A patient on a standard nasal cannula at 4 L/min oxygen has an approximate inspired oxygen concentration of:

At 4 L/min via nasal cannula the approximate FiO2 is 36%, derived from the rule that each litre per minute above room air adds roughly 3 to 4% FiO2 to the baseline of 21%.

The rule that settles the answer

Room air contains 21% oxygen. A nasal cannula is a low-flow, variable-performance device, meaning the exact fraction of inspired oxygen (FiO2) delivered depends on the flow rate set on the flowmeter and on the patient's own breathing pattern. The standard teaching rule is that each 1 L/min increment in oxygen flow raises the FiO2 by approximately 3 to 4% above the 21% baseline. At 1 L/min this gives roughly 24 to 25%, at 2 L/min roughly 28 to 29%, at 3 L/min roughly 32 to 33%, and at 4 L/min roughly 36 to 37%. The figure 36% is the one most commonly cited in textbooks and the one expected on postgraduate entrance examinations.

The calculation is linear only as a rough guide. The true FiO2 at any given flow is not measured at the prongs but determined inside the oropharynx and trachea, where oxygen mixes with room air drawn in around the cannula and through the mouth. This is why the device is called variable performance: the same flow setting delivers different FiO2 values in different patients, and even in the same patient at different tidal volumes.

The upper practical limit of a standard nasal cannula is 6 L/min. Beyond this flow the oxygen jet causes mucosal drying and patient discomfort without a proportional gain in FiO2, and a face mask or high-flow device should be considered instead.

Why the delivered FiO2 is only an estimate

A nasal cannula does not seal the airway. During inspiration the patient draws in oxygen from the cannula and simultaneously entrains room air through the mouth and around the prongs. The ratio of oxygen to entrained air depends on the patient's peak inspiratory flow rate, which in a resting adult is roughly 30 L/min but can rise to 100 L/min or more during respiratory distress. The oxygen flow from the cannula, even at 6 L/min, is a small fraction of the total inspiratory flow, so the FiO2 is heavily diluted.

This is the defining feature of a variable-performance device: the FiO2 is not fixed by the equipment but varies with the patient's minute ventilation, tidal volume, respiratory rate, and inspiratory-to-expiratory ratio. A patient breathing at 30 breaths per minute with a small tidal volume will entrain proportionally more room air and receive a lower FiO2 than a patient breathing slowly and deeply at the same flow setting. By contrast, a fixed-performance device such as a Venturi mask uses the Bernoulli principle to entrain a precise volume of room air, delivering a set FiO2 regardless of the patient's breathing pattern.

The anatomical reservoir also matters. Oxygen from the cannula fills the nasopharynx, oropharynx, and hypopharynx during expiration. This anatomic reservoir of roughly 50 mL, combined with the volume of the nasal and oral cavities, provides a small store of oxygen that is inhaled first at the start of the next breath. Mouth breathing bypasses much of this reservoir, which is why patients who breathe through their mouth receive a lower FiO2 than the flow setting suggests.

Clinical application and when to escalate

The nasal cannula is the default low-flow device for patients who need modest oxygen supplementation, roughly 24 to 44% FiO2. It is well tolerated for prolonged use, allows eating and speaking, and is appropriate for stable patients with mild hypoxaemia, for example a patient with pneumonia whose SpO2 is 89% on room air. It is also the device of choice for patients with chronic type 2 respiratory failure who rely on hypoxic drive, because it allows titration to a target SpO2 of 88 to 92% without the risk of delivering an uncontrolled high FiO2.

The cannula is inadequate when the required FiO2 exceeds 44%, when the patient's peak inspiratory flow far exceeds the delivered flow, or when precise FiO2 control is needed. A simple face mask at 5 to 10 L/min delivers roughly 40 to 60% FiO2 but requires a minimum flow of 5 L/min to flush exhaled carbon dioxide from the mask reservoir and prevent rebreathing. A non-rebreather mask with a reservoir bag at 10 to 15 L/min can deliver 60 to 90% FiO2, though in practice the seal is rarely perfect and the actual FiO2 is lower. For patients who need both high FiO2 and precise control, a Venturi mask or high-flow nasal cannula is the appropriate escalation.

How this topic behaves in the exam

Postgraduate entrance questions on oxygen delivery devices test three things: the FiO2 at a given flow rate, the classification of devices as fixed or variable performance, and the clinical scenario that dictates device choice. The most common format gives a flow rate and asks for the FiO2, exactly as in this question. The second most common format gives a clinical vignette, for example a patient with COPD and chronic hypercapnia, and asks which device is safest, with the answer being a Venturi mask or a low-flow nasal cannula titrated to SpO2 88 to 92%.

A frequently tested comparison is between a nasal cannula and a simple face mask. Students are expected to know that a nasal cannula at 2 L/min and a simple face mask at 5 L/min both deliver roughly 28 to 30% FiO2, but the mask requires the higher minimum flow to prevent CO2 rebreathing. Another common question asks which device delivers a fixed FiO2, with the Venturi mask being the correct answer because it uses colour-coded valves that entrain a set air-to-oxygen ratio.

The rule of 3 to 4% per litre is the single most useful fact to memorise. Some sources use 3% per litre and others use 4%, which is why the exam options are spaced far enough apart that either calculation points to the same answer. At 4 L/min, 21 plus (4 times 3) gives 33% and 21 plus (4 times 4) gives 37%, so 36% is the only plausible option among the choices given.

Why the other options fail

One-glance recall table

Mnemonics

What the exam actually asks

• The FiO2 at 1, 2, 3, 4, 5, and 6 L/min via nasal cannula is asked directly in almost every anaesthesia and medicine paper; memorise the sequence 24, 28, 32, 36, 40, 44.
• Questions that ask which device delivers a fixed FiO2 are answered with the Venturi mask; questions that ask which device is safest for a COPD patient with CO2 retention are answered with a Venturi mask or a low-flow nasal cannula titrated to SpO2 88 to 92%.
• A simple face mask requires a minimum flow of 5 L/min to prevent CO2 rebreathing; a flow below 5 L/min is a distractor in mask selection questions.
• High-flow nasal cannula is classified as a fixed-performance device because it meets or exceeds the patient's peak inspiratory flow, eliminating room air entrainment; this distinction is tested in advanced questions.

Traps that cost marks

• Confusing the FiO2 at 2 L/min (28%) with the FiO2 at 4 L/min (36%); the exam deliberately places the 28% option next to the correct answer to catch this error.
• Assuming the nasal cannula delivers a fixed FiO2; it is a variable-performance device and the stated FiO2 is only an approximation that changes with the patient's breathing pattern.
• Selecting 50% because it is a round number associated with face masks; the device in the stem is a nasal cannula, not a mask, and the FiO2 range is different.

Frequently asked

Why does a nasal cannula deliver a variable FiO2?

A nasal cannula does not seal the airway, so the patient entrains room air around the prongs and through the mouth during inspiration. The ratio of delivered oxygen to entrained room air depends on the patient's peak inspiratory flow, tidal volume, and respiratory rate. Because these vary between patients and within the same patient over time, the FiO2 is not fixed by the flow setting alone. This is the definition of a variable-performance device.

What is the maximum FiO2 achievable with a nasal cannula?

The maximum practical FiO2 is approximately 44 to 45% at 6 L/min. Beyond 6 L/min the oxygen jet causes nasal mucosal drying and discomfort without a meaningful increase in FiO2. If a higher FiO2 is required, the next step is a simple face mask, a non-rebreather mask, or a high-flow nasal cannula, depending on the clinical situation and the need for precise FiO2 control.

How does a Venturi mask differ from a nasal cannula in terms of FiO2 delivery?

A Venturi mask is a fixed-performance device that uses the Bernoulli principle to entrain a precise volume of room air for each litre of oxygen flow. The air-to-oxygen ratio is set by colour-coded valves, each of which delivers a specific FiO2, commonly 24%, 28%, 31%, 35%, 40%, or 50%. The delivered FiO2 remains constant regardless of the patient's breathing pattern, which is why the Venturi mask is preferred when precise oxygen control is needed, as in patients with chronic obstructive pulmonary disease and CO2 retention.

References

• Miller's Anesthesia, 9th. oxygen delivery systems and respiratory monitoring
• Morgan and Mikhail's Clinical Anesthesiology, 6th. oxygen delivery devices and airway management
• West's Respiratory Physiology, 10th. oxygen transport and the effects of supplemental oxygen