Modifying ventilator settings based on arterial oxygen (PaO2) levels

4th Jan 2021

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.

Ventilator with PEEP and FIO2 highlighted and oxygen monitor. Needle for arterial blood gas.

Figure 1. To assess for adequate oxygenation, or intake of oxygen (O2), check the patient’s arterial oxygen level (PaO2 ) on an arterial blood gas (ABG). Then adjust the positive end-expiratory pressure (PEEP) or the fraction of inspired oxygen (FIO2) on the ventilator.

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.

Table of PaO2 results, interpretation, and recommendation for mechanical ventilation settings.

Figure 2. Assessing adequate oxygenation with arterial oxygen level (PaO2) from an arterial blood gas (ABG)—a table of results, their interpretation, and recommended course of action.

 

Now, please note that as you make a change to the FIO2 or PEEP it would be helpful to continuously monitor the oxygen saturation (SpO2) too. Medmastery note.

 

So, suppose we reduced the FIOfrom 100% to 60%. Then we order another ABG and the results reveal a PaOof 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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Recommended reading

  • Esteban, A, Ferguson, ND, Meade, MO, et al. 2008. Evolution of mechanical ventilation in response to clinical research. Am J Respir Crit Care Med. 177: 170–177. PMID: 17962636
  • Hess, D. 2001. Ventilator modes used in weaning. Chest. 120: 474S-476S. PMID: 11742968
  • Tobin, MJ, and Lodato, RF. 1989. PEEP, auto-PEEP, and waterfalls. Chest. 96: 449–451. PMID: 2670461
  • Mechanical ventilation protocol summary. NIH-NHLBI ARDS Clinical Network. http://www.ardsnet.org