CHICAGO – The global practice of giving oxygen to patients having a heart attack may cause more harm than good, results from the AVOID study showed.
Patients who were not hypoxic and received oxygen for ST-segment elevation MI (STEMI) had larger myocardial infarct size, as measured during the first 3 days of hospitalization using cardiac enzymes.
The oxygen arm had a statistically significant 25% increase in creatine kinase, compared with the no-oxygen arm, whether measured as geometric mean peak (1,948 U/L vs. 1,543 U/L) or median peak (2,073 U/L vs. 1,727 U/L), Dr. Stub said.
Cardiac troponin I levels were nonsignificantly higher with oxygen therapy (geometric mean peak, 57.4 mcg/L vs. 48 mcg/L; median peak, 65.7 mcg/L vs. 62.1 mcg/L).
When the preferred treatment approach cardiac magnetic resonance imaging was applied in about a third of patients at 6 months’ follow-up, the median infarct size remained significantly larger, at 20.3 g, in those given oxygen therapy, than in those who did not receive such therapy, whose median infarct size was 13.1 g, Dr. Dion Stub said at the American Heart Association scientific sessions.
“The primary endpoint of infarct size was significantly less without oxygen. That’s an astounding finding and really one that I think will cause many cardiologists to take note and perhaps step back,” said invited discussant Dr. Karl Kern, the Gordon A. Ewy Distinguished Endowed Chair of Cardiovascular Medicine, University of Arizona, Tucson.
“On the other hand, it’s important to realize that is a surrogate endpoint. That is not a mortality or outcome endpoint and the way that this was measured with biomarkers was admirable, but perhaps not today the most accurate. What is accurate was cardiac MR scanning, and the data held up at 6 months as well,” he added.
Although the study was not powered for clinical outcomes, patients receiving oxygen, compared with no oxygen, also had significantly more recurrent MI, at 5.5% and 0.9%, respectively, and major arrhythmia, at 40.4% and 31.4%, at discharge.
Survival at discharge was similar with oxygen versus no oxygen (1.8% vs. 4.5%), Dr. Stub, an interventional cardiologist at St. Paul’s Hospital, Vancouver, B.C., and a researcher at the Baker IDI Heart & Diabetes Institute, Melbourne, reported. .
Oxygen therapy has been used for more than a century in the initial treatment of patients with suspected MI, although there is limited evidence suggesting such therapy is beneficial in patients without hypoxia. A growing body of evidence, however, suggests that even 15 minutes of oxygen can reduce coronary blood flow, increase the production of oxygen-free radicals, and disturb microcirculation, all of which can contribute to reperfusion injury during MI, he said.
The 638 Australian patients in the AVOID (Air Versus Oxygen in ST-Elevation Myocardial Infarction) study were randomized, before hospitalization, by paramedics to oxygen administered via face mask at a flow rate of 8 L/min until patients were stabilized on the ward or to no oxygen, unless oxygen saturation fell below 94%.
Dr. Kern and others pointed out that the oxygen level provided to study participants exceeds the 2-4 L typically given to MI patients in the United States, particularly those who are nonhypoxic.
All parties agreed there is an urgent need for an adequately powered randomized trial to evaluate the effectiveness of oxygen therapy in MI, a conclusion also reached by a recent Cochrane review of the topic (Cochrane Syst. Rev. 2013 August;8:CD007160).