In oxygen therapy, what determines the choice between a nasal cannula and a face mask?

• A. The required FiO2 and oxygen flow as prescribed
• B. Patient preference only
• C. Availability of equipment only
• D. Time of day

Answer: (A)

The device is chosen based on the prescribed amount of oxygen the patient needs and the flow required to deliver it. If the order calls for a relatively low to moderate fraction of inspired oxygen (FiO2) and a modest flow, a nasal cannula is comfortable and sufficient. It typically delivers about 24–44% FiO2 at roughly 1–6 L/min, though the exact delivery depends on the patient’s breathing pattern and canal fit. When a higher FiO2 is necessary, or when more reliable delivery is needed, a simple face mask is used because it can provide a greater FiO2 at higher flow rates (around 6–10 L/min, meaning roughly 35–60% FiO2). In some cases, specialized devices (like Venturi masks) are used for precise FiO2 control, but the common principle remains: the prescribed FiO2 and flow determine which device to use, not patient preference, equipment availability, or time of day.