Oxygen, we all need it! We do not need a lot of it under normal circumstances, with 0.21 being the fraction of inspired oxygen (FiO2) of room air. FiO2 is defined as the concentration of oxygen that a person inhales. The air that we inhale on a day to day basis is made up of 21% of oxygen, 78% of nitrogen and 1% of trace elements such as argon, carbon dioxide, neon, helium and methane. For the purposes of this article, fractions and percentages will be used interchangeably for ease of explanation.
Sometimes, 21% of oxygen may not be enough to maintain adequate oxygen saturations. In these situations, supplemental oxygen can be administered via various oxygen delivery devices ranging from nasal prongs to invasive ventilation. This allows the concentration of oxygen to be increased, potentially increasing the FiO2 to 100%.
In settings outside of critical care areas, FiO2 has historically not received much attention. But things are changing! In standard hospital settings these days, there is an increasing use of humidified high flow oxygen therapy that requires an understanding of the relationship between oxygen flow rate and FiO2. In most clinical areas that require an FiO2 to be documented, you will be able to find a table that outlines an approximate correlation between oxygen flow rate and FiO2, similar to the table below:
It is all well and good to memorise that for every 1 L/min increase of oxygen flow rate, the FiO2 increases by 4%. But it would be better to understand WHY the FiO2 increases in those specific increments! So let’s discuss that…
My first question for you is this: what is the FiO2 of the air you are breathing right now?
If you said 21%, excellent!
My second question for you is this: what is the FiO2 of the oxygen being delivered through the oxygen flow meter as soon as you turn it on?
And this is where people start saying “it depends on the oxygen flow rate”. Despite this being true when we are discussing the FiO2 that the person is inhaling, that is not actually the question that I asked.
Therefore, my third question for you is this: does the oxygen flow rate really change the FiO2 of the PURE oxygen that is being delivered through the oxygen flow meter?
The answer is NO! The oxygen flow meter is connected to either a bottle of oxygen or a medical wall supply of oxygen. This oxygen is PURE, it is 100% oxygen! Therefore, anything that comes out of that oxygen flow meter has an FiO2 of 100%. Consider the following:
If I have the oxygen flow rate set at 1 L/min, I will have 1 L/min of 100% oxygen…
If I have the oxygen flow rate set at 5 L/min, I will have 5 L/min of 100% oxygen…
If I have the oxygen flow rate set at 10 L/min, I will have 10 L/min of 100% oxygen…
If I have the oxygen flow rate set at 15 L/min, I will have 15 L/min of………………….?
If you said 100% oxygen, excellent!
So my fourth question for you is this: why does the table above show different FiO2 values corresponding with these oxygen flow rates that we have just said is always 100% because it is pure oxygen?
This is the point that people start scratching their heads, shrugging their shoulders and backing away slowly while avoiding eye contact with me. Hang in there! The lightbulb will go off very shortly!
The answer to this question comes down to the flow requirements of the patient! What do I mean by that? You are currently breathing air in and out of your lungs while you are reading this blog, hopefully with enough interest to share it with your friends and colleagues after you finish reading it *wink wink*. The air that you are breathing has to get from point A (the atmosphere) to point B (your lungs). If a car was trying to get from point A to point B, it can only do this if you press the accelerator to achieve a certain speed. The faster the speed, the faster you get from point A to point B. The same principle applies to how we breathe, but we refer to this speed as our peak inspiratory flow.
Our normal peak inspiratory flow tends to range between 20 – 30 L/min. Our respiratory muscles are comfortable and do not tire when we breathe at a normal respiratory rate with this peak inspiratory flow. Now consider what your breathing does when you go for a run; or if you are allergic to running like me, imagine what your breathing does! Asides from your respiratory rate increasing, you start sucking in for more air. You are trying to get the air from point A to point B faster, which means that your peak inspiratory flow requirement has increased. The same goes for a person that is “struggling to breathe” or has an “increased work of breathing”, they have a high peak inspiratory flow requirement.
So back to patient flow requirements and FiO2…
If you are breathing in normally at a peak inspiratory flow rate of 30 L/min at room air with an FiO2 of 21%, you can easily calculate the average FiO2 you are breathing in an almost redundant formula:
30 x 21 = 630%
630 ÷ 30 = 21%
Now consider you are receiving 10 L/min of oxygen via a face mask at an FiO2 of 100%. You still have a normal peak inspiratory flow rate of 30 L/min, but 10 L/min if being blown in your face via the face mask. Therefore, you still need another 20 L/min to meet your inspiratory flow requirements. Where are you going to get this from? You are going to suck it in from the surrounding atmosphere with an FiO2 of 21%. So let’s apply the same formula as before:
(10 x 100) + (20 x 21) = 1420%
1420 ÷ 30 = 47%
However, if you had an increased peak inspiratory flow rate of 50 L/min but were still only receiving 10 L/min of oxygen via a face mask at an FiO2 of 100%:
(10 x 100) + (40 x 21) = 1840%
1840 ÷ 50 = 37%
Or a decreased peak inspiratory flow rate of 20 L/min while receiving 10 L/min of oxygen via a face mask at an FiO2 of 100%:
(10 x 100) + (10 x 21) = 1210%
1210 ÷ 20 = 60%
In the above examples, nothing changed with the oxygen flow rate being delivered to the patient. The only thing that has changed was the patient’s inspiratory flow demand and how much that “diluted” the pure oxygen being delivered with the FiO2 of 21% found in room air. If the flow rate being delivered to the patient is greater than their peak inspiratory flow rate, they have no reason to have to suck in atmospheric air and “dilute” the pure oxygen. Consider sticking your head out the car window as you are driving at the maximum legal speed. All that air that is blown in your face makes it a lot easier to breathe, it reduces the effort required to suck in the air. So if you were breathing with a normal peak inspiratory flow rate of 30 L/min but were receiving ≥ 30 L/min of pure oxygen via a high flow oxygen delivery device, you do not need to suck in any more air from the surrounding atmosphere and would therefore be receiving an FiO2 of 100%.
Unless the flow rate being delivered to the patient is more than their peak inspiratory flow demand, it is impossible to know what the patient’s exact FiO2 is because you do not know their exact peak inspiratory flow. The tables utilised to outline a relationship between oxygen flow rate and FiO2 are based on mere estimations of normal peak inspiratory flow rate, ranging between 20 – 30 L/min.
So let’s take this one step further and discuss the practical application of understanding oxygen flow rate and FiO2. As discussed in the blog post titled Respiratory Failure: Type 1 or Type 2, you can have a patient that has a problem with oxygenation or a patient that has a problem with ventilation. If your patient has a problem with oxygenation, they require a higher FiO2 to aid with this. In most settings, this is achieved by turning up the oxygen flow rate in order to subsequently increase the FiO2. If your patient has a problem with ventilation, they require a higher flow rate to aid with this. If we are aiming to set a flow rate higher than their inspiratory flow demand, it is not ideal to use just pure oxygen and deliver an FiO2 of 100% to someone that may not even have an oxygenation problem. They may only require an FiO2 of 21% with a higher flow rate, that can be achieved with a high flow air meter. Or the patient may require something in between these two extremes, which can be achieved with a dual flow adaptor that utilises both an oxygen and an air meter.
For example, 15 L/min of oxygen at an FiO2 of 100% and 15 L/min of air at an FiO2 of 21% to give a total of 30 L/min of flow at a diluted FiO2 of 60%. Or perhaps 15 L/min of oxygen at an FiO2 of 100% and 30 L/min of air at an FiO2 of 21% to give a total of 45 L/min of flow at a diluted FiO2 of 47%. The world is your oyster! Devices such as the AIRVO 2, do all of the above calculations for you. All you need to do is dial up how much total flow you want to set for your patient, and increase the oxygen flow meter to achieve the desired FiO2 to maintain adequate oxygen saturations.
So next time you are looking after that asthmatic patient who is sucking in air like their lives depend on it (pardon the nursing humour), consider making their breathing easier by giving them some additional flow! Imagine how much easier it would be for them to breath in if instead of having to make all of the effort to suck in the air, they had some of that air blown into their face? And next time you are looking after that patient with suboptimal oxygen saturations, do what we always do and turn up the oxygen!
Also remember the following:
- If your patient has a problem with oxygenation, they need more FiO2
- If your patient has a problem with ventilation, they need more flow
- If you patient has a problem with oxygenation AND ventilation, they need more FiO2 AND flow
University of Colorado Denver. (n.d). Rules on oxygen therapy. Retrieved from: http://www.ucdenver.edu/academics/colleges/medicalschool/departments/medicine/intmed/imrp/CURRICULUM/Documents/Oxygenation%20and%20oxygen%20therapy.pdf
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