Difference between nrbm mask and oxygen face mask
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non-rebreather mask vs simple face mask oxygen delivery difference FiO2
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Here is a clear comparison between a Non-Rebreather Mask (NRBM) and a Simple Oxygen Face Mask:
Simple Oxygen Face Mask vs. Non-Rebreather Mask (NRBM)
Simple Oxygen Face Mask
| Feature | Details |
|---|---|
| Design | Plastic mask covering nose and mouth with multiple vent holes on the sides |
| Reservoir | None — the mask body (~200 mL) itself acts as a small reservoir |
| Valves | No one-way valves |
| Flow rate | Minimum 5 L/min to prevent CO₂ rebreathing |
| FiO₂ delivered | ~45–65% at 10 L/min; up to 80–90% at 30 L/min |
| Rebreathing | Some exhaled gas is rebreathed from within the mask before the next breath |
| Mechanism | O₂-enriched air accumulates in the mask; on inspiration, this mixes with room air entrained through vent holes |
Non-Rebreather Mask (NRBM)
| Feature | Details |
|---|---|
| Design | Mask + attached reservoir bag (typically 1 L) |
| Reservoir | Large reservoir bag fills with 100% O₂ from the source |
| Valves | Three one-way valves: one between the bag and mask (prevents exhaled gas entering the bag), two on the side ports (allow exhalation, prevent room air entry) |
| Flow rate | Typically 10–15 L/min (bag must stay at least ½ inflated during inspiration) |
| FiO₂ delivered | ~60–70% at 15 L/min in practice (not 100%, as commonly assumed); up to ~90% at flow rates ≥45 L/min |
| Rebreathing | Minimal — valves prevent exhaled air from re-entering the reservoir |
| Mechanism | Patient inhales directly from the O₂-filled reservoir bag; side valves allow exhalation to atmosphere |
Key Differences at a Glance
| Parameter | Simple Face Mask | NRBM |
|---|---|---|
| Reservoir bag | ✗ No | ✓ Yes |
| One-way valves | ✗ No | ✓ Yes (×3) |
| FiO₂ range | 40–60% | 60–90%+ |
| Min flow rate | 5 L/min | 10–15 L/min |
| Room air entrainment | Yes (via vent holes) | Minimized (valves block) |
| CO₂ rebreathing | Possible at low flow | Prevented by valve |
| Indication | Mild–moderate hypoxemia | Severe hypoxemia, respiratory distress |
Important Clinical Notes
- The "100% O₂" myth: NRBM does not deliver 100% FiO₂ at standard 15 L/min — it delivers roughly 60–70% because room air still leaks around the imperfect mask seal. True FiO₂ ≥90% requires flow rates of ~45 L/min. (Roberts and Hedges' Clinical Procedures in Emergency Medicine)
- Partial non-rebreather mask: Similar to NRBM but one exhalation valve is often removed — allows a small amount of exhaled gas to re-enter the reservoir. Lower FiO₂ than NRBM.
- In respiratory distress, inspiratory flow increases significantly, which dilutes the delivered FiO₂ for both mask types compared to a resting patient.
- Minimum flow rule: Simple mask must have ≥5 L/min to flush CO₂; NRBM bag must never fully collapse on inspiration.
— Roberts and Hedges' Clinical Procedures in Emergency Medicine; Miller's Anesthesia, 10e
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