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.
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
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.
Delivers up to 100% Oxygen!
(continuous positive airway pressure): 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).
(bi-level positive airway pressure): 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%. 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 mucosal drying if prolonged medium/high-flow oxygen therapy is required
Patients at risk of hypercapnic respiratory failure
Patients with severe obstructive lung disease (e.g. COPD) or severe restrictive lung disease (e.g. neuromuscular, severe kyphosis, morbid obesity) may rely on hypoxic pulmonary vasoconstriction to improve their V/Q ratios to improve gas exchange
Administering too much oxygen can counteract this mechanism, leading to a V/Q mismatch which then creates more alveolar dead space, resulting resulting in less CO2 excretion and hypercapnia
The aim of acute oxygen therapy in these patients is to provide enough oxygen but not so much that it worsens hypercapnia and respiratory acidosis – aim for target saturations of 88-92% initially
To avoid over oxygenation, generally start with nasal cannula or 24-28% Venturi mask and titrate up if required; however, if they need more oxygen (e.g. they are in respiratory distress or dangerously hypoxaemic) then give higher flow oxygen initially and titrate down – hypoxia will kill faster than hypercapnia
Hypercapnia (PaCO2 >6 kPa) → continue to titrate to target saturations 88-92%
No hypercapnia (PaCO2 <6 kPa) → consider changing target saturations to 92-96% (but repeat ABG when in range to ensure PaCO2 doesn’t begin to increase)
Repeat ABG 30-60 minutes after any increase in oxygen therapy
If you cannot achieve oxygen sats of 88-92%/PaO2 ≥8 without a hypercapnic respiratory acidosis despite medical management, the patient needs non-invasive ventilation (BiPAP)