Too much of a good thing? Oxygen and asthma

A summary of a Randomised controlled trial comparing high concentration oxygen to titrated oxygen in acute severe asthma.

What was the point?

 There is an increasing body of evidence that high levels of oxygen may be harmful administered for a prolonged period in several circumstances including stroke, MI and Emergency surgery. Studies are being undertaken in the paediatric population.  This study was interesting as focused on short term condition associated with immediate mortality from hypoxia- Acute severe Asthma.

How was it done?

 A Randomised controlled multi-centre trial.


Where and on whom was it done?

  •  3 New Zealand Emergency departments
  • Adults 18-65 years old
  • Diagnosis of both previous and acute asthma with PEFR 50% able to verbally consent
  • Excluded were those with COPD or other conditions with which would prone to raised pCO2
  • Non – blinded study


What were they testing and what did they compare it to?

  •  Comparing control group high flow oxygen (8L/min hudson mask)
  • To oxygen titrating peripheral saturations of 93-95%
  • Both received standard asthma care
  • Primary outcome measured was transcutaneous pCO2 measurement at 60 minutes


The numbers that matter

Statistically significant difference of a rise in  PtCO2 of 4mm or greater in the high oxygen concentration group (RR 2.3 ( CI 1.2-4.4).


Author’s Conclusions

 That the untitrated adminstration of oxygen in acute severe asthma leads to higher CO2 levels.

Good things

  •  Power calcuation with reasonable expected difference
  • Intentional to treat analysis and minimal protocol violations

Less good things

  • Unblinded
  • Change in primary outcome early in study (although well reasoned by author’s)
  • Surrogate non patient orientated outcome
  • I question to clinical significance of a CO2 rise of 4mmg (approximately 0.5 kpa)


My Humble opinion

This is an interesting study adding to the evidence that too much oxygen may be a bad thing. Although uses a non- patient orientated that I am not too familiar with but apparently is accurate (transcutaneous CO2) it will make me thing twice in giving lots of oxygen to those asthmatic patients who are responding well to treatment; although I already aim for sats <98% as a diagnostic method to spot deterioration earlier. In addition, in addition from being excluded from this study,  for the really unwell asthmatics would be keen to hyper-oxygenate to give a bit more wriggle room if things go south.

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