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 0.21 and 1.0 to ensure your patient is getting enough oxygen.

The principles of assessing oxygen need that we have learned in this chapter can be applied when adjusting settings on the ventilator. If the Sp0₂ is less than 92%, 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 increase 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 (see the following animation):

Lonely Teeming Billygoat via GfyCat

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. 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 showings signs of lower SpO₂. Just avoid delivering higher oxygen than you need to for extended periods of time.

SpO₂ only goes as high as 100% 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 100% on FiO₂ 0.60 could potentially still have an SpO₂ of 100% on FiO₂ 0.50. There could be an excess of oxygen present. This excess of oxygen would drive the PaO₂ much higher than the normal targeted range of 80-100mmHg 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 Pa02. As stated, the normal is 80-100mmHg. A person with an SpO₂ of 100% could have an PaO₂ of 100mmHg or upwards of 400mmHg. PaO₂ that are higher than 100mmHg are indicative of over oxygenation, or hyperoxia. High levels greater than 100mmHg increase the risk of oxygen free radicals and lung damage.

Since there is no way to tell from an SpO₂ of 100% 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 92-99% and not let the SpO₂ sit at 100% unless the oxygen is down to minimum (i.e., FiO₂ 0.21).

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 100.

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 97% (a safe level), but not excess oxygen with the partial pressure of oxygen at 100mmHg. 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.