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.
In order to answer this question, I am going to ask you a series of questions!
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 you said 100% oxygen, excellent!
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.
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
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%
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.
Read related: Understanding COPD and the Hypoxic Drive to Breathe