Understanding the Relationship Between Oxygen Flow Rate and FiO2

Oxygen – we all need it! Sometimes we need more of it in order to maintain our oxygen saturations. In these situations, supplemental oxygen can be administered via various oxygen delivery devices, ranging from nasal prongs to invasive ventilation.

There are two important things to consider when delivering supplemental oxygen to your patient: the oxygen flow rate and the FiO₂. The oxygen flow rate is the number that we dial up on the oxygen flow metre, usually between 1-15L/min. FiO₂ is defined as the percentage or concentration of oxygen that a person inhales (the fraction of inspired oxygen). The atmospheric air that we inhale on a day to day basis is made up of 21% oxygen, 78% nitrogen and 1% trace elements such as argon, carbon dioxide, neon, helium and methane.

What is the FiO₂ of the Oxygen You are Delivering to Your Patient?

In order to answer this question, I am going to ask you a series of questions!

1. My first question: what is the FiO₂ of the air you are breathing right now? If you said 21%, excellent!
2. My second question for you is this: what is the FiO₂ of the oxygen being delivered through the oxygen flow metre as soon as you turn it on? This is where people start saying, ‘it depends on the oxygen flow rate’. Despite this being true, when we are discussing the FiO₂, that is not actually correct.
3. 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 metre? The answer is no! The oxygen flow metre 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 metre has an FiO₂ of 100%. Consider the following:

• If I have the oxygen flow rate set at 1L/min, I will have 1L/min of 100% oxygen…
• If I have the oxygen flow rate set at 5L/min, I will have 5L/min of 100% oxygen…
• If I have the oxygen flow rate set at 10L/min, I will have 10L/min of 100% oxygen…
• If I have the oxygen flow rate set at 15L/min, I will have 15L/min of………………….?

If you said 100% oxygen, excellent!

But My Patient Receiving 1L/min of Oxygen via the Nasal Prongs is Not Actually Breathing in an FiO₂ of 100%, Are They?

Of course not! The true FiO₂ the patient is breathing is based on the flow requirements of that patient. What do I mean by that? You are currently breathing air in and out of your lungs. 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 20–30L/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 a patient who is ‘struggling to breathe’ or has an ‘increase work of breathing’. They are breathing faster and deeper to try and get the air from point A to point B faster (Review lung sounds).

This means that their peak inspiratory flow requirement has increased.

What is the Relationship Between Oxygen Flow Rate, FiO₂ and Peak Inspiratory Flow?

If you are breathing in normally at a peak inspiratory flow rate of 30L/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 10L/min of oxygen via a face mask at an FiO₂ of 100%. You still have a normal peak inspiratory flow rate of 30L/min, but 10L/min if being blown in your face via the face mask. Therefore, you still need another 20L/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 50L/min but were still only receiving 10L/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 20L/min while receiving 10L/min of oxygen via a face mask at an FiO₂ of 100%:

(10 x 100) + (10 x 21) = 1210%
1210 ÷ 20 = 60%

How is this Practically Relevant to Me?

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. We can never accurately know what the FiO₂ of a patient is unless we accurately know what their peak inspiratory flow is, or we are delivering high flow oxygen at a rate that is estimated to be greater than their peak inspiratory flow. The quick reference charts that we sometimes see equating an oxygen flow rate to a FiO₂ is assuming a peak inspiratory flow rate of 20-30L/min, but it is merely an approximation.

The Most Important Take Home Points are as Follows:

1. It is an increase in FiO₂ that increases the oxygen saturations of a patient, not the oxygen flow rate
2. As peak inspiratory demands change for a patient, so will the oxygen flow rate demands if we want to maintain a consistent FiO₂
3. Hypoxia is not a good thing, but neither is too much FiO₂; and
4. Effective oxygen therapy is about finding a balance between delivering the lowest FiO₂ in order to achieve normal oxygen saturations for the patient.

Read related: Understanding COPD and the Hypoxic Drive to Breathe

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References

• Bailey, P 2016, Continuous Oxygen Delivery Systems for Infants, Children, and Adults, UpToDate, viewed 2 November 2016, https://www.uptodate.com/contents/continuous-oxygen-delivery-systems-forinfants-children-and-adults
• Courey, AJ & Hyzy, RC 2016, Overview of Mechanical Ventilation: Fraction of Inspired Oxygen, UpToDate, viewed 2 November 2016, https://www.uptodate.com/contents/overview-of-mechanical-ventilation
• Theodore, AC 2016, Oxygenation and Mechanisms of Hypoxemia, UpToDate, viewed 2 November 2016, https://www.uptodate.com/contents/oxygenation-and-mechanisms-of-hypoxemia
• University of Colorado Denver (n.d.), Rules on Oxygen Therapy, University of Colorado, Denver, USA, viewed 2 November 2016, http://www.ucdenver.edu/academics/colleges/medicalschool/0therapy.pdf

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