The two masks can look similar from across a room, but they solve opposite oxygen-delivery problems. A Venturi mask is selected when the concentration needs to be known and controlled. A non-rebreather mask — also called a reservoir mask — is selected when a high concentration is needed promptly while the patient is being assessed and a definitive plan is made.
Neither mask treats the cause of low oxygen. Both are interfaces connected to an oxygen source, and both should be used against a prescribed target saturation and clinical plan.
The difference at a glance
| Feature | Venturi mask | Non-rebreather mask |
|---|---|---|
| Main purpose | Deliver a selected, controlled FiO₂ | Deliver a high oxygen concentration quickly |
| Typical source flow | Printed on the adapter; varies by percentage | 10–15 L/min; commonly 15 L/min in acute care |
| Approximate delivered oxygen | Usually 24–60%, depending on adapter | Commonly above 60%; variable with fit, flow and breathing pattern |
| Reservoir bag | No | Yes |
| One-way valves | No | Usually present around the reservoir/side ports |
| Performance type | Fixed-performance when total flow meets demand | Variable-performance |
| Common setting | Controlled oxygen, including patients at risk of hypercapnia | Severe acute hypoxaemia or critical illness |
FiO₂ means the fraction of inspired oxygen. Room air is approximately 21% oxygen. The number printed on a Venturi adapter is an intended concentration, not a flow rate.
How a Venturi mask controls oxygen
A Venturi adapter sends oxygen through a narrow jet. The fast jet entrains a calculated amount of room air through side ports, creating a predictable oxygen–air mixture. A 28% adapter therefore does not deliver “28 L/min”; it is designed to deliver approximately 28% oxygen when connected at or above the source flow printed on that adapter.
The source-flow number matters because it powers the entrainment system. If the patient is breathing very rapidly or deeply, their peak demand can exceed the total gas flow generated by the adapter. Current British Thoracic Society guidance says the oxygen flow may be increased above the adapter’s minimum — by up to 50% when respiratory rate is above 30 breaths per minute — without changing the nominal percentage. The extra source flow increases total flow available to the mask; it does not turn a 28% adapter into a 35% one.
Read the full Venturi colour and flow chart before relying on adapter colour, because colour conventions and minimum flows can vary between manufacturers.
How a non-rebreather provides high-concentration oxygen
A non-rebreather stores oxygen in a reservoir bag between breaths. During inspiration, the patient draws from that bag; one-way valves reduce the amount of exhaled gas and room air entering the circuit. Its performance depends on four practical conditions:
- The reservoir bag is inflated before the mask is placed.
- Source flow is high enough that the bag does not collapse substantially during inspiration.
- The mask fits reasonably well around the nose and mouth.
- The valves are present, unobstructed and moving correctly.
If any of these fail, room-air dilution rises and the delivered FiO₂ falls. That is why “15 L/min” alone does not guarantee a particular percentage. The British Thoracic Society describes reservoir-mask oxygen at 15 L/min as the initial interface for acutely breathless patients with saturations below 85% when they are not in a controlled-oxygen pathway, with urgent reassessment and step-down once stable.
Which mask is used for COPD?
The diagnosis “COPD” does not automatically select a mask. The question is whether the person is at risk of hypercapnic respiratory failure and what their current saturation, blood gas and clinical condition show.
For a patient with known or suspected risk of hypercapnia, a common initial controlled-oxygen approach is 24% or 28% Venturi oxygen, titrated to the prescribed saturation range while an arterial or arterialised blood gas is obtained. The commonly cited provisional target is 88–92%, but an individual oxygen alert card or prior specialist plan takes precedence.
Severe hypoxaemia is still an emergency. Oxygen should not be withheld from a critically ill patient because of fear of carbon dioxide retention. The correct response is monitored oxygen, urgent blood-gas assessment and escalation when ventilation is failing — sometimes to CPAP, BiPAP, NIV or a ventilator — rather than leaving the patient dangerously hypoxaemic.
When a non-rebreather is the bridge, not the destination
A reservoir mask buys time. It is commonly used during ambulance transport, emergency assessment, severe acute hypoxaemia and preparation for a higher-support interface. If adequate saturation cannot be maintained, or work of breathing, consciousness or carbon dioxide worsens, the answer is not simply a tighter mask. The patient needs urgent senior assessment for high-flow nasal oxygen, non-invasive ventilation or invasive ventilation according to the cause.
At home, an unexpected need for a non-rebreather is an emergency signal. Do not improvise one as a long-term upgrade from a nasal cannula or connect it to a concentrator that cannot sustain the required flow and purity.
Common setup mistakes
Venturi mask mistakes
- Choosing by colour without reading the printed percentage and flow.
- Covering or obstructing the air-entrainment ports with bedding.
- Running below the adapter’s stated flow.
- Assuming that turning up flow changes the selected FiO₂.
- Using an adapter whose total flow cannot match a very tachypnoeic patient.
Non-rebreather mistakes
- Applying the mask before pre-inflating the reservoir.
- Letting the bag collapse on every breath.
- Using a mask with a missing or stuck valve.
- Connecting to a source that cannot deliver the required continuous flow.
- Treating the interface as definitive therapy while the patient deteriorates.
Takeaway
Choose a Venturi mask when the clinical question is “what controlled oxygen concentration should this patient receive?” Choose a non-rebreather when the question is “how do we deliver a high concentration immediately while urgent assessment and escalation occur?” The Venturi adapter’s printed percentage and flow must be followed; the non-rebreather reservoir must remain inflated.
This article explains equipment, not an individual prescription. New severe breathlessness, confusion, blue lips, chest pain or a saturation below the person’s emergency threshold requires urgent medical care.
Primary references: British Thoracic Society oxygen guideline; BTS home-oxygen equipment appendix; European Respiratory Society oxygen-device review.