2.5 Oxygen on a Ventilator: Setting the FiO2

Oxygen is always set on a mechanical ventilator—regardless of the mode or type of breath. Since the air delivered by a mechanical ventilator compromises all of the air the patient will be breathing in, the composition or percentage of oxygen must be set on the ventilator. On the ventilator, oxygen is set by the FiO₂. You would adjust the FiO₂ between [latex]0.21[/latex] and [latex]1.0[/latex] to ensure your patient is getting enough oxygen.

The principles of assessing oxygen need that we have previously learned in our respiratory therapy training can be applied when adjusting settings on the ventilator. If the SpO₂ is less than [latex]92\%[/latex], increase the FiO₂ the ventilator is supplying to help increase the oxygen available to the lungs, and by extension you will increase the amount of oxygen in the body. Increasing the FiO₂ will increase the partial pressure of oxygen in the alveoli of the lungs, which in turn increases the amount diffusing across the alveolar-capillary membrane to attach to hemoglobin and also increases the amount of oxygen dissolving into the bloodstream. Remember, air moves from high pressure to low pressure. So, if the partial pressure of oxygen is higher in the alveoli then the blood, a higher pressure difference will increase the movement of oxygen from the high end of the gradient to the low end—moving the oxygen from the alveoli across the alveolar-capillary membrane into the bloodstream. Watch the following video from 5:22 to 5:44 for a demonstration:

Oxygen Free Radicals

But don’t start thinking that’s all there is to it! You cannot simply set the oxygen and walk away. First, it is important to note that oxygen is classified as a drug and, like any other medication, it has its own negative side effects. Medical research has concluded that exposure to high amounts of oxygen for extended periods of time can cause damage to the lungs. A high level of oxygen in the body creates a byproduct called oxygen free radicals that cause scarring for the delicate alveoli-capillary membrane and harden it, making it more difficult for oxygen to get through. More details on these complications will be covered in a later chapter. The best way of minimizing exposure to oxygen free radicals is ensuring that patients get just enough oxygen to support their needs, but not so much that they have extra oxygen circulating in the body.

It’s easy to misunderstand this concept of oxygen causing damage. One of the most pervasive and misunderstood ideas is the “danger” of oxygen delivery leading many medical professionals to avoid giving oxygen. Remember that oxygen is the food for every cell of the body. Without this food, cells will die! Hypoxia can damage the brain in approximately four minutes. Damage caused by exposure to too much oxygen is a much slower process. It is essential to understand that you must give more oxygen if levels are low and do not withhold oxygen if your patient is showing signs of lower SpO₂. Just avoid delivering higher oxygen than you need to for extended periods of time.

Object Lesson

It’s like the children’s story of “Goldilocks and the Three Bears”. With oxygen, we really don’t want “too little,” which leads to hypoxia, causing tissue damage. We also don’t want “too much,” especially for too long, which leads to hyperoxia, causing release of oxygen free radicals and damage to cellular membranes. We really want it to be “just right,” with an SpO₂ greater than [latex]92\%[/latex] but less than [latex]100\%[/latex]. Oxygen levels delivered can be titrated up or down, which means that it will be increased or decreased slowly in a step-wise fashion, to ensure SpO₂ is [latex]92\text{ - }99\%[/latex] for healthy individuals.

 

 

 

SpO₂ only goes as high as [latex]100\%[/latex] once all the hemoglobin is bound to oxygen. Any additional oxygen that is in the lungs is not captured by hemoglobin. This means that a person with an SpO₂ of [latex]100\%[/latex] on FiO₂ [latex]0.60[/latex] could potentially still have an SpO₂ of [latex]100\%[/latex] on FiO₂ [latex]0.50[/latex]. There could be an excess of oxygen present. This excess of oxygen would drive the PaO₂ much higher than the normal targeted range of [latex]80\text{ - }100\text{ mmHg}[/latex] since the high concentration of oxygen in the alveoli would drive more oxygen to diffuse into the plasma.

You can test for excess oxygen by taking an arterial blood gas (ABG sample) and test the PaO₂. As stated, the normal is [latex]80\text{ - }100\text{ mmHg}[/latex]. A person with an SpO₂ of [latex]100\%[/latex] could have an PaO₂ of [latex]100\text{ mmHg}[/latex] or upwards of [latex]400\text{ mmHg}[/latex]. PaO₂ that are higher than [latex]100\text{ mmHg}[/latex] are indicative of over oxygenation, or hyperoxia. High levels greater than [latex]100\text{ mmHg}[/latex] increase the risk of oxygen free radicals and lung damage.

Since there is no way to tell from an SpO₂ of [latex]100\%[/latex] whether the person is getting just enough oxygen or too much, an easy way to avoid this situation entirely is to target an SpO₂ of [latex]92\text{ - }99\%[/latex] and not let the SpO₂ sit at [latex]100\%[/latex] unless the oxygen is down to minimum (i.e., FiO₂ [latex]0.21[/latex]).

Remember that oxygen content (CaO₂) equation? You may recall that PaO₂ was part of the dissolved oxygen portion. As long as the individual has adequate hemoglobin that is functioning appropriately, high PaO₂s do not contribute a significant amount of oxygen compared to PaO₂ of [latex]100[/latex].

Let’s compare two examples of the content of oxygen in the blood. Both examples have the same amount of hemoglobin saturated within a safe range. Patient X is getting “just enough” oxygen to saturate their hemoglobin to [latex]97\%[/latex] (a safe level), but not excess oxygen with the partial pressure of oxygen at [latex]100\text{ mmHg}[/latex]. Comparatively, Patient Y is getting too much oxygen. Their hemoglobin is fully saturated with the partial pressure of oxygen in their blood reading above the normal levels. Compare the oxygen content of these two patients below.

Compare the calculated oxygen content (CaO₂) for both patients. The calculated CaO₂ of both of these patients are very close though they are receiving very different amounts of oxygen. This model shows that, in people with normal hemoglobin, delivering just enough oxygen for saturating hemoglobin adequately is the most important aspect of oxygenation in the blood. Over-delivering oxygen does not significantly contribute to improving the amount of oxygenation if the saturation of hemoglobin does not change. Therefore, the damage of delivering high levels of O₂ and the creation of oxygen free radicals far outweighs the benefits of dissolving more oxygen into the blood, and it should be avoided.

“Oxygen on a Ventilator: Setting the FiO2” from Basic Principles of Mechanical Ventilation by Melody Bishop, © Sault College is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.