CVR Physiotherapy › Technique cards › Oxygen and aerosols
Oxygen therapy: devices, targets and hazards
Oxygen was asked as low-flow oxygen therapy (S20, 7 marks), methods of delivery (S23), hazards (S25) and hypoxic drive (W25). Learn the device ladder with flows and approximate FiO₂, the two target ranges (94–98% and 88–92%), and the hazards.
- Syllabus
- Practical: medication delivery including oxygen; Ch 9(b) ICU monitoring
- Priority
- Moderate. In 4 of 20 papers (2016–2026), 3 of them since 2023.
- Asked as
- 7m S20 3m S23S25W25
- Usual split
- 7-mark low-flow oxygen therapy; 3-mark methods of delivery or hazards
- Checked against
- BTS guideline for oxygen use in healthcare and emergency settings (2017, with later updates); ATS home oxygen guideline (2020). Full list under Sources.
- Content checked
- 19 October 2026. Next review due October 2027.
- My revision
-
1What oxygen is for
Oxygen is a drug given to correct or prevent hypoxaemia (low oxygen in the blood). It does not treat breathlessness itself, and it should be prescribed with a target saturation.
| Patient | Target |
|---|---|
| Most acutely ill adults | 94–98% |
| At risk of high CO₂ (COPD, severe obesity, chest wall deformity, neuromuscular disease, cystic fibrosis with high CO₂) | 88–92%, adjusted with blood gases |
| Critically ill, peri-arrest, carbon monoxide poisoning | High concentration first (reservoir mask 15 L/min), then titrate |
- Indications: low SpO₂ in acute illness (pneumonia, asthma, heart failure, pulmonary embolism, after surgery); critical illness; carbon monoxide poisoning; desaturation on exercise; long-term oxygen in chronic hypoxaemia (criteria on the COPD page).
- Not routinely for breathless patients with normal saturation, or in heart attack when SpO₂ is 90% or more.
2Methods of delivery (3 marks in S23)
Devices are grouped by whether they supply all the air the patient breathes in. Low-flow (variable-performance) devices give less than the patient's inspiratory flow, so room air mixes in and the FiO₂ changes with the breathing pattern. High-flow (fixed-performance) devices supply all of it, so the FiO₂ stays fixed.
| Device | Flow | Approximate FiO₂ | Points |
|---|---|---|---|
| Nasal cannula (prongs) | 1–4 L/min (up to 6) | About 24–40% (roughly 4% more per litre) | Comfortable; can eat and talk; FiO₂ falls with fast or mouth breathing |
| Simple face mask | 5–10 L/min | About 35–60% | Never below 5 L/min (CO₂ is rebreathed); not for patients at risk of high CO₂ |
| Reservoir (non-rebreather) mask | 15 L/min, bag kept inflated | About 60–90% | Emergencies, critical illness, carbon monoxide |
| Venturi mask (fixed performance) | The flow printed on each colour-coded valve | Fixed: 24, 28, 31, 35, 40 or 60% | Best for patients at risk of high CO₂ (start 24% or 28%); raise the flow by half if breathing faster than 30 |
| High-flow nasal cannula | Up to about 60 L/min, heated and humidified | 21–100%, set | Comfortable; gives a little PEEP and washes out CO₂ from the upper airway; type 1 respiratory failure, after extubation |
| Tracheostomy mask or T-piece | Humidified flow | Set by the system | Patients with a tracheostomy or tube breathing on their own |
| Head box, incubator | Set flow | Measured by an analyser | Newborns |
| NIV, CPAP, ventilator | Oxygen blended into the circuit | 21–100%, set | With respiratory support |
FiO₂ figures for low-flow devices are approximate and vary between sources, because they depend on the patient's breathing. Sources of oxygen: piped wall supply, cylinders, oxygen concentrators at home.
3Low-flow oxygen therapy (7 marks in S20)
- Definition: oxygen given at a flow lower than the patient's own inspiratory flow, so the gas breathed is a mix of oxygen and room air and the FiO₂ varies.
- Devices: nasal cannula, simple face mask, reservoir mask, tracheostomy mask (oxygen catheter in older texts).
- Uses: mild to moderate hypoxaemia in stable patients; home and ambulatory oxygen; during exercise in patients who desaturate.
- Advantages: simple, cheap, comfortable (cannula), patient can eat, talk and walk.
- Disadvantages: FiO₂ is not known exactly (higher with slow, shallow breathing, lower with fast breathing), so it is less suitable when the CO₂ level must be controlled precisely; nasal dryness at higher flows.
- Humidification: not needed for low-flow oxygen; consider it for high flows used for more than about 24 hours, for a tracheostomy, or if the nose and mouth feel dry.
- Monitoring: SpO₂ against the prescribed target, breathing rate, drowsiness (rising CO₂), ABG in those at risk.
4Hazards of oxygen therapy (3 marks in S25)
| Hazard | Details |
|---|---|
| CO₂ retention (hypercapnic respiratory failure) | In COPD and other at-risk patients, uncontrolled high oxygen raises CO₂ (worse V/Q matching and the Haldane effect; reduced drive plays a small part): drowsiness, confusion, acidosis. See hypoxic drive |
| Absorption atelectasis | High FiO₂ washes nitrogen out of alveoli; the oxygen is absorbed and poorly ventilated alveoli collapse |
| Oxygen toxicity | Lung injury from prolonged high concentrations (FiO₂ above about 0.6 for long periods) |
| Retinopathy of prematurity | High oxygen damages developing retinal vessels in preterm babies |
| Fire | Oxygen feeds fire: no smoking, no flames or chulha nearby, no oil or grease on fittings; burns of the face in smokers using oxygen |
| Masking deterioration | A good SpO₂ on oxygen can hide worsening breathing; watch the rate and the patient |
| Local problems | Dry nose and mouth, nosebleeds, pressure sores behind the ears and on the nose; masks cause claustrophobia and aspiration risk if vomiting |
| Equipment | Trips over tubing; empty cylinders during walks; heavy cylinders |
5The physiotherapist's role
- Check the prescription and target before treatment; never stop or change oxygen without it.
- Exercise: measure SpO₂ during walking; give oxygen (as prescribed) to keep SpO₂ above about 88–90% during training; use portable cylinders or concentrators for ambulatory oxygen.
- Keep oxygen on during airway clearance and positioning when needed; watch SpO₂ after turns and suction.
- Teach long-term oxygen users: at least 15 hours a day, safety (no smoking), cleaning the cannula, and pacing.
- Pulse oximetry limits: wrong readings with cold fingers, poor circulation, movement, nail polish and carbon monoxide (falsely normal); oximeters can over-read in darker skin, so act on the whole clinical picture.
6Exam corner
Past questions (MUHS, 2016–2026)
- S20 Q2(c), 7 marksLow-flow oxygen therapy.
- 3-mark questionsMethods of delivery of oxygen therapy (S23). Hazards of oxygen therapy (S25). Hypoxic drive in COPD (W25; see the COPD page).
Test yourself
3 mList three low-flow and two high-flow oxygen devices with flows.
Low-flow: nasal cannula 1–4 L/min; simple face mask 5–10 L/min; reservoir mask 15 L/min. High-flow (fixed performance): Venturi mask (flow printed on the valve, FiO₂ 24–60%); high-flow nasal cannula (up to about 60 L/min).
MCQA COPD patient at risk of CO₂ retention needs oxygen. The best first choice is:A. Reservoir mask 15 L/minB. Simple face mask 3 L/minC. 24% or 28% Venturi mask, target SpO₂ 88–92%D. No oxygen
C. A fixed, low FiO₂ with the 88–92% target; a simple mask below 5 L/min risks rebreathing CO₂.
MCQWhich hazard of oxygen is caused by nitrogen washout?A. Absorption atelectasisB. Retinopathy of prematurityC. NosebleedsD. Fire
A. Without nitrogen to hold them open, poorly ventilated alveoli collapse as the oxygen is absorbed.
Sources
- MUHS BPTh syllabus (2012) and MUHS IV BPTh question papers 2016–2026 (muhsonline.com, unofficial archive; Dhole Patil College of Physiotherapy past-paper page).
- O'Driscoll BR, et al. BTS guideline for oxygen use in adults in healthcare and emergency settings. Thorax 2017;72 Suppl 1:i1–90. brit-thoracic.org.uk
- Jacobs SS, et al. Home oxygen therapy for adults with chronic lung disease: an official ATS clinical practice guideline. Am J Respir Crit Care Med 2020.
- Abdo WF, Heunks LMA. Oxygen-induced hypercapnia in COPD: myths and facts. Crit Care 2012;16:323.
- Byrne RA, et al. 2023 ESC Guidelines for the management of acute coronary syndromes (oxygen only if SpO₂ below 90%).
- Structure and classical content: Hough, Physiotherapy in Respiratory and Cardiac Care; Main & Denehy (2016).
Educational notes for exam preparation. Not medical advice.
Version 1. Content checked 19 October 2026. Next review October 2027.