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: RR or VT

If the pH is abnormal and you are going to try to normalize it by increasing or decreasing the pCO₂ level, you know V_T and RR 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 RR? Medical providers with basic ventilator knowledge are encouraged to start with an RR between 14-18 bpm, with a caution to not go above an RR of 24 bpm without guidance from physicians or RRTs due to the impact on adequate time to exhale and the danger of causing patient asynchrony. If the RR is above 14 bpm and below 18 bpm, you have flexibility to go up or down safely within the range of 10-24 bpm 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 6-8 ml/kg. Remember, we do not go above 8 ml/kg—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 SpO₂ >92% and a pO₂ at normal ranges (80-100 mmHg). FiO₂s of higher than 0.5 can lead to oxygen damage to the lungs (See Chapter 2).

For PEEP, there is a hard minimum of 5 cmH₂0—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 35 cmH₂0—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 FiO₂ 0.50, and then slowly increase PEEP by 1-2 cmH₂0 until you get to a moderate level (10-12 cmH₂0) 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 FiO₂ >0.50.

When weaning FiO₂, go in reverse. Wean FiO₂ until you are below 0.5; then, slowly decrease PEEP until you reach approximately 8 cmH₂0 before weaning FiO₂ again.

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

Let’s go back to our example patient…

Patient | 7.31/57/68/24

Let’s say your patient is being ventilated in volume control. Their settings are: RR 16 bpm, V_T 420 ml, PEEP 5, FiO₂ 0.5. Their IBW is calculated at 52 kg.

What settings would be most correct to adjust to fix the issues of acidotic pH, high pCO₂ and low O₂?

Looking at their RR, you definitely have room to adjust it up as the patient is only at 16 bpm, and you can safely increase the RR up as high as 24 bpm with minimal concerns. This would be an appropriate change.

Looking at their V_T of 420 ml, compare it to the safe V_T range of 6-8 ml/kg of IBW. The safe tidal volume range for a 52kg person is 312 ml –  416 ml. You are already at the high range for this patient, and are ventilating at 8 ml/kg. In this case, you cannot increase the V_T to help blow off the CO₂ and correct the ABG. This would be an inappropriate change.

What about the low pO₂? The patient is getting a PEEP 5 and FiO₂ 0.5. PEEP is at the minimum, and we definitely have the ability to increase that number to assist. Before you decide to take this action, it’s a good idea to check the FiO₂.

What about the FiO2? Though we can increase the FiO₂ up to 1.0, we know that oxygen damage can occur at levels higher than 0.50. So, in this case, a change to PEEP would be a better option.

A word of caution here: We know that severe hypoxia needs to be fixed. All of our cells need oxygen to survive. FiO₂ has a rapid effect within a few breaths, while PEEP takes approximately an hour to start to fully work. If there is moderate to severe hypoxia (pO₂ is less than 60 mmHg). Increasing the FiO₂ to get immediate results while increasing PEEP, and then dialing back the FiO₂ when SpO₂ recovers would be the most correct. In this case, the hypoxia is only mild (60-79 mmHg), so increasing the PEEP only might be enough as long as the patient maintains SpO₂ >92%. If needed, you can also make a small increase to FiO₂ while the PEEP increase is starting to work, and then wean the FiO₂ back down when the SpO₂ improves enough.

Making sense so far? There is only one more question to work through…

If you’re struggling to keep up, don’t forget that there is guidance and extra support available for students of the Basic Principles of Mechanical Ventilation micro-credential offered online by Sault College.