Supplemental oxygen is one of the most commonly prescribed treatments in medical care. At any given time, up to a quarter of hospitalized patients receive oxygen therapy and data from the United Kingdom suggest that 34% of all ambulance transports involve the delivery of oxygen.

Although guidelines suggest that oxygen is not a treatment for breathlessness in the absence of hypoxaemia, in a Victorian Emergency Department audit, the most common reasons for oxygen administration were shortness of breath, chest pain and hypoxaemia (in that order). The efficacy of oxygen therapy depends on the fraction of inspired oxygen (FiO2), which in turn depends on the oxygen supply and delivery method. Low-flow oxygen delivery systems deliver oxygen at flow rates below the patient's inspiratory flow rate, entrain room air and provide a variable FiO2.

Nasal cannulae-delivering (low) flow rates of 2–4 L/min or more are provided to patients almost automatically in a range of common clinical situations, without the oxygen even necessarily being ‘prescribed’. Rule of thumb suggests that for patients with a normal rate and depth of breathing, each litre per minute increase of nasal oxygen increases the fraction of inspired oxygen (FiO2) by approximately 4%. ...