This has appeared in Question 15.3 from the first paper of 2012, and in Question 22.1 from the first paper of 2010. In brief, it is a plastic mask with a bag acting as an oxygen reservoir, which is held affixed to the patient's face with elastic. The defining feature of this device is the presence of unidirectional valves. When the patient inhales, the valves prevent the inspiration of room air - the patient will only breathe from the oxygen reservoir. When the patient exhales these valves prevent the movement of expired gas back into the reservoir, so as to prevent the re-breathing of expired gas (hence "non-rebreather").
- 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.
The actual college description is as follows:
- 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