Oxygen therapy

Oxygen masks

Oxygen comes out of a wall tap/canister at 100% concentration, but you can adjust the flow rate on the tap/dial (0-15L/minute). Different delivery devices tolerate different flow rates. The FiO2 (percentage of oxygen delivery) is determined by the flow rate and delivery device.

Nasal cannula

  • Delivers 24-30%
  • Comfortable
  • Flow rate 1-4L/minute
  • Used for mild hypoxaemia and use in non-acute wards

Hudson simple face mask

  • Delivers 30-40%
  • Flow rate 5-10L/minute
  • Delivers slightly more oxygen than a nasal cannula but the precise FiO2 cannot be determined so a Venturi mask is often used instead

Venturi (air entrapment) mask

  • Delivers 24-60%
  • Oxygen delivery depends on mask: this is marked on the side of the mask, along with the appropriate flow rate setting
  • Often used in patients with COPD/type 2 respiratory failure so you know the precise FiO2 you are delivering
Types of Venturi Mask
ColourFlow rate (L/min)Oxygen delivery (%)

Non-rebreather mask

  • Delivers 85-90% with 15L/minute flow rate
  • Mask with a reservoir bag and valve which stops almost all rebreathing.
  • Used for acutely unwell hypoxaemic patients

High flow nasal oxygen (e.g. Optiflow)

  • Delivers up to 100% with up to 60L/minute flow rate
  • The very high flow rate also creates a small positive airway pressure effect similar to CPAP
  • Used in type 1 respiratory failure as an alternative to CPAP or a non-rebreather mask

Non-invasive ventilation 

CPAP (continuous positive airway pressure)

  • Delivers up to 100% oxygen
  • Air/oxygen delivered through a tight-fitting mask at constant positive pressure to keep alveoli open
  • Used in type 1 respiratory failure (e.g. due to sleep apnoea or acute LVF)

BiPAP (bi-level positive airway pressure) 

  • Delivers up to 100% oxygen
  • Same system but with a high positive pressure on inspiration and a lower positive pressure on expiration
  • Used in type 2 respiratory failure with respiratory acidosis or exhaustion (e.g. due to COPD or neuromuscular diseases)

Invasive ventilation 

Invasive ventilation

Delivers up to 100% oxygen. A ventilation bag or machine is attached to an artificial airway to ventilate lungs. Used in intensive care and theatre. 

General points
  • Intubate if GCS ≤ 8 (risk of airway not protected)
  • Aim for oxygen saturations of 92-96% in most patients, but 88-92% in those at risk of hypercapnic respiratory failure (e.g. COPD)
  • If patient is requiring the maximal level of ward-based oxygen therapy (i.e. 15L non-rebreather mask or higher-flow Venturi) to maintain saturations, or they are in type 2 respiratory failure, or oxygen levels are not improving, involve seniors and/or intensive care for consideration of non-invasive or invasive ventilation 
  • Do an ABG on any patient with oxygen saturations of <92% or high oxygen requirements 
  • Humidified oxygen helps secretions and prevents

Test your knowledge

What is the difference between hypoxia and hypoxaemia?

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Which patients are at risk of hypercapnic respiratory failure?

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What is the risk of over-oxygenation in such patients?

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How would you approach oxygen therapy in patients are at risk of hypercapnic respiratory failure?

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Try some OSCE stations that require oxygen therapy

  1. Asthma exacerbation
  2. Acute assessment – shortness of breath II
  3. Anaphylaxis
  4. Sepsis
  5. Status Epilepticus

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