A non-rebreather mask is a device used in medicine to assist in the delivery of oxygen therapy. An NRB requires that the patient can breathe unassisted, but unlike low-flow nasal cannulae, the NRB allows for the delivery of higher concentrations of oxygen. An ideal non-rebreather mask does not permit air from the surrounding environment to be inhaled, hence an event of a source gas failure is life threatening.
Design
The non-rebreather mask covers both the nose and mouth of the patient and attaches with the use of an elastic cord around the patient's head. The NRB has an attached reservoir bag, typically one liter, that connects to an external oxygen tank or bulk oxygen supply system. Before an NRB is placed on the patient, the reservoir bag is inflated to greater than two-thirds full of oxygen, at a rate of 15 liters per minute. Approximately ¹⁄₃ of the air from the reservoir is depleted as the patient inhales, and it is then replaced by the flow from the O2 supply. If the bag becomes completely deflated, the patient will no longer have a source of air to breathe. Exhaled air is directed through a one-way valve in the mask, which prevents the inhalation of room air and the re-inhalation of exhaled air. The valve, along with a sufficient seal around the patient's nose and mouth, allows for the administration of high concentrations of oxygen, approximately 60% - 80% O2. Many textbooks report higher oxygen concentrations, however formal studies reporting these levels are not referenced to research. The patient must partially deflate the reservoir bag during inspiration or the high oxygen concentration will not be achieved, and the mask will provide only the flow rate setting on the flowmeter. Ideally, a non-rebreather mask would not permit air from the surrounding environment to be inhaled. However, due to safety concerns regarding anti-suffocation protection in the event of a source gas failure, one of the two one-way valves is normally removed, allowing inhalation of outside air to a significant degree. However, as almost all non-rebreathing masks are disposable, and manufactured in one adult size, most do not provide a good seal with an individual patient's face, thus permitting the inflow of large amounts of ambient air, and diluting the oxygen provided. Hence, very few patients receive anything close to 100% oxygen. Very high flows from the oxygen flowmeter are required to partially overcome room air dilution. Further, the larger the patient's inspiratory flow rate, the greater the dilution from air. Very little effort is required by most patients, to inspire at flow rates in excess of 50 LPM. Partial rebreather masks are designed to capture the first 150ml of the exhaled breath into the reservoir bag for inhalation during the subsequent breath. This portion of the breath was initially delivered at the end of inhalation and was therefore delivered to the "deadspace" anatomy where gas exchange did not occur. Therefore, there would be no depletion of oxygen nor gain of carbon dioxide during the rebreathing component.
