SEPT 10 Built from the handwritten pages you uploaded on 10 September 2026.
Oxygen delivery devices
Fixed versus variable performance, the Venturi colours, and why the numbers on a nasal cannula are only ever approximate.
The governing principle
Peak inspiratory flow in a breathless adult is 30–60 L/min — far higher than any low-flow device delivers.
A device that delivers less than peak inspiratory flow is variable performance: the patient entrains room air to make up the difference, so the delivered FiO₂ falls as their respiratory effort rises.
A device that delivers more than peak inspiratory flow is fixed performance: the FiO₂ is what the device says, regardless of the patient's breathing.
Nasal cannulae — variable performance
Flow (L/min)
Approximate FiO₂
1
25%
2
29%
3
33%
4
37%
5
41%
6
45%
Roughly 4% per litre. Above 6 L/min the nose dries painfully without humidification, and the FiO₂ gains little. The patient can eat, drink and talk — the main advantage.
Simple face mask — variable performance
Flow (L/min)
Approximate FiO₂
6
35%
7
41%
8
47%
9
53%
10
60%
Never run a simple face mask below 6 L/min. The mask body is a reservoir of exhaled gas — below 6 L/min the flow is insufficient to flush it, and the patient rebreathes CO₂.
Non-rebreathing (reservoir) mask
Flow 10–15 L/min, delivering roughly 80–100% in practice.
A reservoir bag plus one-way flaps that prevent entrainment of room air on inspiration and prevent exhaled gas entering the bag.
Realistic delivered FiO₂ depends on the flaps: both flaps removed 80–85% · one flap removed 85–90% · both flaps in place 95–100%.
Ensure the reservoir bag stays inflated throughout inspiration — if it collapses, the flow is too low.
Venturi masks — fixed performance
Air is entrained in a fixed ratio through a jet, so total flow exceeds peak inspiratory flow and the FiO₂ is genuinely fixed.
Colour
FiO₂
O₂ flow
Blue
24%
2 L/min
White
28%
4 L/min
Orange
31%
6 L/min
Yellow
35%
8 L/min
Red
40%
10 L/min
Green
60%
15 L/min
Two points that get examined
The entrainment ratio falls as FiO₂ rises — the 24% valve entrains about 25 volumes of air per volume of oxygen, the 60% valve only about 1. So at 60% the total flow may no longer exceed peak inspiratory flow, and the "fixed performance" claim weakens at the top of the range.
Use a Venturi in the patient at risk of hypercapnic respiratory failure (target 88–92%), where a predictable FiO₂ matters more than a high one.
Quick recall
4% per litre nasal cannula
Never <6 L simple mask
Reservoir 10–15 L, 80–100%
Blue 24 White 28 · Orange 31
Yellow 35 Red 40 · Green 60
88–92% CO₂ retainer target
Built from your oxygen delivery equipment and Venturi valve tables.