Myocardial infarction occurs when heart muscle cells die as a consequence of oxygen deprivation. Occasionally this can happen when the oxygen-carrying capacity of the blood is reduced, and there is general hypoxaemia, for example in acute carbon monoxide poisoning. More commonly, however, it follows acute coronary artery occlusion and the resultant abrupt reduction in the delivery of otherwise well-oxygenated blood to a localised segment of the myocardium.
In either case oxygen therapy would seem a reasonable treatment for those with suspected or confirmed heart attack, and inhaled oxygen has been so used for over 100 years . In fact, so widespread and accepted has this practice become that both National Guidelines groups and authors of influential textbooks , while aware of the limited scientific evidence supporting such use, have sanctioned the administration of supplemental oxygen to all patients in the early phase of myocardial infarction (even those with normal blood oxygen levels when breathing air). As Cabello and colleagues report in their Cochrane Systematic Review , only three randomised controlled trials involving just 387 patients have been published. Meta-analysis shows no benefit of routine inhaled oxygen therapy, in terms of either mortality or pain relief. In fact there were more deaths in the group receiving oxygen – albeit this might be a chance finding.
What does this tell us? Perhaps it suggests that practising clinicians (including ambulance paramedics), while keen to be seen to be doing something for patients with suspected heart attack, are unlikely to question the practice of preceding generations; particularly when such an intervention (inspired oxygen) is simple, convenient, inexpensive, and appears innocuous. Perhaps it also tells us that large-scale randomised trials in clinical practice are complex to organise and difficult to perform. Without significant financial support – and the experience of cardiovascular medicine is that such funding most often comes from the pharmaceutical industry – they are unlikely to be started, let alone completed. So while randomised trials including many thousands of patients are required to persuade clinicians, based on an understanding of the risks and benefits, to introduce novel expensive therapies, existing interventions, based on limited trials of a few hundred patients, are used without question. After all, “it can’t hurt”!
But this relaxed view of possible harm is questionable. While most clinicians are aware that oxygen therapy can be hazardous in patients with chronic pulmonary disease, reversing hypoxaemia-induced respiratory drive and paradoxically leading to respiratory failure, it is little appreciated that hyperoxia (higher than physiologically normal levels of oxygen) is associated with effects that might be detrimental in patients whose hearts are ischaemic. Over 40 years ago it was shown that inhaled oxygen successfully and substantially increased arterial oxygen levels during acute myocardial infarction, yet it was associated with a fall in cardiac output, a rise in blood pressure and an increased resistance to blood flow .
So there are indeed mechanisms through which oxygen could exert deleterious effects in these patients, and the findings of excess deaths in the randomised trials of oxygen therapy take on more worrying implications. In this regard it is interesting to note that a large (n = 6326) observational study of patients admitted to intensive care units following resuscitation from cardiac arrest recently showed that those with high blood oxygen levels had higher in-hospital death rates compared with those whose oxygen levels were normal or low .
The authors of the systematic review join the clamour for a randomised trial of oxygen treatment . In the meantime, those who adhere to the advice to “above all, do no harm” would be best advised to avoid oxygen in patients with acute myocardial infarction, unless the patient has demonstrably low oxygen levels, and then only deliver sufficient to avoid hyperoxia.
Reader in Clinical Medicine, School of Medicine, Swansea University, Singleton Park, Swansea SA2 8PP, UK (email@example.com)
How to cite: Weston C. Oxygen therapy in acute myocardial infarction – too much of a good thing? [editorial]. Cochrane Database Syst Rev. 2010 June 16;2011:ED000006. http://www.thecochranelibrary.com/details/editorial/742329/Oxygen-therapy-in-acute-myocardial-infarction--too-much-of-a-good-thing--by-Dr-C.html
Competing interests: The author has completed the Unified Competing Interest form at www.icmje.org/coi_disclosure.pdf (available upon request) and declares (1) no receipt of payment or support in kind for any aspect of the article; (2) no financial relationships with any entities that have an interest related to the submitted work; (3) that the author/spouse/partner/children has no financial relationships with entities that have an interest in the content of the article; and (4) that, as a result of working on this editorial, the author intends to collaborate with the authors of the accompanying systematic review in developing a randomised trial of oxygen therapy in acute myocardial infarction.
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Image Credit: Oscar Burriel/Science Photo Library.
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