By Martina McGrath, MD
November 3, 2017
Acute myocardial infarction occurs where there is insufficient supply of oxygenated blood to an area of the heart, leading to myocardial injury and cell death. For decades, clinical guidelines have recommended the administration of supplemental oxygen as a first-line therapy for all patients experiencing myocardial ischemia, regardless of oxygen saturation.1 For patients with hypoxemia, the benefits of supplemental oxygen are clear and not under debate. However, for those with normal oxygenation and acute myocardial ischemia, there has been controversy in recent years regarding the benefit of additional oxygen. While it seems like a common sense approach (giving more oxygen will get more oxygen to the ischemic area), like so many things in medicine, the simple explanation is not borne out by detailed studies.
Several recent studies point to a lack of benefit at best, and possible risk at worst. The AVOID trial, published in 2015, enrolled 441 patients with ST elevation MI and randomized them to supplemental oxygen at 8 litres per minute versus room air.2 More severe early myocardial injury (higher biomarker levels) and increased infarct size were seen in those treated with oxygen as compared those on room air.2,3 Similarly, a Cochrane review published in 2016 did not show evidence to support routine use of supplemental oxygen in myocardial infarction.4
Adding to the evidence, the DETO2X-AMI study was recently published in NEJM.5 This multicenter clinical trial comprised 6,629 patients across 69 cardiac care facilities in Sweden. Enrolled patients presented with symptoms suggestive of myocardial ischemia, and either ischemic EKG changes or elevated levels of cardiac troponin T. At the time of randomization, median oxygen saturation was 97% and patients continued on supplementation oxygen for an average of 11.6 hours. The primary end point was death within one year after randomization, analyzed by intention-to-treat analysis. At the end of one-year follow up, no differences in outcomes were observed between the groups, with one-year mortality of 5.0% versus 5.1%. Similarly, no differences were seen in 30-day rehospitalization, peak troponin level, or 30-day mortality5.
Why could this be? Several mechanisms have been proposed. Supraphysiological oxygen levels may cause coronary vasoconstriction, exacerbating the demand/supply mismatch.3,6 Similarly; additional oxygen can contribute to the development of reactive oxygen species, with potentially detrimental effects on the ischemic area. Further studies may be required to define the exact mechanism, and clarify if there is potential risk as opposed to lack of benefit to oxygen administration. However, trial data increasingly indicate that supplemental oxygen is not beneficial to patients with myocardial ischemia and normal oxygenation, a major change to clinical dogma stretching back for decades.7
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- Steg PG, James SK, et al. ESC Guidelines for the management of acute myocardial infarction in patients presenting with ST-segment elevation. Eur Heart J. 2012;33:2569-619.
- Stub D, Smith K, Bernard S, et al. Air Versus Oxygen in ST-Segment-Elevation Myocardial Infarction. Circulation 2015;131:2143-50.
- Loscalzo J. Is Oxygen Therapy Beneficial in Acute Myocardial Infarction? Simple Question, Complicated Mechanism, Simple Answer. N Engl J Med. 2017;377:1286-7.
- Cabello JB, Burls A, Emparanza JI, Bayliss SE, Quinn T. Oxygen therapy for acute myocardial infarction. Cochrane Database Syst Rev. 2016;12:CD007160.
- Hofmann R, James SK, Jernberg T, et al. Oxygen Therapy in Suspected Acute Myocardial Infarction. N Engl J Med. 2017;377:1240-9.
- McNulty PH, King N, Scott S, et al. Effects of supplemental oxygen administration on coronary blood flow in patients undergoing cardiac catheterization. Am J Physiol Heart Circ Physiol. 2005;288:H1057-62.
- Steele C. Severe angina pectoris relieved by oxygen inhalations. BMJ 1900, 2:1568-1568.
Dr. Martina McGrath is an Instructor in Medicine at Harvard Medical School, and a member of the Renal Division, Department of Medicine, at Brigham and Women’s Hospital, both in Boston. Dr. McGrath is the Medical Editor for the Trends in Medicine blog.