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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 

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 in type 1 respiratory failure (e.g. due to sleep apnoea or acute LVF).


  • (bi-level positive airway pressure): 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 

Delivers up to 100%. 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 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
  • Perform ABG:
    • 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)