Low-Flow Oxygen Devices

A nasal cannula at 6 L/min and a simple face mask at 6 L/min deliver very different FiO2 levels — knowing the math behind each device prevents both under-oxygenation and CO2 retention.

Low-flow oxygen devices deliver oxygen at rates below the patient's total inspiratory demand, so the actual FiO2 varies with respiratory rate and tidal volume because room air mixes in with each breath. The nasal cannula is the workhorse: each liter per minute adds roughly 4% to the baseline 21% room air, giving a range of approximately 24–44% FiO2 at 1–6 L/min. Flow above 6 L/min dries and damages nasal mucosa without meaningfully raising FiO2. A simple face mask requires 5–8 L/min (never below 5 — exhaled CO2 accumulates in the mask) and delivers 40–60% FiO2. A partial rebreather mask (6–15 L/min, 60–75% FiO2) has a reservoir bag that should never fully deflate during inspiration; keep it at least one-third inflated. A nonrebreather mask (10–15 L/min, 80–95% FiO2) has one-way valves preventing room air entrainment and is the highest-FiO2 low-flow device — used for emergencies like carbon monoxide poisoning. Humidification is required at flows above 4 L/min to prevent mucosal drying.

Key Distinctions

Don't confuse low-flow devices (variable FiO2, depends on breathing pattern) with high-flow devices like a Venturi mask (precise, fixed FiO2) — this is the defining distinction, not the liter flow number. Students mix up the nonrebreather with the partial rebreather: the nonrebreather has one-way valves and delivers up to 80–95% FiO2; the partial rebreather lacks those valves. Never run a simple mask below 5 L/min — below that threshold it becomes a CO2 trap, not an oxygen source.

Clinical Pearl

4-4-4 rule for nasal cannula: each 1 L/min adds ~4% FiO2, max 6 L/min, humidify above 4 L/min.