Modifying ventilator settings based on arterial oxygen (PaO2) levels

If you remember, in order to determine if our initial tidal volume (V_T) and respiratory rate (RR) settings provide adequate ventilation, we analyze the arterial carbon dioxide levels (P_aCO₂) 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, P_aO₂.

Before moving forward, we must determine which ventilator settings affect the P_aO₂. We already know that the V_T and RR control ventilation, that is the P_aCO₂. 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 (F_IO₂), or both.

So now, let’s take our hypothetical ventilated patient with an initial PEEP of 5 cmH₂O and an initial F_IO₂ of 100% to see if we are providing adequate oxygenation. The ABG reveals that the P_aO₂ is 400 mmHg. Well, for a normal lung, the acceptable range for P_aO₂ is about 80–100 mmHg, so clearly this P_aO₂ is too high, and therefore we’ll need to reduce the amount of F_IO_2.

So, suppose we reduced the F_IO₂ from 100% to 60%. Then we order another ABG and the results reveal a P_aO₂ of 40 mmHg. Knowing that an acceptable normal range for P_aO₂ is 80–100 mmHg, what should we do? Well, this is a little tricky because at first, you may consider raising the F_IO₂— and that certainly is an option—but you’re already using a pretty high amount of F_IO₂, and it seems to be fairly ineffective in normalizing the P_aO₂. So, instead, you could increase the PEEP.

In this scenario, the new ABG returns a P_aO₂ of 90 mmHg, which falls well within the acceptable P_aO₂ range, indicating we should maintain the current PEEP and F_IO₂ 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 V_T and RR are used to correct ventilation, while PEEP and F_IO₂ are primarily used to correct oxygenation.

But is normalizing oxygenation and ventilation the most important goal? That depends.