College Answer

Reservoir oxygen mask / non-rebreather or partial rebreather oxygen mask

• • Fresh gas flow attached to reservoir bag and adjusted to ensure bag remains 2/3 full at all times
  • One-way valve between reservoir bag and patient preventing expired gas entering reservoir bag
  • One or two valves on side ports in mask close in inspiration reducing entrainment of room air and open in expiration to prevent rebreathing. (The presence of two valves requires close monitoring of the patient to ensure adequate fresh gas flow from the reservoir bag)
  • FiO2 varies from 60-80% depending on presence of valves on side ports and mask fit

Discussion

That image comes from www.acesurgical.com; no permission whatsoever has been granted for its use, and it remains in place only until somebody complains.

Anyway. The basics:

• The reservoir fills with 100% oxygen
• The patient inhales, entraining the reservoir oxygen from the bag
• One-way valves in the mask prevent the entrainment of room air
• The patient exhales, and the one-way valve prevents expired air from entering the reservoir
• The expired air instead escapes through side-vents and around the sides of the mask
• One optimistic article suggests that NRBMs may be capable of 90% FiO₂ at 10L flow rate.