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Common Oxygen therapy exam questions for medical finals, OSCEs and MRCP PACES


Question 1.

What saturations should I aim for?

  • Aim for oxygen saturations of 94-98% in non-COPD patients
  • Aim for oxygen saturations 88-92% in patients with COPD
    • The targeted approach is associated with decreased mortality in COPD patients and less respiratory acidosis


Question 2.

What are the dangers of uncontrolled high-flow oxygen?

  • When a patient is acutely unwell it is reasonable to start them on high flow oxygen immediately.
    • Hypoxia will kill before any other respiratory issue so this should be treated first if it is a possibility.
  • However, keeping patients on high-flow oxygen is dangerous.
    • It has been demonstrated to be harmful in
    • As such, keep saturations between 94-98%
    • Do not keep patients on any oxygen therapy with saturations of 100%


Question 3.

What are CO2 retainers?

  • CO2 retainers are those who tend have higher CO2 concentrations and, if given oxygen, will retain more CO2 potentially leading to hypercarbic respiratory failure.
    • Possible CO2 retainers include, obstructive lung disease (10 % of COPD, bronichiectasis, CF) and severe restrictive lung diseases (neuromuscular, severe kyphoscoliosis, severe obesity).
    • The most common are those with COPD


Question 4.

Why do these people retain CO2?

  • The tradition teaching was that COPD patients’ respiratory drive comes their hypoxia. Giving oxygen therefore lowers respiratory drive leading to hypercapnia. THIS IS LARGELY NOT TRUE. The real explanation is:


  • V/Q mismatch (main reason)
    • In COPD, patients optimise their gas exchange by hypoxic vasoconstriction leading to altered alveolar ventilation-perfusion (V/Q) ratios
    • Excessive oxygen administration overcomes this, leading to increased blood flow to poorly ventilated alveoli
      • This increases the V/Q mismatch and increases dead space, increasing CO2 levels
  • The Haldane effect (minor reason)
    • Deoxygenated hemoglobin (Hb) binds CO2 with greater affinity than oxygenated hemoglobin (HbO2)
    • Oxygen therefore induces a rightward shift of the CO2 dissociation curve, which is called the Haldane effect
    • In patients with severe COPD who cannot increase minute ventilation, the Haldane effect accounts for about 25% of the total PaCO2 increase due to O2 administration


Question 5.

How should you give oxygen in patients with COPD who may be CO2 retainers?

  • If patients are in respiratory distress, give 100% oxygen at 15L via a non-rebreather mask initially
    • Never withhold oxygen from a seriously ill hypoxic patient due to fear of cause hypercapnic respiratory failure!
  • If patients are not in distress but are hypoxic, perform baseline ABG (preferably on room air):
    • Hypoxia with hypercapnia (PaCO2 > 5.3kPa) = continue 24-28% initially
    • Hypoxia without hypercapnia (PaCO2 < 5.3kPa) = can have higher concentrations that 28%
  • If not able to ABG immediately, start on 24-28% (via Venturi mask)
    • This is very helpful in determining baseline respiratory function.
  • If significant hypoxia continues try increasing the oxygen level in small increments and repeating ABG after 30 minutes of next stage up
    • If hypoxic and hypercapnia & acidosis worsening with increased oxygen: consider non-invasive ventilation (BiPAP)
    • If hypoxic and hypercapnia & acidosis stable or improving: scope to increase oxygen cautiously
  • In other words, if you cannot get enough oxygen into patients to maintain sats 88-92% without causing a hypercapnic acidosis, then they need likely need non-invasive ventilation (BiPAP)


  • Note that these guidelines are general. All patients should be treated individually as there is wide interpatient variability. If they have been admitted before a previous ABG is invaluable.