How does a non-rebreather (NRB) mask work?

How a Non-Rebreather Mask Works

A non-rebreather mask (NRB) delivers high-concentration oxygen (60-90%) by using a reservoir bag that fills with pure oxygen during exhalation, one-way valves that prevent exhaled air from mixing back into the reservoir, and side ports that allow exhaled CO2 to escape while blocking room air entry—but it requires a minimum flow rate of 15 L/min to function properly and prevent dangerous CO2 rebreathing. 1, 2

Core Mechanism Components

The Reservoir Bag System

• The reservoir bag attached to the mask fills with 100% oxygen from the oxygen source during the patient's exhalation phase 3
• During inspiration, the patient draws oxygen primarily from this pre-filled reservoir bag, which contains pure oxygen rather than room air 3
• The bag must remain at least one-third to two-thirds inflated throughout the respiratory cycle to ensure adequate oxygen supply 4

One-Way Valve System

• A one-way valve sits between the mask and reservoir bag, allowing oxygen to flow from the bag into the mask during inspiration but preventing exhaled air from entering the reservoir 5
• Side ports on the mask body contain one-way flap valves (or in some designs, a safety vent) that open during exhalation to allow CO2-rich exhaled air to escape 3
• These same side valves close during inspiration to prevent room air from being drawn into the mask, maintaining high oxygen concentration 3
• The most effective designs use three valves total (one at the reservoir and two at the side ports) rather than a safety vent, which can achieve inspired oxygen fractions up to 97% 3

Critical Flow Rate Requirements

Minimum 15 L/min Requirement

• NRB masks absolutely require oxygen flow rates of 15 L/min to function safely and effectively 1, 2
• This flow rate must exceed the patient's minute ventilation (typically 6-10 L/min in adults) to prevent the reservoir bag from collapsing and to avoid CO2 rebreathing 4
• The British Thoracic Society specifically recommends 12 L/min via NRB for acute cluster headache treatment, with appropriate equipment to ensure high flow delivery 5

Dangers of Inadequate Flow

• Using flow rates below 10-15 L/min with an NRB creates a serious risk of CO2 rebreathing and can precipitate CO2 narcosis 4
• At low flow rates (e.g., 4 L/min), the reservoir bag collapses during inspiration, forcing the patient to draw in exhaled CO2 that hasn't been adequately flushed from the mask 4
• This is particularly dangerous in COPD patients with small tidal volumes, where inadequate flow dramatically increases CO2 rebreathing risk 4
• One case report documented a COPD patient developing CO2 narcosis requiring intubation when an NRB was inappropriately used at only 4 L/min 4

Oxygen Concentration Delivery

Achievable FiO2 Levels

• With proper technique (15 L/min flow, tight mask seal, three-valve design), NRB masks can deliver inspired oxygen concentrations of 85-97% 3
• Standard NRB designs with safety vents typically deliver 60-80% oxygen concentration 1
• Performance is significantly enhanced by: using 15 L/min flow (versus 10 L/min), ensuring a tight facial seal, and using three-valve designs rather than safety vent designs 3

Comparison to Other Devices

• NRB masks outperform simple face masks, which deliver only 40-60% oxygen at 5-10 L/min 1, 2
• When using flush rate oxygen (40-60 L/min), NRB masks achieve mean expired oxygen fractions of 81-83%, which is noninferior to bag-valve masks and superior when mask leaks occur 6
• The nonrebreathing outlet valve must not jam with oxygen flows up to 30 L/min 5

Clinical Application Considerations

Appropriate Use Scenarios

• NRB masks are most suitable for trauma and emergency situations requiring immediate high-concentration oxygen delivery 1
• They are recommended for acute cluster headache attacks at 12 L/min flow 5
• For pregnant women with major trauma or severe hypoxemia, high-concentration oxygen via NRB is appropriate 5

When NOT to Use NRB

• Do not connect NRB masks to oxygen concentrators with maximum output of 10 L/min—use simple face masks or nasal cannulae instead 2
• For patients at risk of hypercapnic respiratory failure, nasal cannulae at 1-2 L/min or simple face masks at 5 L/min are safer alternatives 4
• For home oxygen therapy at ≤4 L/min, nasal cannulae should be the first-choice delivery device 2

Proper Fitting Technique

• The mask must create a tight seal covering both mouth and nose to prevent room air entrainment 3
• Masks should be transparent material to allow detection of regurgitation 5
• Tightening the mask seal significantly improves oxygen delivery performance 3

Common Pitfalls to Avoid

• Never use flow rates below 10-15 L/min with an NRB mask—this is the most critical safety error 4
• Do not assume the reservoir bag can remain deflated; if it collapses completely during inspiration, increase the flow rate immediately 4
• Avoid using NRB masks for patients requiring precise oxygen titration (use Venturi masks instead) 1, 2
• Do not use NRB masks for prolonged therapy when oxygen cylinders are the source, as they deplete rapidly at 15 L/min—consider alternative delivery methods for sustained use 2
• Monitor arterial blood gases regularly in COPD patients, as even with proper flow rates, there remains risk of CO2 retention 4