Nursing care
Oxygen Delivery Devices, explained for the bedside and the exam
Written and reviewed by Dana Whitfield, RN, MSN · 5 min read · Updated September 2026
Short answer
Oxygen delivery devices are matched to flow rate and required precision: nasal cannula runs 1 to 6 L/min for mild hypoxaemia, simple face mask 5 to 10 L/min, non-rebreather mask 10 to 15 L/min with the reservoir bag kept inflated, and Venturi mask when an exact FiO2 is required. Choosing the wrong one wastes oxygen or under-treats the patient.
Defining it precisely
Oxygen delivery devices fall into two families: low-flow systems that mix room air with supplemental oxygen, and high-flow systems that deliver a fixed, predictable FiO2 regardless of the patient's breathing pattern. Nasal cannula, simple face mask, and non-rebreather mask are all low-flow; the Venturi mask is the classic high-flow device used at the bedside.
Flow rate and FiO2 are not the same thing, and mixing them up is the single most common error. Nasal cannula runs 1 to 6 L/min, roughly raising FiO2 by 3 to 4% per litre above room air's 21%. Simple mask runs 5 to 10 L/min. Non-rebreather mask runs 10 to 15 L/min, and the reservoir bag must stay inflated during use; if it collapses on inspiration, the patient is rebreathing CO2 and the flow needs increasing, not the mask removing.
The exceptions that matter
Below 5 L/min on a simple face mask, exhaled CO2 can pool in the mask and be rebreathed, so simple masks are never set below 5 L/min. This is the opposite problem from the non-rebreather's bag rule but the same underlying physics: stale gas has to be flushed out faster than the patient can rebreathe it.
Venturi masks are the exception to the imprecision of low-flow devices. Colour-coded adaptors deliver a fixed FiO2, commonly 24, 28, 31, 35, 40 or 60%, independent of how deeply or quickly the patient breathes. This makes it the device of choice for COPD patients who are CO2 retainers and need a reliably low, titratable FiO2 rather than the variable delivery a nasal cannula gives.
Using it to prioritise
Device choice tells you how sick the patient is before you even check the sats. A patient on 2 L nasal cannula who suddenly needs a non-rebreather at 15 L/min has decompensated, and that escalation is itself a reportable finding, not just a documentation update.
In triage and assignment questions, a patient on a high-flow device with a reservoir bag outranks a patient on nasal cannula for a nurse's next check, because that device signals a higher oxygen requirement and closer monitoring need. Recognising which device a patient is on lets you infer acuity fast, which is exactly why the exam tests it as a prioritisation cue rather than just a fact to recall.
Traps in exam wording
Questions often give a flow rate and ask you to identify the device or the approximate FiO2, expecting you to know that 6 L/min nasal cannula delivers roughly 44% FiO2 while the same 6 L/min on a simple mask is too low and unsafe. The number alone is meaningless without the device attached to it.
Another trap: a non-rebreather mask reservoir bag that has collapsed is written into a stem to test whether you catch that this means insufficient flow, not a broken device to discard. And COPD questions frequently push high-flow oxygen as a distractor; the safe answer is usually the lowest FiO2 that corrects hypoxaemia, delivered by Venturi mask for precision.
Examples from practice
A post-operative patient with mild desaturation to 92% on room air is typically started on nasal cannula at 2 L/min, titrated to a target range set by the provider. A patient in acute respiratory distress with sats in the low 80s is more likely to need a non-rebreather at 10 to 15 L/min while the underlying cause is worked up.
A COPD patient admitted with pneumonia is a good example of the Venturi mask's purpose: the nurse needs an FiO2 precise enough to correct hypoxaemia without suppressing the hypoxic drive that some CO2 retainers rely on. Starting at 24 or 28% and titrating based on ABGs, rather than guessing with a nasal cannula, is the safer approach in that patient.
Summary
Match flow rate to device, not the other way round: nasal cannula 1 to 6 L/min, simple mask 5 to 10 L/min, non-rebreather 10 to 15 L/min with the bag inflated, Venturi mask when the FiO2 must be exact. Each threshold exists for a physiological reason, not an arbitrary cutoff.
Use the device as a quick read on acuity and escalation, keep the reservoir bag inflated on a non-rebreather, never run a simple mask below 5 L/min, and reach for a Venturi mask whenever the patient's CO2 status makes a precise, controlled FiO2 the priority.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our respiratory practice questions are the closest set to what this page covers.
Common questions
What flow rate is used for a non-rebreather mask?
10 to 15 L/min, set high enough to keep the reservoir bag inflated throughout the respiratory cycle. If the bag collapses on inspiration, increase the flow rather than removing the mask.
Why can't a simple face mask run below 5 L/min?
Flow below 5 L/min is not enough to flush exhaled carbon dioxide out of the mask, so the patient ends up rebreathing it. Simple masks are always set at 5 L/min or above for that reason.
Which device gives the most precise FiO2?
The Venturi mask. Its colour-coded adaptors deliver a fixed, high-flow FiO2 regardless of the patient's breathing pattern, which is why it is preferred for COPD patients who need tightly controlled oxygen levels.
What FiO2 does nasal cannula deliver at 6 L/min?
Roughly 44%, using the common estimate of a 3 to 4% FiO2 increase per litre above room air's 21%. It is an approximation, not an exact figure, because nasal cannula is a low-flow, variable-performance device.
Why is nasal cannula flow capped around 6 L/min?
Above about 6 L/min the airflow starts to dry and irritate the nasal mucosa without meaningfully raising FiO2 further, since the device can't reliably deliver higher concentrations. A patient needing more oxygen than that is switched to a mask-based device instead.