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Oxygen masks and nasal cannulae – how to choose?
High flow and low flow systems
Oxygen is a widely used intervention in medicine. There are many devices available to do the job. Broadly speaking there are two types of devices: high flow and low flow delivery systems. How do you decide on which device to use for a particular patient?
This often comes down to the patient’s peak inspiratory flow rate (demand) and the inspired fraction of oxygen (FiO2) needed to maintain normal physiology. Pulse oximetry helps us to titrate the FiO2 to target saturations (a surrogate of normal physiology) in real-time.
What is peak inspiratory flow rate?
This is nothing but the maximum flow rate required to deliver a tidal volume breath. It is measured as the volume of air (litres) that whizzes past the nostril per unit time (minute). At rest we normally have a flow rate of 20-30 L/min. In disease states the demand (flow rate) sometimes goes up to 50 -60 L/min!
Now look at the flow rate of oxygen in your hospital that is delivered via the flow meter at the wall where the oxygen port is. Quite often the maximum that we can get here is 15 L/min which is lower than what we need at rest! The oxygen that comes out from here however has a concentration of 100%. For ease of understanding this is equivalent to an FiO2 of 1.0. But this is not always what the patient eventually gets and we shall soon see why?
You might ask, “How does the patient compensate for the extra flow rate required?”
He or she draws in (entrains) air from outside the face mask or nasal cannula which results in dilution of the oxygen that we are delivering at the wall (Figure 1). The resulting oxygen concentration is now much lower than 100%.
How much lower?
That depends on how much air is drawn in. If we can match the patient’s flow rate then the oxygen concentration delivered becomes constant.
Figure 1
Nasal Cannula
The following table gives an approximate FiO2 for the corresponding flow rate via a nasal cannula. However, we have already discovered that this is not what the patient eventually gets! An easy way to remember this table is to start with 24% for 1 L/min and keep adding 4% to every litre rise in flow rate 😉
| Flow Rate via Nasal Cannula | Approximate FiO2 Delivered |
|---|---|
| 1 L/min | 24% |
| 2 L/min | 28% |
| 3 L/min | 32% |
| 4 L/min | 36% |
| 5 L/min | 40% |
| 6 L/min | 44% |
| 7 L/min | 48% |
| 8 L/min | 52% |
| 9 L/min | 56% |
| 10 L/min | 60% |
Figure 2 shows how different devices deliver a set oxygen concentration. As you can see the higher the flow rate that can be achieved the more likely it is that the patient will receive a fixed oxygen concentration.
Figure 2
Venturi Masks
Venturi masks are high flow delivery devices. They have valves which deliver a fixed concentration of oxygen e.g. 28%, 35% etc. Each valve is colour coded (Figure 3) although newer valves can be set at the desired FiO2 in a single unit. A 28% valve will have a smaller hole than a 40% valve and so on. Venturi valves deliver the following fixed FiO2s: 24%, 28%, 31%, 35%, 40%, 50% to 60%. Every valve comes with a minimum flow rate that is engraved on it.
Figure 3
If we have a 28% valve, with a 4L/min flow rate at the wall, the oxygen concentration delivered within the mask is 28%. Even if you increase the flow rate at the wall to 6, 8 or 10 L/min the valve will only deliver 28% FiO2! However, if you drop the flow rate to less than what has been prescribed on the valve, the FiO2 will drop to less than 28%. The advantage of using a venturi valve is that owing to Bernoulli’s principle, the delivered flow rate increases, going up to 40L/min even though the FiO2 remains 28%.
With increasing oxygen concentration valves e.g. 35% or 40%, the flow rates drop to 10-15L/min because the size of the hole in the valve, from which oxygen is escaping, starts getting bigger (Bernoulli’s principle). We are once again back to the same problem of mismatched flow rates.
We know that the actual FiO2 the patient receives, varies with the flow rate at which the patient is breathing. In the example here, if the patient’s demand is 60L/min, they will entrain air from the side ports of the mask to compensate, thereby diluting the final FiO2.
High Flow Nasal Cannula
High flow nasal cannula (HFNC), as the name suggests, delivers oxygen at a high flow rate up to 60L/min. The oxygen and air are blended before being delivered to the patient. This mixture is humidified which reduces bronchoconstriction and improves mucociliary clearance thereby reducing the work of breathing. The higher flow rate reduces any further dilution ensuring a more accurate oxygen concentration and therefore an accurate FiO2 being delivered.
Generally speaking, intubation results in a closed circuit that ensures delivery of 100% oxygen if needed, with an equivalent FiO2 of 1.0.
What happens when someone is hypoventilating (under-breathing) for example with an inspiratory flow rate of 15L/min?
If you give oxygen via a non-rebreather mask the higher flow rates at the wall (15L/min) which match their demand mean that the final FiO2 they receive is closer to 1.0 (there is no dilution!). When this happens in a patient who is prone to hypercapnic respiratory failure (e.g. COPD) they get pushed into it owing to ventilation perfusion mismatch. A venturi mask is a better alternative here provided they are not in respiratory acidosis. The latter requires non-invasive ventilation.
In general we can summarise that high flow delivery systems provide a much more accurate FiO2 especially in diseased states than low flow delivery systems (Figure 2).
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