Common Oxygen therapy exam questions for medical finals, OSCEs and MRCP PACES
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
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%
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
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
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)
- LOOK FOR PREVIOUS ABGS IF POSSIBLE.
- 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.