Should oxygen be given routinely in the treatment of MI?
The review originally set entry criteria that trials had to be randomised and placebo controlled with clinical outcome data such as mortality. Only 2 of 51 trials identified met these criteria and only one study reported the number of deaths in each group. Therefore a formal meta-analysis could not be performed.
The single study with mortality data involved the randomisation of 200 patients to either high-flow oxygen or compressed air for the first 24 hours of a suspected uncomplicated MI. A total of 43 patients were subsequently excluded when MI was not confirmed.
There were nine deaths in the oxygen group and three in the air group (RR 2.9; 95% CI 0.8-10.3; p=0.08). The serum AST level was used as a surrogate for infarct size with higher peak values in the oxygen group (p=0.05). Finally, there was a non-significant increase in the incidence of ventricular tachycardia in the oxygen group.
Although the primary outcome of mortality did not reach statistical significance and the study was small and underpowered it does provide food for thought. Certainly there is a distinct lack of evidence in favour of the routine use of oxygen in MI and the little evidence that is available suggests that it may actually be harmful with a possible increase in infarct size and mortality. Such harmful effects could be mediated by hyperoxia-induced coronary vasoconstriction and increased free radical generation following reperfusion by thrombolysis or primary percutaneous coronary intervention.
I have often wondered if there is any evidence for the routine practice of providing oxygen to patients suspected of having an acute MI. The American Heart Association guidelines state: "It is reasonable to administer supplementary oxygen to all patients with uncomplicated ST elevation myocardial infarction."
It is clear that further studies are needed in this area and in the meantime I would not routinely reach for the oxygen cylinder when treating a patient with a suspected MI.
Wijesinghe M, Perrin K, Ranchord M et al. Routine use of oxygen in the treatment of myocardial infarction. Heart 2009; 95:198-202Reviewer
Dr Peter Savill
GPwSI Cardiology, Southampton