Supplemental oxygen therapy is a common treatment used in emergency and clinical settings for patients with difficulty breathing or low blood oxygen levels. The air we normally breathe contains a fraction of inspired oxygen (FiO2) of approximately 21%. Various delivery devices exist to increase this concentration, ranging from simple nasal cannulas to more complex mask systems. The device chosen depends on the patient’s condition and the required oxygen concentration.
Defining High-Concentration Oxygen Delivery
The Non-Rebreather Mask (NRB) is a specialized device designed for delivering the highest possible concentration of oxygen outside of mechanical ventilation. Although classified as a low-flow device, it provides an FiO2 generally ranging from 60% to 90%. The NRB achieves this high concentration using a face mask, a reservoir bag, and a system of one-way valves.
The reservoir bag connects to the oxygen source and holds pure, 100% oxygen before the patient inhales. A one-way valve between the reservoir bag and the mask ensures the patient draws oxygen only from this reserve. One or two other one-way valves are located on the mask’s side ports. These side valves open during exhalation to allow spent air to escape and close during inhalation, preventing the entrainment of room air that would dilute the concentrated oxygen.
The Required Minimum Flow Rate for NRBs
The operational requirement for a Non-Rebreather Mask is based on the mechanical necessity of the mask, not the patient’s clinical saturation. To function correctly and maintain the intended high FiO2, the NRB requires a substantial flow of oxygen into the reservoir bag. The standard minimum flow rate required for an NRB is set between 10 and 15 Liters per Minute (L/min).
In most acute care settings, 15 L/min is the common setting used to ensure safety and effectiveness. This high flow rate ensures the oxygen supply continuously exceeds the patient’s peak inspiratory flow demand. The flowmeter is adjusted until the reservoir bag remains adequately inflated throughout the breathing cycle. Maintaining this oxygen reserve directly translates to delivering the highest oxygen concentration to the patient.
Why the Reservoir Bag Must Remain Inflated
The need for the reservoir bag to remain inflated is the primary reason for the high oxygen flow rate. The bag serves as a buffer, holding a volume of high-concentration oxygen ready for the patient’s next breath. During inhalation, a patient may draw a large volume of gas that exceeds the flow meter’s setting. If the reservoir bag is not sufficiently full, the patient’s inspiratory effort will collapse the bag.
A collapsed bag forces the patient to draw air from around the mask’s edges, significantly diluting the oxygen concentration and lowering the intended FiO2. The one-way valve system prevents exhaled air from entering the reservoir, directing it out through the side ports. If the flow rate is too low, the fresh oxygen flow may not be sufficient to wash out exhaled carbon dioxide (CO2) from the mask space, leading to the risk of CO2 rebreathing. Therefore, the constant high flow rate ensures the bag is at least two-thirds inflated, maintaining an oxygen reserve and preventing CO2 rebreathing.
Clinical Indicators for Non-Rebreather Mask Use
The Non-Rebreather Mask is reserved for acute, life-threatening situations where the rapid delivery of maximal oxygen concentration is necessary. The primary clinical indication is acute severe hypoxemia, meaning dangerously low blood oxygen levels. Conditions often requiring an NRB include severe trauma, various types of shock, and acute severe respiratory distress.
It is also the preferred device for toxic exposures, such as carbon monoxide poisoning or smoke inhalation, where high FiO2 is needed to displace the toxic gas from the bloodstream. The mask is used as a temporary measure, providing a bridge until the patient stabilizes or transitions to another form of respiratory support. Healthcare providers must be vigilant during NRB use, as reliance on continuous high flow means any interruption to the oxygen source can be immediately life-threatening.