Introduction
Oxygen is a drug — it must be prescribed, documented, and monitored like any other medication. Inappropriate oxygen therapy is a well-documented cause of harm: under-treatment risks hypoxic organ damage, whilst over-treatment in susceptible patients can precipitate hypercapnic respiratory failure. Oxygen prescribing is a high-yield OSCE station that integrates clinical knowledge, safe prescribing principles, and practical skills.
Oxygen Is a Drug
In the UK, oxygen must be prescribed on the drug chart (or electronic prescribing system) except in emergencies where it can be administered without a prescription. The British Thoracic Society (BTS) guidelines form the basis of safe practice. Always document:
- Delivery device (mask type or nasal cannulae)
- Flow rate (litres per minute)
- Target saturation range
- Prescriber name, signature, date and time
🧠 Mnemonic
VITAL — Safe Oxygen Prescribing
V — Verify the indication and confirm hypoxaemia
I — Identify patients at risk of hypercapnic respiratory failure (COPD, obesity hypoventilation, chest wall deformity)
T — Target saturations: 94–98% for most patients; 88–92% for at-risk patients
A — Appropriate delivery device for the required FiO₂
L — Label correctly on the drug chart with device, flow rate, and target range
Target Saturations
| Patient Group | Target SpO₂ | Rationale |
|---|---|---|
| Most acutely ill patients | 94–98% | Standard BTS target — prevents hypoxaemia without hyperoxia |
| COPD (known or suspected type 2 RF) | 88–92% | High O₂ may worsen CO₂ retention |
| Acute MI or stroke | 94–98% | Hyperoxia harmful in normoxic patients — only treat if SpO₂ <94% |
| CO poisoning | As high as possible (100%) | CO displaces O₂ — pulse oximetry unreliable |
| Premature neonates | 91–95% | Retinopathy of prematurity risk |
| Palliative / breathlessness | Comfort-based | Target saturation may be less relevant than symptom relief |
⚠️ Red Flag
Do not assume all patients with COPD need a low oxygen target. Only patients with confirmed or suspected type 2 respiratory failure (hypercapnia on ABG) should have a target of 88–92%. If in doubt, start at 94–98% and obtain an ABG within 30–60 minutes to guide ongoing therapy.
Oxygen Delivery Devices
Nasal Cannulae
- Flow rate: 1–6 L/min
- Approximate FiO₂: 24–44% (1 L/min ≈ 24%, each additional litre adds approximately 4%)
- Use: mild hypoxaemia, chronic supplemental oxygen, patients who need to eat, drink, or communicate easily
- Limitations: FiO₂ variable with breathing pattern; dries nasal mucosa above 4 L/min
Simple Face Mask
- Flow rate: 5–10 L/min (minimum 5 L/min to flush CO₂ from reservoir)
- Approximate FiO₂: 35–55%
- Use: moderate hypoxaemia not requiring precise FiO₂
- Limitations: imprecise FiO₂, poorly tolerated, interferes with eating and communication
Non-Rebreather Mask (NRM)
- Flow rate: 10–15 L/min
- Approximate FiO₂: 60–90%
- Use: emergency oxygen in critically unwell patients — major haemorrhage, shock, post-cardiac arrest, severe sepsis, CO poisoning
- Important: ensure the reservoir bag is inflated before applying to the patient
💎 Clinical Pearl
The non-rebreather mask is the emergency oxygen mask — reach for it first in any critically unwell patient with SpO₂ <94% who is not known to have type 2 respiratory failure. You can always titrate down once an ABG confirms CO₂ retention.
Venturi Mask (Controlled Oxygen Therapy)
The Venturi mask delivers a precise, fixed FiO₂ regardless of respiratory rate or tidal volume — this is the device of choice for patients needing controlled oxygen therapy.
| Venturi Colour | FiO₂ | Flow Rate Required |
|---|---|---|
| Blue | 24% | 2 L/min |
| White | 28% | 4 L/min |
| Yellow | 35% | 8 L/min |
| Red | 40% | 10 L/min |
| Green | 60% | 15 L/min |
💡 Tip
Memorise the Venturi colours with "Big White Yaks Run Green" — Blue 24%, White 28%, Yellow 35%, Red 40%, Green 60%. In the OSCE, state the colour, the FiO₂, and the required flow rate when prescribing.
The Hypercapnic Drive — Common OSCE Misconception
⚠️ Red Flag
The hypoxic drive explanation is a simplification. In chronic type 2 respiratory failure, patients' CO₂ receptors become desensitised and peripheral hypoxic chemoreceptors contribute more to respiratory drive. High-flow oxygen abolishes this hypoxic stimulus, leads to increased CO₂ retention (Haldane effect and V/Q mismatch), and worsens hypercapnia. The harm is real, but the mechanism is more complex than commonly taught.
How to Prescribe Oxygen on a Drug Chart
When completing the oxygen section in an OSCE:
- 1Drug name: Oxygen
- 2Dose/flow rate: e.g. 2 L/min (or 10–15 L/min in emergency)
- 3Route: Inhalation
- 4Device: e.g. Nasal cannulae / Venturi 28% / Non-rebreather mask
- 5Target SpO₂: e.g. 88–92% OR 94–98%
- 6Frequency: Continuous
- 7Signature, date, time, bleep/GMC number
💡 Tip
Stating *"I am prescribing oxygen because the patient has SpO₂ of X% and I am targeting 94–98% using a Y device at Z litres per minute, and I will reassess in 15–30 minutes with a repeat SpO₂"* demonstrates clinical reasoning that separates a pass from a high pass.
Monitoring After Prescribing
- Repeat SpO₂ after 5–10 minutes to confirm target range achieved
- Obtain an ABG within 30–60 minutes in any patient who may have CO₂ retention
- If PaCO₂ is rising on ABG, reduce FiO₂ or consider NIV — seek respiratory team input
- Document response and reassessment in the notes
FAQs
"Should I prescribe oxygen to all patients who feel breathless?"
No. Breathlessness without hypoxaemia (SpO₂ ≥94%) does not benefit from supplemental oxygen — in normoxic patients with MI, oxygen may even be harmful. Always check the SpO₂ before prescribing. Treat the breathlessness symptomatically if the saturation is normal.
"Can I give a patient with COPD oxygen in an emergency without worrying about hypercapnic drive?"
Yes — always prioritise acute hypoxaemia first. In a crashing COPD patient with SpO₂ <88%, commence controlled oxygen via Venturi mask at 24–28%, obtain an ABG urgently, and involve the respiratory team for possible NIV. The risk of acute hypoxia outweighs the risk of CO₂ retention in the immediate term — but titrate down as soon as possible.
"What flow rate must I use with a simple face mask?"
Always use at least 5 L/min with a simple face mask. Below this, the mask's dead space fills with exhaled CO₂ and the patient rebreathes it, causing hypercapnia. This is a common error and an OSCE mark-losing mistake.
"How is a Venturi mask different from a simple mask?"
A Venturi mask uses the Bernoulli principle — high-velocity oxygen flow through a narrow jet entrains a fixed proportion of room air, delivering a precise FiO₂ regardless of the patient's breathing pattern. A simple mask's FiO₂ varies with tidal volume and respiratory rate, making it imprecise. Use Venturi masks whenever accurate FiO₂ control matters.
"What should I do if a patient's SpO₂ drops despite maximum oxygen therapy?"
Escalate immediately using SBAR. Reassess the patient with an ABCDE approach, ensure the device is correctly applied and the reservoir bag inflated, obtain an urgent ABG, contact the registrar or critical care outreach, and consider whether NIV, high-flow nasal oxygen, or intubation is needed. Document your actions and the time.
Related Posts
- A–E Assessment OSCE — using the ABCDE framework to guide oxygen therapy in the acutely unwell patient
- Prescribing Safety OSCE — drug chart completion, common prescribing errors, and safe documentation
- Blood Results Interpretation OSCE — interpreting ABG in the context of oxygen therapy and respiratory failure