A non-rebreather mask, or NRB, is a device used in medical emergencies that require oxygen therapy. An NRB requires that the patient can breathe unassisted, but unlike low flow nasal cannula, the NRB allows for the delivery of higher concentrations of oxygen.
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 1 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 (lpm). 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. Please note: 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 liter flow rate setting on the flowmeter.
Ideally, a non-rebreather mask would not permit air from the surrounding environment to be inhaled. However, safety concerns regarding anti-suffocation protection in the event of a source gas failure (i.e. the oxygen cylinder allowed to drain completely) one of the two one-way valves are normally removed, allowing inhalation of outside air to a significant degree.
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.
The non-rebreather mask is utilized for patients with physical trauma, chronic airway limitation/chronic obstructive pulmonary diseases, cluster headache, smoke inhalation, and carbon monoxide poisoning, or any other patients who require high-flow oxygen, but do not require breathing assistance. Patients uncomfortable with having a mask on their face, such as those with claustrophobia, or patients with injuries to the mouth are more likely to benefit from a nasal cannula, or passive ("blow-by") oxygen. Patients who are unable to breathe on their own would require an active breathing device, such as a bag valve mask or endotracheal tube.
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