If you remember, in order to determine if our initial tidal volume (VT) and respiratory rate (RR) settings provide adequate ventilation, we analyze the arterial carbon dioxide levels (PaCO2) from the arterial blood gas (ABG). Here, we’ll determine whether or not the initial settings are sufficient to provide adequate oxygenation. And, in order to monitor oxygenation, we will analyze the arterial oxygen levels, or, PaO2.
Which ventilator settings affect oxygenation and PaO2?
Before moving forward, we must determine which ventilator settings affect the PaO2. We already know that the VT and RR control ventilation, that is the PaCO2. But, to control oxygenation—and ensure the adequate delivery of oxygen—we will need to adjust either the positive end-expiratory pressure (PEEP), or the fraction of inspired oxygen (FIO2), or both.
So now, let’s take our hypothetical ventilated patient with an initial PEEP of 5 cmH2O and an initial FIO2 of 100% to see if we are providing adequate oxygenation. The ABG reveals that the PaO2 is 400 mmHg. Well, for a normal lung, the acceptable range for PaO2 is about 80–100 mmHg, so clearly this PaO2 is too high, and therefore we’ll need to reduce the amount of FIO2.
So, suppose we reduced the FIO2 from 100% to 60%. Then we order another ABG and the results reveal a PaO2 of 40 mmHg. Knowing that an acceptable normal range for PaO2 is 80–100 mmHg, what should we do? Well, this is a little tricky because at first, you may consider raising the FIO2— and that certainly is an option—but you’re already using a pretty high amount of FIO2, and it seems to be fairly ineffective in normalizing the PaO2. So, instead, you could increase the PEEP.
In this scenario, the new ABG returns a PaO2 of 90 mmHg, which falls well within the acceptable PaO2 range, indicating we should maintain the current PEEP and FIO2 settings.
Okay, so now we know that after placing a patient on acceptable initial settings, we should analyze the ABG and adjust to achieve optimal ventilation and oxygenation. And we know that VT and RR are used to correct ventilation, while PEEP and FIO2 are primarily used to correct oxygenation.
But is normalizing oxygenation and ventilation the most important goal? That depends.
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- Mechanical ventilation protocol summary. NIH-NHLBI ARDS Clinical Network. http://www.ardsnet.org