What FiO₂ is 4 litres by nasal cannula?

Roughly 37% by the usual estimate of 21% plus 4% per litre. The true value varies with the patient's breathing pattern and can be considerably lower in someone breathing rapidly.

What is the maximum FiO₂ from a nasal cannula?

Around 40 to 44% at 6 L/min. Higher flows through a standard cannula mostly add discomfort and drying rather than concentration.

Why is a venturi mask more accurate?

It entrains air and oxygen in a fixed ratio at a flow that exceeds the patient's inspiratory demand, so the delivered concentration does not change when the breathing pattern does.

Why is the SpO₂ target lower in COPD?

In patients who chronically retain CO₂, generous oxygen can worsen hypercapnia — largely by increasing dead space through reversal of hypoxic pulmonary vasoconstriction. A target of 88 to 92% is commonly used.

Clinical Tool

FiO₂ Estimator

O₂ device → estimated FiO₂

Oxygen delivery device

Flow rate (L/min) — max 6

Estimated FiO₂

29%

≈ 0.29 fraction

Quick Reference

Device	Flow	FiO₂
Nasal Cannula	1–6 L/min	24–44%
Simple Mask	5–10 L/min	35–55%
Partial Rebreather	6–10 L/min	60–80%
Non-Rebreather	≥10–15 L/min	90–100%
Venturi Mask	Variable	24–60% (set)
HFNC	1–60 L/min	21–100% (dial)

▸Nasal cannula rule: Room air (21%) + 4% per L/min. At 6 L/min ≈ 44%.

▸Venturi is the most precise, fixed-FiO₂ device — ideal for COPD patients needing controlled O₂.

About the FiO₂ estimator

Low-flow oxygen devices do not deliver a set oxygen concentration. They add oxygen to the air a patient is already breathing, so the concentration that reaches the alveoli depends as much on the patient as on the flowmeter.

Why the estimate is only an estimate

The common bedside rule adds roughly 4% for every litre per minute by nasal cannula, starting from 21%. It holds only for a patient with a normal, unhurried breathing pattern.

A patient breathing fast and deep entrains more room air around the same fixed oxygen flow, which dilutes it and lowers the delivered concentration — the sickest patients are the ones for whom the estimate is least accurate. Mouth breathing, nasal obstruction and a high inspiratory flow demand all move the real value away from the estimate.

Fixed-performance devices

A venturi mask is a fixed-performance device: it entrains a set ratio of air to oxygen, so the delivered concentration holds even when the breathing pattern changes. That predictability is the reason it is preferred where a specific concentration matters, such as in a patient who retains CO₂ and needs a controlled target.

High-flow nasal cannula achieves something similar by exceeding the patient's own inspiratory flow, so there is little room-air entrainment left to dilute the oxygen.

Titrate to the patient

Because the delivered concentration cannot be known exactly on a low-flow device, oxygen is titrated to a measured target — usually an SpO₂ range — rather than to a calculated FiO₂. In a patient with chronic CO₂ retention that target is deliberately lower, commonly 88 to 92%.

Common questions

What FiO₂ is 4 litres by nasal cannula?: Roughly 37% by the usual estimate of 21% plus 4% per litre. The true value varies with the patient's breathing pattern and can be considerably lower in someone breathing rapidly.
What is the maximum FiO₂ from a nasal cannula?: Around 40 to 44% at 6 L/min. Higher flows through a standard cannula mostly add discomfort and drying rather than concentration.
Why is a venturi mask more accurate?: It entrains air and oxygen in a fixed ratio at a flow that exceeds the patient's inspiratory demand, so the delivered concentration does not change when the breathing pattern does.
Why is the SpO₂ target lower in COPD?: In patients who chronically retain CO₂, generous oxygen can worsen hypercapnia — largely by increasing dead space through reversal of hypoxic pulmonary vasoconstriction. A target of 88 to 92% is commonly used.

Educational reference for respiratory therapy students and clinicians preparing for the NBRC examinations. It supports clinical reasoning and does not replace institutional protocol, a device's instructions for use, or the judgement of the team caring for the patient.