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Home | Articles | Respiratory | Oxygen Therapy

Oxygen Therapy

Last updated 2nd March 2023

Pulse Oximetry Targets

  1. Oxygen is a treatment for hypoxaemia, not breathlessness.
  2. Aim for sats >94% in acutely unwell patients, except those at high risk of Type 2 Respiratory Failure (T2RF), who should have a target of 88-92% or who have COVID-19, who should have a target of 92-96%.
  3. Oxygen targets should be prescribed on HEPMA and the NEWS chart.

Type 2 Respiratory Failure – Who is at Risk?

  1. Severe COPD – previous T2RF, home oxygen, home NIV, clinically severe (not able to climb a flight of stairs)
  2. Severe chest wall deformity
  3. Neuromuscular disease affecting respiratory muscles (MND, myasthenia gravis, Guillaine Barre)
  4. Morbid obesity
  5. Drugs causing CNS depression

NB in severe metabolic alkalosis, patients hypoventilate in order to compensate for their alkalosis, leading to a compensatory increase in PaCO2.  This is not the same as hypercapnic respiratory failure.

The Hypoxic Patient

  1. A hypoxic patient will typically be breathless however they may also present with confusion or tachycardia
  2. Patients may appear cyanotic when their sats are <85%, however this can be difficult to assess (e.g. due to lighting, skin tone etc).
  3. SpO2 <94% is abnormal – assess all of these patients with a systematic approach
  4. f a patient has low saturations but looks well, remember to look at the morphology of the SpO2 trace and consider that they may be peripherally cold. In such cases, use an ear probe.

When Should I Check an Arterial Blood Gas?

  1. All critically ill patients
  2. Any patient with a new oxygen requirement
  3. Any patient at high risk of T2RF who becomes acutely SOB or develops features of CO2 retention
  4. Patients who need a VBG (e.g. to check lactate or assess for acidaemia), but who have evidence of respiratory compromise, i.e. breathlessness.
  5. Consider in patients with unexplained agitation, confusion, tachycardia or tachypnoea.

N.B venous gases – Sometimes it isn’t possible to get into the radial artery. VBG gives a reasonable estimate of H+ and an estimate of PaCO2 that is, on average, 0.77kPa higher than an arterial sample.

Assessing ABG results for respiratory failure

  1. Hypoxaemic respiratory failure (T1RF): PaO2 <8 with normal or low PaCO2
  2. Hypercapnic respiratory failure (T2RF):  PaO2 < 8 with PaCO2 >6

Management of T2RF

  1. This requires careful titration of oxygen to ensure patients are adequately oxygenated without becoming hypercapnic and acidotic.  This will usually be done using a Venturi system
  2. Patients at high risk of T2RF may still require oxygen – this should be controlled/low dose.
  3. If acidotic – may need NIV. If unstable and for full escalation, then discuss with anaesthetics early for consideration of invasive ventilation.
  4. If not acidotic monitor closely especially if increasing oxygen requirement and consider repeat gases.

When to give 15 litres non-rebreather

  1. Cardiac arrest/peri-arrest e.g. shock, severe sepsis, major trauma, drowning, anaphylaxis, major pulmonary haemorrhage, status
  2. Carbon monoxide poisoning
  3. Conservatively-managed pneumothorax
  4. In patients at risk of T2RF with any of the above, give 15L initially and get an ABG.

Oxygen delivery devices and flow rates for use on the general ward

  1. High Concentration Reservoir Mask – can deliver oxygen at a concentration of up to around 90%, and should be set at a flow rate of 15l/min. Most suitable for trauma and emergency use in all critically ill patients pending an SpO2 and appropriate titration.
  2. Simple Face Mask – should be set at flow rates between 5 and 10l/min. Flow rates <5l/min not advised due to theoretical risk of CO2 build up within the mask. Suitable for patients with hypoxaemic respiratory failure though many such patients can be managed more easily by nasal cannula as these are better tolerated.
  3. Nasal Cannulae – should be set at flow rates of 1-6 l/min. Studies have shown they are just as effective in mouth breathers. Most patients prefer nasal cannulae to simple face masks because that aren’t claustrophobic though some dislike the flow of oxygen into the nose, especially at flow rates >4l/min.
  4. Venturi Mask – fixed performance device designed to deliver a particular percentage of oxygen, which is independent of the flow rate. Used when it is important to control the amount of oxygen that is given precisely. Available as 24,28,31,35,40 and 60% oxygen using different Venturi masks.
  5. High flow humidified oxygen via nasal cannulae – alternative delivery device for patients requiring high flow oxygen. Requires a delivery device, a flow generator and a humidifier. Advantages may be delivery of increased inspired oxygen, a low level of CPAP and greater comfort compared to reservoir face masks

N.B. Humidification is only required during high dose oxygen treatment for >24hrs and those who report upper airway dryness
*Insert table showing O2 devices and FiO2 output*

Oxygen and Nebulisers

  1. If the patient requires 4-8 litres to achieve their target SpO2 then this can be used to drive the nebuliser, irrespective of whether they have COPD etc.
  2. If they need more then they will also need nasal cannulae in addition.
  3. If they need less then use a compressor and deliver the supplemental oxygen via nasal cannulae


  1. Compared with standard oxygen delivery systems, HFNO reduces the need for intubation.
  2. Oxygen is humidified and warmed.
  3. Can deliver oxygen at rates up to 60 L/min


  1. NIV is a form of oxygen delivery via a facemask by positive pressure, used in respiratory failure. The term NIV is often used interchangeably with the trade name BiPAP (Bi-level Positive Airway Pressure).
  2. NIV delivers differing air pressure depending on inspiration and expiration. The inspiratory positive airways pressure (iPAP) is higher than the expiratory positive airways pressure (ePAP). Therefore, ventilation is provided mainly by iPAP, whereas ePAP recruits underventilated or collapsed alveoli for gas exchange and allows for the removal of the exhaled gas.
  3. In the acute setting, NIV is used in type 2 respiratory failure eg COPD exacerbation with respiratory acidosis
  4. Indications – persistent hypercapnia with acidosis for >30 minutes despite appropriate treatment.
  5. Click here for page on NIV in CCU.  Click here for page on NIV in ED.


  1. CPAP supplies constant fixed positive pressure throughout inspiration and expiration. It, therefore, is not a form of ventilation, but splints the airways open. It allows a higher degree of inspired oxygen than other oxygen masks.
  2. In the chronic setting it is used for severe obstructive sleep apnoea (splinting the upper airway) and in the acute setting for type 1 respiratory failure eg ARDS

Indications for Considering Invasive Ventilation

  1. Critically unwell patient with respiratory failure for full ventilation.
  2. Persistent hypoxia – not maintaining sats >90% despite high flow oxygen (in patients with scale 1 target)
  3. Patients with significantly reduced conscious level
  4. RR <10 or >40


Content updated by Jacob Williams & Emily Roberts