Air Verses Oxygen In myocarDial Infarction Study

Coronary artery disease (CAD) is a leading cause of morbidity and mortality in Australia[1]. In particular, many patients with CAD present with ST-elevation myocardial infarction (STEMI) as a result of acute thrombotic coronary artery occlusion. The optimal treatment for patients presenting with STEMI is reperfusion therapy either with primary percutaneous coronary intervention (PCI) or administration of a thrombolytic drug[2, 3].

Current guidelines recommend additional treatments for patients with STEMI prior to reperfusion therapy, such as oxygen, aspirin and nitrates[4]. Whilst there is supportive evidence from clinical trials for the administration of aspirin[5] and nitrates[6], there is no data from prospective, randomised, controlled clinical trials to support the use of routine supplemental oxygen.

For many years, the administration of supplemental oxygen has been considered beneficial for the treatment of patients with acute myocardial infarction largely based on experimental laboratory data. For example, in a laboratory study, anaesthetised dogs underwent coronary artery occlusion and were then administered either 21% oxygen, 40% oxygen or 100% oxygen. In the 40% oxygen group, there was decreased myocardial injury and infarct size compared with the air or 100% oxygen groups[7].

In another laboratory study, two groups of dogs underwent 90 minutes of coronary occlusion followed by 72 hours of reperfusion[8]. One group received 100% inspired oxygen from 20 minutes before reperfusion and three hours after reperfusion whereas the air group received room air. The infarct size in the oxygen group was reduced by 38% and left ventricular ejection fraction was improved compared with the dogs receiving room air. This data suggests that high concentrations of inspired oxygen may be of benefit in acute myocardial infarction followed by reperfusion therapy.

However, there is increasing clinical data that suggests that oxygen administration before and during reperfusion in patients with STEMI may be harmful.

For example, the hemodynamic effects of inhalation of oxygen in high concentration has been investigated in 50 patients with acute myocardial infarction[9]. This resulted in adverse effects including a fall in cardiac output, a rise in blood pressure and an increase systemic vascular resistance. The latter would be expected to increase myocardial work and increase myocardial ischaemia.

There have been three prospective, controlled trials of supplemental oxygen compared with no supplemental oxygen in patients with myocardial infarction.

In a double-blind, randomised in-hospital study, two-hundred patients with myocardial infarction were allocated to receive supplemental oxygen or air administered by face mask for the initial 24 hours in hospital[10]. The two groups were comparable at baseline. There was no significant difference in mortality, incidence of arrhythmias or use of analgesics between the groups. There was a higher incidence of sinus tachycardia in the patients given oxygen. This study suggested that there was no benefit from the routine administration of oxygen in uncomplicated myocardial infarction.

In a second study, 50 patients were allocated to either supplemental oxygen or room air[11]. The main outcome measure was the requirement for analgesia with 16 of 22 patients (72.7%) in the oxygen group using opiates for pain relief compared with 18 of 20 patients (90%) in the air group. This study did not report mortality rate.

In a third study conducted in Russia, 137 patients were allocated to either supplemental oxygen (4-6L/Min) or air[12]. Complications including heart failure, pericarditis and rhythm disorders occurred less frequently in the air group (Risk ratio 0.45: 95% CI 0.22 to 0.94). One patient out of 58 died in the oxygen group and none out of 79 participants in the air group.

A meta-analysis analysed the outcomes in the 387 patients included in these three studies[13]. The pooled risk ratio of death for patients allocated to oxygen administration was 2.88 (95% CI 0.88 to 9.39) in an intention-to-treat analysis and 3.03 (95% CI 0.93 to 9.83) in patients with confirmed myocardial infarction. While suggestive of harm, the small number of deaths recorded meant that this finding did not reach statistical significance. Pain was measured by analgesic use and the pooled risk ratio for decreased use of analgesics in the oxygen group was 0.97 (95% CI 0.78 to 1.20).

In addition to the above studies, other clinical studies have examined the use of novel techniques for additional oxygen delivery to the ischemic myocardium during reperfusion. In a clinical trial testing the role of hyperbaric oxygen (HBO) in myocardial infarction, 112 patients with STEMI were allocated to either HBO or usual supplemental oxygen (40% by mask or 6L/min by nasal prongs) during thrombolysis[14]. There was no significant difference between the groups in creatinine kinase levels at 24 hours or left ventricle ejection fraction (LVEF) on discharge. Overall, there was no overall benefit with this approach found in this study.

In a clinical trial testing coronary artery reperfusion with hyperoxic blood during reperfusion, 269 patients with acute AMI undergoing PCI were randomly assigned to receive hyperoxemic blood reperfusion or normoxemic blood reperfusion by catheter into the area of reperfused myocardium[15]. At 30 days, there was no significant difference in the infarct size, ST-segment resolution, or regional wall motion score. Although improvement in cardiac function was seen in patients with anterior MI who were reperfused within 6 hours, this finding was a post-hoc analysis.

A meta-analysis of all studies of hyperoxic myocardial reperfusion found that this treatment caused a significant reduction in coronary blood flow, an increase in coronary vascular resistance and a significant reduction in myocardial oxygen consumption[16]. This data appears to confirm that supplemental oxygen may be harmful.

Given the lack of clinical data of the efficacy of oxygen administration, European guidelines for the management of acute coronary syndromes do not now include a recommendation for supplemental oxygen[17]. Whilst the recent American Heart Association guideline for the management of acute coronary syndromes does recommend oxygen, they note that there is no clinical trial evidence to support this recommendation[4]. The most recent addendum to the 2006 Australian National Heart Foundation Guidelines does not recommend the routine use of supplemental oxygen[18].

In summary, whilst there is some laboratory evidence of benefit for supplemental oxygen during STEMI and reperfusion, the available clinical data suggests that oxygen may be of no use or even harmful. Since oxygen is currently used routinely in many ambulance services and hospitals in the treatment of acute coronary syndromes, prospective clinical trials comparing supplemental oxygen with no supplemental oxygen in this condition are required[19, 20].

Ambulance Victoria is uniquely placed to undertake this research. The Mobile Intensive Care Ambulances (MICA) of Ambulance Victoria are equipped with 12 lead ECG capability and pulse-oximetry monitors. In Melbourne, MICA attends approximately 400 STEMI patients per year.

We therefore propose to undertake a randomised, controlled trial comparing supplemental oxygen therapy with air in patients without hypoxia who present with STEMI to determine the effect on the size of the myocardial infarct at hospital discharge.

• References

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