Low flow oxygen delivery quiz


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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?

 Question 1


What is it?

Nasal cannula.

How does it work?

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.

How much can it deliver?

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.

What are the pros and cons?

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.

  Question 2


What is it?

Hudson mask.

How does it work?

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 COis allowed to escape. Air flow is controlled by the patient’s inspiratory flow and so the oxygen mix varies.

How much can it deliver?

The inspired FiO2 varies according to patient’s inspiratory flow – but a rough guide is shown in the table.


Flow FiO2
5-6 L/min 0.35-0.5
6-10 L/min 0.5-0.6
10-12 L/min 0.6-0.65


What are the pros and cons?

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.

 Question 3


What is it?

Non-rebreather mask.

How does it work?

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.

How much can it deliver?

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.

What are the pros and cons?

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.

  Question 4


What is it?

Venturi or percentage mask.

How does it work?

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. 

How much can it deliver?

It provides varying oxygen concentrations, depending on which attachment is used.

What are the pros and cons?

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.


Oxygen and bronchodilator delivery

Oxygen delivery, Royal Children’s Hospital, Melbourne


About the authors

  • Tessa Davis is a Consultant in Paediatric Emergency Medicine at the Royal London Hospital and a Senior Lecturer at Queen Mary University of London.


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