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How to prescribe oxygen in COPD

Prescribing oxygen to patients with COPD for doctors, medical student exams, finals, OSCES and MRCP PACES

 

Basics of oxygen prescribing

  • Oxygen is a drug and should always be prescribed
  • There are relevant places on drug charts to do this, usually defined with two options: 88-92% or >94%

 

Target saturations in patients without COPD

  • Aim for oxygen saturations of 94-98% in non-COPD patients
    • There is no point in aiming for sats of 100%
    • In fact saturations of 100% are less helpful than 98-99% as they can reflect a normal PaO2 (10-13 kPa) but may also reflect an inappropriately high kPa of e.g. 30 (in an over-oxygenated patient)

 

Why is too much oxygen dangerous in COPD?

  • The reason for aiming for lower oxygen sats in those with COPD are in case they are CO2 retainers
  • Possible CO2 retainers include:
    • Severe obstructive lung disease (10% of COPD, bronichiectasis, CF)
    • Severe restrictive lung diseases (neuromuscular, severe kyphoscoliosis, severe obesity)
  • So why is too much oxygen dangerous for CO2 retainers?
  • The Lie
    • The traditional explanation is that oxygen administration to CO2 retainers causes loss of hypoxic drive, resulting in hypoventilation and therefore type 2 respiratory failure.
    • This is not true. Patients suffering from COPD exacerbations, regardless of whether they have CO2 retention, actually have supra-normal respiratory drive (unless there is impending hypercapnic coma)
  • The Truth
    • Reason 1: V/Q mismatch [major reason]
      • Patients with COPD 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, increasing the V/Q mismatch and therefore increasing physiological dead space
      • This increase in V/Q mismatch occurs in both CO2 retainers and non-retainers but the difference seems to be more profound in certain patients
    • Reason 2: The Haldane Effect
      • Deoxygenated hemoglobin (Hb) binds CO2 with greater affinity than oxygenated hemoglobin (HbO2). Oxygen therefore induces a right 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

 

Video explaining V/Q mismatch in CO2 retention

 

How to prescribe oxygen in COPD

  • In COPD, you should aim for sats of 88-92%
    • Mark the target sats on the drug chart
    • This target is associated with decreased mortality in COPD patients and less respiratory acidosis
  • Initially start on 24-28% (via Venturi mask) if no ABG results
  • Click here for details on oxygen delivery devices
  • Remember, however, to never withhold oxygen from a seriously ill hypoxic patient (with or without COPD) due to fear of cause hypercapnic respiratory failure; hypoxia is more dangerous than hypercapnia acutely

 

How to titrate oxygen in COPD

  • Once on 24-28%, perform baseline ABG and then titrate oxygen as follows:
  • Hypoxia without hypercapnia (PaCO2 < 5.3kPa)
    • = hypoxic drive unlikely
    • Can use higher flow oxygen
  • Hypoxia with hypercapnia but IMPROVING acidosis
    • = no hypoxic drive
    • Room to increase oxygen: if significant hypoxia continues increase the oxygen level in increments and repeat ABG after 30 minutes of next stage up
  • Hypoxia with hypercapnia and STABLE acidosis (PaCO2 > 5.3kPa)
    • = hypoxic drive possible
    • Continue 24-28% initially but consider you may need to switch to non-invasive ventilation if no improvement
  • Hypoxia with hypercapnia and WORSENING acidosis
    • = hypoxic drive
    • Reduce oxygen and discuss with ITU/HDU regarding non-invasive ventilation if oxygen levels stay dangerously low

 

  • Summary
    • The aim of giving oxygen in COPD is to prevent hypoxia while not leading to an increased CO2
    • If you cannot get enough oxygen into the patient to maintain sats 88-92% without causing a hypercapnic acidosis, then they need non-invasive ventilation (BiPAP)

 

When should you do an ABG?

  • In those with COPD or other known lung disease, always get a baseline ABG if the admission problem involves low GCS, chest pain or shortness of breath
  • It is particularly important to check ABGs promptly if a patient with COPD has been brought in as emergency by an ambulance: ambulance crews have to give high-flow oxygen if a patient is hypoxic, regardless of previous history
  • In those without lung disease, get an ABG if sats are under 94%

 

When should you re-ABG?

  • Always measure ABGs within 60 minutes (ideally in 30 minutes) of starting supplemental oxygen or changing its concentration
    • See above for how to titrate oxygen in COPD
  • Check ABGs on air before discharge in those who presented with a low PO2 and/or hypercapnia to guide later formal assessment for long term oxygen therapy (LTOT)
  • 4- to 6-week follow-up should include consideration of LTOT assessment (not before, as the patient needs to be clinically stable)

 

Extra tips on oxygen prescribing

  • Intubate if GCS is less than (or equal to) 8
    • Such patients cannot protect their own airway so will need airway manoeuvres, airway adjuncts and bag-valve-masking until a definitive airway can be secured
  • Do an ABG on any patient with presumed normal lungs and oxygen saturations of <94%
  • If O2 therapy is being used maximally and oxygen levels continue to drop, involve intensive care with a view to non-invasive ventilation or intubation and ventilation
  • Humidified oxygen can help with secretions and if prolonged oxygen therapy is required
  • Always give oxygen to a seriously ill hypoxic patient, whether or not they have COPD. Do not worry about hypercapnic respiratory failure in this case; the hypoxia is more dangerous than the hypercapnia acutely!

 

Now click here for questions on oxygen prescribing

Perfect revision for medical student exams, finals, OSCEs and MRCP PACES