We usually choose one value to change at a time and then test the ABGs and SpO₂ again to assess the changes. But, how much do we change each setting by to see changes? Many ventilation textbooks will suggest using formulas to calculate changes. In actual practice, these calculations can mostly be done by using simple logic. A good approach to stepwise changes is the following:

Let’s return once more to the example patient.

Patient | 7.31/57/68/24

Vent settings: RR 16bpm, V_T 420mL, PEEP 5, FiO₂ 0.5

Known information: IBW 52 kg. When calculated using the safe V_T range of 6-8 ml/kg equals a safe tidal volume range of 312-416 mls for this patient.

When answering the previous questions, you already decided that you needed to fix the pH by blowing off more CO₂. Though the two options would be increasing the RR or increasing the tidal volume (think of the analogy of the rising water and needing to bail either faster or with a bigger bucket), you determined you cannot increase the V_T since the patient is at the maximum tidal volume of 8 ml/kg. Therefore, the only change you can make is increasing the RR to decrease the pCO₂ and correct the pH. A practitioner would most likely choose to increase the RR +2 bpm and set it at 18 bpm.

In addition, you already know that your patient has mild hypoxia, and you decided that although you can increase either PEEP or FiO₂ to fix this issue, since we are already at an FiO₂ of 0.5, it might be worthwhile to just increase the PEEP as long as the SpO₂ is >92%. A practitioner would most likely increase the PEEP to 7 cmH₂0. If the SpO₂ is less than 92%, the FiO₂ might be increased to 0.6 for about 30 minutes and weaned as soon as the PEEP change starts to impact the patient.

You have just learned the typical amounts each setting is adjusted to impact a change on an ABG. Sometimes, ABGs will show mild imbalances, while other times, the issues are quite significant. If the numbers on the ABG are profoundly off, it might be worthwhile to do two steps of changes.

When completing two stages of changes, the same rules still apply—only some changes will be appropriate for your patient. What this approach looks like in practice is changing two settings (if able) or doing two-step changes to the same settings. For example, a significant issue would be if your pH is less than 7.3 or greater than 7.5 and your pO₂ is less than 55. In these cases, if the RR was the only change you could make, you might consider changing it to 20 bpm (16 +2 +2 =20 bpm), and you would definitely increase your FiO₂ +0.1+0.1 and consider your PEEP +2 as well. Remember, PEEP is slower to work, and the hope is you would bring the FiO₂ back down as soon as PEEP starts to work).

Weaning: Ventilation settings to progress towards extubation

What if your ABG is normal? In the world of mechanical ventilation, a normal ABG is considered weanable! You would consider decreasing your sedation and ventilator settings utilizing the same rules as above.

We treat a normal ABG the same as an over-vented ABG (removing too much CO₂) as long as the patient is vitally stable enough to decrease the support, allowing them to start doing more work on their own. If you notice your patient is triggering breaths above the set rate and vitally stable, this is also the time that you could consider changing from a control mode to a spontaneous mode—utilizing the initial settings described in Chapter 6. ABGs would still be used to assess the effectiveness of this change, and then PSV settings would be decreased down to minimal settings to work towards extubation.