Tessa Davis. Low flow oxygen delivery quiz, Don't Forget the Bubbles, 2013. Available at:
Any doctor or nurse working a hospital should be familiar with mechanisms for delivering low flow oxygen. So you should know all the answers to this quick quiz…shouldn’t you?
These deliver oxygen via prongs in the nose. The standard prongs come in 3 sizes: neonatal, paediatric and adult. The humidified prongs come in 5 sizes.
To size the prongs: they should be roughly half the diameter of the nostril.
Nasal cannula can deliver 1-8 L/min with varying FiO2 (24% at 1 L/min and 44% at 6 L/min).
For flows over 4L, use the humidified nasal prongs.
A low flow meter will be needed if you are using flow rates of less than 0.5 L/min.
At flows of over 4L the nasal mucosa can become irritated and dry so they should be attached to a humidifier. There is a risk of pressure areas to the face. And it’s not that useful for mouth breathers. If you’re using high flows then keep an eye on the PaCO2 as sats can remain normal in high flow in spite of a rising PaCO2.
However, nasal cannulae allow the patient to talk, eat and drink while keeping the oxygen on.
It’s a vented mask that goes over the mouth and nose (make sure you get a good fit). The vents on the sides allow room air to enter so the oxygen source is diluted and CO2 is allowed to escape. Air flow is controlled by the patient’s inspiratory flow and so the oxygen mix varies.
The inspired FiO2 varies according to patient’s inspiratory flow – but a rough guide is shown in the table.
If you use too low a flow (usually less than 5 L/min) with a Hudson mask, this can cause rebreathing of CO2, leading to an increase in PaCO2. It can also dry the mouth leading to discomfort.
The pros are that a Hudson mask can deliver a higher FiO2 than nasal prongs, but consider a non-rebreather for achieving an FiO2 greater than 0.5.
This mask has a reservoir bag attached to it which is filled with 100% oxygen – this is fitted with a one-way valve to prevent CO2 from being exhaled into the bag in order to delivery the highest oxygen concentration possible.
It can deliver up to 15 L/min from the wall which will supply an FiO2 of 0.6-0.8 (minimum flow of 10 L/min).
Make sure the bag remains fully inflated during the respiratory cycle.
It can deliver high oxygen volumes with no risk of rebreathing.
But, just like the Hudson mask, it needs close skin contact to be effective.
It can provide a particular percentage of oxygen depending on the flow required (24-60 %). There are different attachments that determine the FiO2 – these are entrainment devices.
We often use this in paediatrics when giving a nebuliser.
It provides varying oxygen concentrations, depending on which attachment is used.
Rebreathing of expired gas is not a problem because of the high flow rates.
However, it’s not very effective for achieving an FiO2 over 0.5, so consider a non-rebreather in this case.
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