High time to omit oxygen therapy in ST elevation myocardial infarction

Supplemental oxygen (O2) therapy in patients with chest pain has been a cornerstone in the treatment of suspected myocardial infarction (MI). Recent randomized controlled trials have, however, shown that supplemental O2 therapy has no positive nor negative effects on cardiovascular functions, mortality, morbidity or pain in normoxic patients with suspected MI and foremost patients with ST Elevation Myocardial Infarction (STEMI). O2 therapy in normoxic STEMI patients should therefore be omitted. More studies are needed in discussing hemodynamically unstable STEMI patients, as well as patients with non-STEMI, unstable angina and other emergency conditions.

ST Elevation Myocardial Infarction (STEMI) is the most serious manifestation of Acute Coronary Syndrome (ACS) and is by the Fourth Universal Definition of Myocardial Infarction deemed to be a Type I myocardial infarction, caused by an acute atherothrombotic coronary artery disease (CAD) [1]. The consequence of CAD is a partial or complete occlusion of a coronary artery thus contributing to the termination of oxygen (O 2 ) supply to the myocardium, giving rise to ischemia [1,2].
Because of its seriousness, prompt and rapid diagnosis and treatment is of high importance to reduce both mortality and morbidity [3]. The most important treatment in STEMI is Percutaneous Coronary Intervention (PCI), which should be performed as soon as possible after that the condition is diagnosed [4][5][6].
International guidelines also emphasize on treatment with dual antiplatelets in the emergency phase before the start of the PCI [4][5][6]. Previous guidelines also recommended the immediate administration of O 2 to patients with diagnosed or suspected ACS, without any respect to the blood O 2 saturation [7][8][9][10]. In discussing current guidelines, only the 2017 ESC Guidelines for the management of patients with STEMI [5] states that O 2 should not routinely be administrated to patients with STEMI, and that only those with a blood O 2 saturation < 90% or PaO 2 < 60 mmHg should receive O 2 therapy.
With an overwhelming scientific evidence that O 2 therapy has no positive (nor negative) effects in patients with STEMI, all international, regional and local guidelines should be updated and omit O 2 therapy in normoxic (≥ 90%) STEMI patients.

Oxygen therapy
The history of O 2 as a medicine dates back to 1775 when the British chemist Joseph Priestly discovered O 2 and stated that it could be used as a medicine [11]. It was, however, in 1900 that the first publication on the role of O 2 therapy in patients with chest pain was published. It was a short letter by Dr. Charles Steele, in which he deemed that O 2 therapy had relieved chest pains in one single patient he believed to have angina [12]. Ever since this letter by Dr. Steele, several studies have tried to answer how supplemental O 2 therapy in both healthy and ill patients affect their cardiovascular system.
The rationale behind O 2 therapy has been that by adding O 2 to the patient's blood, the myocardium can be oxygenated, which in turn will contribute to a diminished ischemic area and infarct size, thereby minimizing the risk for lethal arrhythmias [13,14]. Studies on canines [14][15][16] have given some support to this theory, showing that O 2 therapy decreases infarct size and ischemia in these animals. A recent study on swine, however, showed that hyperoxemia can aggravate and worsen myocardial ischemia [17].
In healthy individuals, experimental studies have shown that hyperoxemia because of supplemental O 2 therapy, may contribute to negative cardiovascular effects like a decrease in coronary blood flow, arterial vasoconstriction, diminished cardiac output, an increase in the systemic vascular resistance as well as impaired blood flow to organs and tissues [18][19][20][21][22][23][24][25][26].
In patients with suspected as well as confirmed myocardial infarction, the role of O 2 were for a long time highly inconclusive. Our knowledge gap in this matter was not because of lack of studies, but rather because of the poor methodologies used in these studies. Ever since 1900, several studies have been published on the role of O 2 in patients with chest pain, coronary artery disease, cardiac failure as well as suspected and confirmed myocardial infarction. All of them have unfortunately had serious limitations and have therefore not been able to correctly answer the question of how O 2 therapy affects the cardiovascular system in both healthy patients and patients with myocardial infarction and cardiac failure. The studies have either been case studies or small reports including only a few patients, thus not being generalizable, or small studies [18][19][20][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43]. Furthermore, the vast majority of the studies was conducted in the pre-PCI era and even before Troponin was used as an important part in the diagnosis of myocardial infarction.
Because of the above limitations, a Cochrane report from 2013 [44] called for randomized controlled trials to once for all answer the question about which role supplemental O 2 therapy should have in patients with chest pain and suspected myocardial infarction. The authors of the report stated that "A definitive randomised controlled trial is urgently required […]." The two first studies were conducted in the pre-PCI era. While Wilson et al. [42] found no significant differences between the patients with myocardial infarction randomized to supplemental O 2 or air, Rawles et al. [45] showed that patients with myocardial infarction receiving supplemental O 2 , had a larger infarct size as measured with serum aspartate aminotransferase.

Randomized controlled trials
The study by Ukholkina et al. [46] is the only randomized study showing a positive effect of supplemental O 2 therapy in patients with myocardial infarction. The study is thus highly biased because of a limited methodology [44].
Rancord et al. [47] have a sound methodology, and should be considered the first modern RCT on the role of supplemental O 2 in patients with STEMI. The authors found no significant differences between the two arms (supplemental O 2 vs titrated O 2 ) with regard to infarct size as measured by cardiac Troponin T, as well as cardiac MRI (CMRI) close to one month after inclusion.
The main publication of the AVOID study was conducted by Stub et al. [48] in which 441 STEMI patients were randomized to supplemental O 2 therapy or air. Even though the study found no significant difference in infarct size as measured by cardiac Troponin, a subset of the patients undergoing CMRI after six months, showed that those randomized to supplemental O 2 therapy, had a larger infarct size as measured in absolute mass but not in percent of the left ventricle. A sub study [49] of the AVOID trial showed later that patients randomized to the O 2 arm, had significantly higher cardiac Troponin rates than those randomized to the air arm.
The SOCCER study was conducted in Sweden by Khoshnood et al. and aimed to evaluate the effects of supplemental O 2 in normoxic first-time STEMI patients accepted for PCI. Patients were randomized to either supplemental O 2 therapy or air. All patients underwent CMRI, while only a subset of patients underwent echocardiography. Their chest pain was scored and assessed prehospital and in-hospital with the Visual Analog Scale (VAS) [50]. Ninety-four patients underwent CMRI which showed no significant difference between the two arms in discussing infarct size, myocardium at risk and myocardial salvage index [51]. Of the 87 patients undergoing echocardiography, no significant differences could be measured between the two arms in discussing left ventricular ejection fraction and wall motion score index [52]. In a recently published sub study, 111 patients were assessed in regard to chest pain to evaluate the analgesic effect of O 2 therapy. Those randomized to the supplemental O 2 group had significantly higher median VAS and also received significantly higher amounts of morphine. The study could not show that supplemental O 2 diminished chest pain [53].
The DETO2X study was also conducted in Sweden. The main publication by Hofmann et al. included more than 6000 patients and evaluated the one-year-all-cause mortality in normoxic patients with suspected myocardial infarction randomized to supplemental O 2 therapy or ambient air. The study found no significant differences between the two arms in regard to mortality nor morbidity [55]. A sub study on patients with only STEMI (n = 2807) did not show any significant differences between the two arms in regard to one-year all-cause mortality, or morbidity like Before the randomization, patients in the O 2 group had a significantly higher VAS and also received significantly more morphine. No significant differences between the two groups in regard to VAS at the start of the PCI or median VAS decrease from randomization to PCI.
A considerable amount of the patients missed VAS rates and were therefore excluded. This may be a source of bias.

Omit supplemental O 2 therapy in STEMI
The above RCTs clearly show that O 2 therapy has so positive nor negative cardiovascular effects, when used in normoxic patients with STEMI both prehospital and in-hospital. Two recent reviews and meta-analysis on the role of supplemental O 2 therapy in acute myocardial infarction, showed also no benefit of using O 2 therapy in these patients [58,59]. In discussing supplemental O 2 therapy in normoxic STEMI patients, the evidences are clear and consistent, why all guidelines must be reformed to state that supplemental O 2 therapy in these patients should be omitted. It is, however, of high importance to point that patients diagnosed with STEMI, and who have a low blood oxygen saturation, should receive supplemental O 2 . It is the routine use of O 2 therapy, with no respect to blood oxygen saturation, that should be omitted (Fig. 1). With this said, it is important to point out that the RCTs presented above does also have some limitations as the majority of them have had a small cohort, and the focus have been stable and normoxic STEMI patients. These limitations might reduce the generalizability of the studies. More studies are therefore needed in discussing supplemental O 2 therapy in hemodynamic unstable STEMI patients, patients with non-STEMI as well as unstable angina. This is especiCally of importance since some studies argue that supplemental O 2 therapy administrated to acutely ill patients can be toxic and increase mortality and morbidity [60,61].

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Author's contribution The author read and approved the final manuscript.
Authors' information Dr. Ardavan Khoshnood is a senior resident in Emergency Medicine, and holds a PhD in Clinical Medicine, Emergency Medicine. He is a lecturer at the Medical Faculty at Lund University, and also serves as an Associate Editor for the BMC Emergency Medicine.
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Competing interests
Dr. Khoshnood have authored several articles on the role of supplemental O 2 therapy in patients with myocardial infarction and STEMI. Dr. Khoshnood is also an Associate Editor at the BMC Emergency Medicine.

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