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.
Flow rate 1-4L/minute
Used for mild hypoxaemia and use in non-acute wards
Hudson simple face mask
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
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
Flow rate (L/min)
Oxygen delivery (%)
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 intype 1 respiratory failure as an alternative to CPAP or a non-rebreather mask
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 intype 1 respiratory failure (e.g. due to sleep apnoea oracute 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 intype 2 respiratory failure with respiratory acidosis or exhaustion (e.g. due to COPD or neuromuscular diseases)
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.
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?
Which patients are at risk of hypercapnic respiratory failure?
What is the risk of over-oxygenation in such patients?
How would you approach oxygen therapy in patients are at risk of hypercapnic respiratory failure?
Try some OSCE stations that require oxygen therapy