01Purpose and principlesWhat the treatment does and how it fits into care.
Oxygen is a drug with indication, target, route, response and toxicity. Both hypoxaemia and unnecessary hyperoxaemia can harm.
The saturation target is the stable clinical instruction; device and flow change to achieve it. This avoids leaving a patient on the initial emergency device after physiology improves.
Oxygen-induced hypercapnia is multifactorial, including worsened V/Q matching and the Haldane effect; 'loss of hypoxic drive' is an incomplete explanation and should not justify withholding oxygen.
Pulse oximetry has limitations in poor perfusion, movement, dyshemoglobinaemia and skin-pigmentation-related bias. When the clinical picture and display disagree, check the waveform/site and obtain a blood gas or co-oximetry.
Key points
- Oxygen treats hypoxaemia, not breathlessness. Prescribe a target saturation, device/flow and monitoring plan rather than a fixed dose with no endpoint.
- For most acutely ill adults, target SpO2 94–98%. For known COPD or another risk of hypercapnic respiratory failure, target 88–92% pending blood gases.
- Risk factors for hypercapnia include prior hypercapnic failure, severe COPD, morbid obesity, neuromuscular disease, chest-wall deformity, cystic fibrosis and fixed airflow obstruction in bronchiectasis.
- Critical illness or SpO2 below about 85% usually starts with a reservoir mask at 15 L/min while help and blood gases are obtained; titrate down promptly once reliable data are available.
- A 24% or 28% Venturi mask provides controlled FiO2 and is often the initial device in COPD/hypercapnia risk; use the manufacturer-marked minimum flow, not a guessed universal flow.
- Repeat blood gases after 30–60 minutes in patients at risk of hypercapnic failure, even if the first PaCO2 is normal, and sooner if consciousness or breathing worsens.
- If PaCO2 rises or pH falls, reduce oxygen to the minimum that maintains 88–92% and assess for NIV; never stop oxygen abruptly.
- A simple face mask must run at least 5 L/min to avoid CO2 rebreathing; nasal cannulae provide variable FiO2 and are useful for lower, titratable requirements.
- Carbon monoxide poisoning is an exception: pulse oximetry is unreliable; give 100% oxygen via reservoir mask irrespective of the apparent SpO2 while measuring co-oximetry and obtaining toxicology advice.
- Every transfer needs enough oxygen supply, a functioning delivery system, continued monitoring and a handover of target, device and remaining cylinder time.
02Indications, selection and cautionsWho may benefit, who needs urgent treatment and important alternatives.
Cardiac arrest, shock, major trauma, sepsis with critical illness or severe hypoxaemia needs immediate oxygen and airway assessment before precise titration.
Known COPD, previous NIV/IMV for CO2 retention, oxygen alert card, obesity hypoventilation, neuromuscular or chest-wall disease: start controlled oxygen and obtain gases.
New drowsiness, headache, a rising PaCO2 or falling pH after oxygen escalation requires controlled step-down, repeat gas and ventilatory assessment—not abrupt withdrawal.
Poor waveform, cold/shocked periphery, nail products, movement, carbon monoxide or methaemoglobinaemia can make the number unreliable.
Empty cylinder, disconnected tubing, kinked line, uninflated reservoir bag or wrong Venturi flow can cause sudden desaturation and must be checked at the bedside.
03Assessment before treatmentTests and checks that guide safe selection.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
Pulse oximetry with waveform/signal checkFirst step - Why
- Titrate oxygen and trend response.
- Interpretation and limitations
- Use a reliable trace and interpret with perfusion and clinical signs; a number without signal quality may be false.
- 02
Arterial blood gas - Why
- Measure PaO2, PaCO2 and pH when ventilation failure is possible or precise oxygen assessment is needed.
- Interpretation and limitations
- Record device and FiO2. Rising PaCO2 with falling pH is acute hypercapnic failure and may require NIV.
- 03
Repeat gas 30–60 minutes after oxygen adjustment - Why
- Detect delayed CO2 retention and confirm safe titration.
- Interpretation and limitations
- A normal first PaCO2 does not remove the need for repeat testing in a high-risk patient.
- 04
Co-oximetry - Why
- Measure carboxyhaemoglobin or methaemoglobin when standard oximetry is unreliable.
- Interpretation and limitations
- Do not wait for confirmation to give 100% oxygen in suspected carbon monoxide poisoning.
- 05
Device and oxygen-source check - Why
- Confirm delivered therapy matches the prescription.
- Interpretation and limitations
- Inspect source, connections, humidification where used, flow setting, Venturi entrainment and reservoir inflation; recalculate cylinder duration before transfer.
- 06
Underlying-cause assessment - Why
- Treat why the patient is hypoxaemic.
- Interpretation and limitations
- CXR/ultrasound, ECG and laboratory tests are selected by presentation; oxygen response does not diagnose pneumonia, oedema, PE or pneumothorax.
04Treatment approachPreparation, options, escalation and aftercare.
01Most adultsTarget 94–98%First stepAcutely ill adult without known risk of hypercapnic respiratory failure.+
- 1Prescribe SpO2 94–98% and select the lowest-intensity device expected to achieve it.
- 2If critically ill or SpO2 is below about 85%, start reservoir mask 15 L/min while assessing airway/breathing and obtaining senior help; titrate down once stable.
- 3EscalationRecheck saturation, respiratory effort and cause frequently; if target cannot be maintained, escalate respiratory support rather than simply accepting maximal oxygen.
02CO2 riskTarget 88–92% pending gasesCOPD or another recognised risk factor for hypercapnic respiratory failure.+
- 1Start controlled oxygen—often 24% or 28% Venturi, or 1–2 L/min nasal cannulae—and prescribe 88–92% pending ABG.
- 2Obtain an ABG promptly. If PaCO2 and pH are normal and there is no previous hypercapnic failure/usual low baseline, BTS permits adjustment toward 94–98% with repeat gas.
- 3Repeat ABG after 30–60 minutes regardless of a normal initial PaCO2. If CO2 rises or pH falls, maintain 88–92%, optimise the cause and assess urgently for NIV.
03Excess oxygenControlled step-downHypercapnia/acidosis develops after oxygen therapy or saturation is well above target.+
- 1Reduce FiO2 gradually using a controlled device until SpO2 is 88–92%; do not disconnect oxygen abruptly.
- 2Repeat ABG and clinical assessment promptly; treat bronchospasm, secretions, infection or sedative effect and obtain NIV/critical-care review when acidotic.
- 3Document the event and future target; provide an oxygen alert plan/card when recurrent hypercapnic episodes are established.
04Special exceptionSuspected carbon monoxide poisoningExposure history, multiple affected people, headache/confusion/collapse or soot/fire context.+
- 1Remove from exposure safely and give 100% oxygen by reservoir mask at 15 L/min regardless of pulse-oximeter reading.
- 2Obtain co-oximetry, ECG, blood gas/lactate and toxicology/hyperbaric advice based on symptoms and exposure; assess for cyanide in smoke inhalation when appropriate.
- 3EscalationContinue high-concentration oxygen until specialist advice and clinical/carboxyhaemoglobin response support de-escalation.
05Regimens, contraindications and interactionsTreatment details and the circumstances that modify them.
Medical oxygen—reservoir mask
15 L/min initially in critical illness, severe hypoxaemia or carbon monoxide poisoning; ensure the reservoir bag remains inflated and titrate to the prescribed target when the exception no longer applies.Hyperoxia and CO2 retention when left uncontrolled; fire risk. Positive saturation response does not treat the underlying cause.
Medical oxygen—Venturi mask
Choose the labelled concentration, commonly 24% or 28% for hypercapnia risk, and set at least the flow printed on that manufacturer's valve; increase flow if inspiratory demand collapses the mask flow.Do not assume colours or flow requirements are identical across manufacturers. Facial fit, high minute ventilation and blocked entrainment alter delivery.
Medical oxygen—nasal cannulae
Commonly 1–2 L/min initially in hypercapnia risk or 2–6 L/min in other adults, then titrate to target; delivered FiO2 is variable.Mouth breathing and minute ventilation change FiO2; nasal dryness occurs. Do not use escalating cannula flow as a substitute for senior review in severe failure.
06Complications, monitoring and follow-upAdverse effects, response and longer-term review.
- Record SpO2 target, device, flow/concentration and saturation on the observation chart; change the device to stay within range.
- Check the patient and the whole delivery chain after every unexplained saturation change, not just the oximeter number.
- Repeat ABG after 30–60 minutes in hypercapnia-risk patients and sooner for drowsiness, worsening work of breathing or falling saturation.
- During transfers, monitor continuously when unstable and calculate oxygen supply with a safety margin; hand over cylinder status and target.
- Wean and stop oxygen when stable in the target range on low-dose therapy; recheck on room air and reinstate if saturation falls below target.
07Special situationsVariants, exceptions and circumstances that change the usual approach.
Target, not device, is the prescription
A Venturi valve or 2 L/min is a means to an endpoint; illness and ventilation change, so the device must be titrated.
CO2 retention is not permission for hypoxaemia
Treat severe hypoxaemia immediately, then use gases and controlled step-down to manage hypercapnia safely.
FiO2 estimates need context
Nasal-cannula FiO2 varies with flow, respiratory pattern and mouth breathing; document the actual device and flow with every blood gas.
Carbon monoxide breaks the usual oximeter logic
Standard pulse oximeters cannot distinguish oxyhaemoglobin from carboxyhaemoglobin, so an apparently normal SpO2 is falsely reassuring.
Transfers are oxygen procedures
Interruption or depletion during transport can be fatal; source, duration, connections and monitoring belong in the plan.
08Common pitfallsFrequent interpretation and management errors.
- 01
Writing 'oxygen as required' without a target range.
- 02
Withholding oxygen from a severely hypoxaemic COPD patient because of fear of CO2 retention.
- 03
Leaving a reservoir mask at 15 L/min after the emergency has stabilised without reassessment.
- 04
Stopping oxygen abruptly when hypercapnia develops.
- 05
Running a simple face mask below 5 L/min or guessing a Venturi valve's required flow.
- 06
Trusting a normal pulse-oximeter reading in suspected carbon monoxide poisoning.