, if you are at either limit, then changing the tidal volume would not be an option to correct the problem.

Oxygenation issues: FiO₂ or PEEP?

When it comes to changing either FiO₂ or PEEP, you need to think about the impacts of either of these values on the body. Remember, high levels of oxygen can cause lung damage, and we are always targeting the lowest FiO₂ to maintain [latex]\text{SpO}_2>92\%[/latex] and a pO₂ at normal ranges [latex]\text{(80 - 100 mmHg)}[/latex]. FiO₂s of higher than [latex]0.5[/latex] can lead to oxygen damage to the lungs (See Chapter 2).

For PEEP, there is a hard minimum of [latex]5\text{ cmH}_2\text{O}[/latex]—you will never decrease the PEEP below that number. Conversely, increasing PEEP too high can start to negatively impact the body. Remember Chapter 2? We talked about how high levels of PEEP can negatively impact the compliance of the lungs—just like balloons that are already inflated with pressure could lose elasticity and not be able to inflate as easily. High PEEPs will also increase the pressure in the alveoli that, when added to the additional volume or pressure applied with every breath, could increase your patient beyond safe pressure levels and put the lungs at risk of barotrauma. We need to maintain pressures of less than [latex]35\text{ cmH}_2\text{O}[/latex]—and, ideally, the lower, the better.

PEEP also increases the pressure in the thorax (chest) of the patient. Other than the lungs, this cavity also houses your heart and important vessels like the aorta and vena cava. Increased intrathoracic pressure will increase the pressure on these vessels as well. This pressure could squeeze the heart, decreasing the blood flow back into the heart and the pumping effectiveness of the heart. The medical terms for these situations are venous return and ejection fraction of the heart. Both conditions will show an impact in the blood pressure of the patient.

When selecting FiO₂ or PEEP as the setting to change, a good rule of thumb when treating hypoxia is to increase FiO₂ until you approach [latex]\text{FiO}_2\;0.50[/latex], and then slowly increase PEEP by [latex]\text{1 - 2 cmH}_2\text{O}[/latex] until you get to a moderate level [latex]\text{(10 - 12 cmH}_2\text{O)}[/latex] to facilitate oxygenation. If your patient is still hypoxic, continue to increase FiO₂. Remember to consult a physician or the most responsible health care professional if requiring higher PEEPs and [latex]\text{FiO}_2>0.50[/latex].

When weaning FiO₂, go in reverse. Wean FiO₂ until you are below [latex]0.5[/latex]; then, slowly decrease PEEP until you reach approximately [latex]8\text{ cmH}_2\text{O}[/latex] before weaning FiO₂ again.

At any time, if increasing PEEP causes a decrease in blood pressure below acceptable levels ([latex]20\%[/latex] below starting levels), return it to the previously documented amount and titrate FiO₂ to achieve SpO₂.

Let’s go back to our example patient…

Making sense so far? There are only two more question to work through…

“Which change(s) would be most correct for my patient?” 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.