Intended for healthcare professionals

Editorials

Should oxygen be given in myocardial infarction?

BMJ 2010; 340 doi: https://doi.org/10.1136/bmj.c3287 (Published 17 June 2010) Cite this as: BMJ 2010;340:c3287

This article has a correction. Please see:

  1. Dan Atar, professor and head of cardiology
  1. 1Department of Cardiology B, Oslo University Hospital Ullevål and Faculty of Medicine, University of Oslo, 0407 Oslo, Norway
  1. dan.atar{at}online.no

    On the basis of physiological reasons and no trial evidence of harm: yes

    A systematic review published this week found no evidence that giving inhaled oxygen to people with acute myocardial infarction improves pain and survival, and that it may even do harm.1 Undoubtedly the medical community will take note of such a conclusion, but are the results reliable and what do they mean for clinical practice?

    The review identified three randomised controlled trials that compared giving air with giving oxygen in people with an acute myocardial infarction; 387 people were studied and 14 died. The pooled relative risk of death was 2.88 (95% confidence interval 0.88 to 9.39), and this risk was 3.03 (0.93 to 9.83) in an intention-to treat analysis. Pain was also not significantly different between the groups (pooled relative risk 0.97, 0.78 to 1.20).

    Methodology was poor in all three of the analysed articles, however. Two studies were performed unblinded,2 3 one was reported in a foreign language with only an abstract accessible in English,3 and one randomised double blind study—the largest of the three trials—was published 34 years ago at a time when reperfusion treatment for infarction did not exist.4

    The most important outcome in trials of myocardial infarction is mortality, for which none of the three trials found a significant difference. The largest study reported three deaths in the oxygen group and nine in the pure air group.4 In another trial, which focused on pain relief, only one death occurred, and ironically it was not even reported in the publication, and when asked later the authors did not remember in which arm of the study the death had occurred.2 The final study reported one death in the oxygen group.3 In the meta-analysis of these studies the result remained non-significant.1 In conclusion, these studies provide no evidence that oxygen increases mortality.

    Other aspects are worth considering. Oxygen therapy in stable angina pectoris is a cornerstone of treatment because this disease is caused by a lack of oxygen supply to the ischaemic myocardium. Its role in this context is undisputed. Importantly, evidence shows that overt or silent ischaemia is detected after myocardial infarction in high proportion of patients, even in the era of reperfusion treatments. For example, the Swiss Interventional Study on Silent Ischemia Type II (SWISSI-II), performed before stents were used, assessed 1057 patients after myocardial infarction and found imaging evidence of silent ischaemia in 411 patients (39%).5 Similarly, the Danish Multicenter Randomized Study of Invasive versus Conservative Treatment in Patients with inducible Ischemia after Thrombolysis in Acute Myocardial Infarction (DANAMI) investigated an ischaemia driven reperfusion strategy in more than 1000 patients after thrombolysis and estimated that more than 8% of the entire population of patients presenting with infarction would later have residual ischaemia.6

    Another aspect to consider is the evidence on the use of hyperbaric oxygen in myocardial infarction. These studies were correctly excluded in the recent meta-analysis because they looked at a different system of giving oxygen. Nevertheless, a systematic review published in 2005 concluded that hyperbaric oxygen may improve pain relief and reduce major complications in myocardial infarction.7 If inhaled oxygen truly was harmful an equally adverse effect from hyperbaric oxygen would be expected, yet the opposite seems to be true.

    Finally, the topic of reperfusion injury in the course of acute myocardial infarction is still not settled. Initial mechanistic theories have built on the oxygen free radical pathway.8 Oxidative stress is thought to occur on successful reperfusion, conferring an additional necrotic stimulus that leads to worse outcomes after infarction. Unfortunately, many studies in animals and patients that have tried to circumvent this phenomenon by scavenging oxygen free radicals have been ineffective. This speaks indirectly against a harmful effect of oxygen.

    What does this all mean for practising clinicians? To date, no contemporary high quality study has investigated inhaled oxygen as part of the treatment of myocardial infarction, and this should be remedied. Because this systematic review found no significant difference in mortality between people taking pure air or oxygen it is reasonable to continue giving oxygen to people with acute myocardial infarction. Pathophysiological reasoning together with trial evidence of residual ischaemia after infarction support this approach.

    Notes

    Cite this as: BMJ 2010;340:c3287

    Footnotes

    • Competing interests: The author has completed the Unified Competing Interest form at www.icmje.org/coi_disclosure.pdf (available on request from DA) and declares: (1) No support for the submitted work; (2) No relationships with any company that might have had an interest in the submitted work in the previous three years; (3) No spouse or children with financial relationships that may be relevant to the submitted work; and (4) No non-financial interests that may be relevant to the submitted work.

    • Provenance and peer review: Commissioned; not externally peer reviewed.

    References