The nasal cannula (NC) is a device used to deliver supplemental oxygen or increased airflow to a patient or person in need of respiratory help. This device consists of a lightweight tube which on one end splits into two prongs which are placed in the nostrils and from which a mixture of air and oxygen flows. The other end of the tube is connected to an oxygen supply such as a portable oxygen generator, or a wall connection in a hospital via a flowmeter. The cannula is generally attached to the patient by way of the tube hooking around the patient’s ears or by an elastic headband.

A nasal cannula is generally used wherever small amounts of supplemental oxygen are required, without rigid control of respiration, such as in oxygen therapy. Most cannulae can only provide oxygen at low flow rates—up to 5 litres per minute (L/min)—delivering an oxygen concentration of 28–44%. Rates above 5 L/min can result in discomfort to the patient, drying of the nasal passages, and possibly nose bleeds (epistaxis). Also with flow rates above 6 L/min, the laminar flow becomes turbulent and the oxygen therapy being delivered is only as effective as delivering 5–6 L/min.

The nasal cannula is often used in elderly patients or patients who can benefit from oxygen therapy but do not require it to self respirate. These patients do not need oxygen to the degree of wearing a non-rebreather mask. It is especially useful in those patients where vasoconstriction could negatively impact their condition, such as those suffering from strokes.

A nasal cannula may also be used by pilots and passengers in small, unpressurized aircraft that do not exceed certain altitudes. The cannula provides extra oxygen to compensate for the lower oxygen content available for breathing at the low ambient air pressures of high altitude, preventing hypoxia. Special aviation cannula systems are manufactured for this purpose.

A non-rebreather mask (NRB, non-rebreather, non-rebreather facemask, etc.) is a device used in medicine to assist in the delivery of oxygen therapy. A NRB requires that the patient can breathe unassisted, but unlike low-flow a nasal cannula, 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 (i.e. the oxygen cylinder being drained completely) is life threatening.

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 (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% – 90% 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 (i.e. the oxygen cylinder being drained completely), 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 (from decades of clinical observation) do not provide a good seal with an individual patient’s face, thus permitting the inflow of large amounts of ambient air (most air follows the path of least resistance), and diluting the oxygen provided.

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, cluster headache, smoke inhalation, and carbon monoxide poisoning, or any other patients who require high-concentration 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 invasive or noninvasive mechanical ventilation.