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Kit and numbers
🫁 Oxygen delivery
Device, flow rate, rough FiO₂ — and which ones are actually fixed
Oxygen is a drug and it is prescribed — to a target saturation range, not to a flow rate. 94–98% for most people; 88–92% for those at risk of hypercapnic respiratory failure.
The percentages below are approximate for every device except the Venturi. A variable-performance device delivers whatever the patient's own inspiratory flow dilutes it to, so the same 4 L/min gives different oxygen to a calm patient and a breathless one.
Variable performance — FiO₂ is an estimate
| Nasal cannulae | 1–6 L/min · ~24–45%Comfortable, allows eating and talking. Above 4 L/min it dries the nose and is not better tolerated. |
|---|---|
| Simple face mask | 5–10 L/min · ~40–60%Never below 5 L/min — the mask becomes a rebreathing space and CO₂ accumulates. |
| Non-rebreathe reservoir | 15 L/min · ~60–90%The emergency mask. Inflate the bag before applying. Reassess quickly — it is a holding measure, not a plan. |
Fixed performance — Venturi, colour-coded
| Blue | 24% · 2–4 L/minThe starting point in known or suspected CO₂ retention. |
|---|---|
| White | 28% · 4–6 L/min |
| Yellow | 35% · 8–10 L/min |
| Red | 40% · 10–12 L/min |
| Green | 60% · 12–15 L/min |
Targets
| Most acutely unwell adults | 94–98% |
|---|---|
| At risk of hypercapnia | 88–92%COPD, obesity hypoventilation, chest wall or neuromuscular disease, cystic fibrosis, bronchiectasis. |
| Critical illness | Highest available firstCardiac arrest, shock, sepsis, major trauma, anaphylaxis — reservoir mask now, titrate down once stable. |
The 88–92% target is not a reason to withhold oxygen from someone who is hypoxic. Hypoxia kills faster than hypercapnia. Give what is needed, then get a blood gas and titrate — do not leave a patient at 78% because they have COPD.
The Venturi flow printed on the valve is the minimum. A patient breathing hard needs more than the stated flow to keep the delivered percentage fixed, which is the whole point of the device.
Source: BTS Guideline for oxygen use in adults in healthcare and emergency settings (2017)