AVOIDing Oxygen in Suspected STEMI Tied to Smaller Infarcts, Less Recurrent MI
CHICAGO, IL—Oxygen therapy has long been standard practice in patients with suspected ST-segment elevation myocardial infarction (STEMI), but results from a late-breaking trial presented November 19, 2014, at the American Heart Association Scientific Sessions suggest that eliminating this strategy may lead to smaller infarcts and less recurrent MI at 6 months.
“While these results certainly need to be confirmed in large randomized controlled trials powered for clinical outcomes, we ask that everyone review their current practice of oxygen therapy,” said Dion Stub, MBBS, PhD, of the Baker IDI Heart and Diabetes Institute (Melbourne, Australia).
For the multicenter AVOID (Air Versus Oxygen In ST-elevation MyocarDial Infarction) trial, Dr. Stub and colleagues randomized 638 patients with suspected STEMI in-ambulance to oxygen therapy (8L/minute via face mask; n = 318) or no oxygen (unless O2 levels fell below 94%, then minimum titrated O2 via mask; n = 320). Once a physician confirmed STEMI in-hospital and primary PCI was initiated, those in the oxygen therapy arm (n = 218) continued to receive oxygen (8L/minute) in the cath lab and those randomized to no oxygen (n = 223) did not.
Baseline characteristics between the study cohorts were well balanced. Mean age was 63 years, more than three-quarters were men, and median oxygen saturation when the paramedics arrived was 98%. About 4% and 5% of patients presented with cardiac arrest and cardiogenic shock, respectively, and the median time from contact by paramedics to hospital arrival was about 56 minutes.
The primary endpoint was myocardial infarct size on both creatine kinase and troponin I at 72 hours. There was an increase in mean peak creatine kinase in the oxygen group compared with the no-oxygen group (AUC P = .04), but mean peak troponin I was similar in the oxygen and no-oxygen groups (AUC P = .12; table 1).
Dr. Stub said his team had issues with troponin compliance at a few sites early in the study, so that may reflect the “low numbers.”
At hospital discharge, there was an increase in the rate of recurrent MI in the oxygen group compared to the no-oxygen group (5.5% vs 0.9%, P < .01) and an increased frequency of significant arrhythmia (40.4% vs 31.4%; P = .05). At 6 months, there was no mortality difference between the groups (3.8% vs 5.9%; P = .32), but those who received oxygen had increased myocardial infarct size on cardiac magnetic resonance (20.3 vs 13.1 g; P = .04) and tended to have higher rates of recurrent MI (7.6% vs 3.6%; P =.07) and MACCE (21.9% vs 15.4%; P = .08).
In “hypothesis-generating” subgroup analyses, Dr. Stub said that patients older than 65 years, women, and those with either symptom-to-intervention times of longer than 180 minutes or pre-intervention TIMI flow of 2 or 3 benefitted from no oxygen therapy.
Time to ‘Break Up’ with MONA?
Discussing the “provocative” study, Karl B. Kern, MD, of University of Arizona (Tucson, AZ), referenced the long-taught MONA (morphine, oxygen, nitroglycerine, and aspirin) protocol. “She’s our old friend,” he said. “[But] the real question is, is MONA irrelevant?”
Dr. Kern highlighted several unanswered questions. Standard US hospital protocol is to use 2 to 4 liters of oxygen with a nasal prong, he reported, while the study used at least 6 to 8 liters with a face mask. Dr. Stub said AVOID was designed around contemporary Australian practice, but Dr. Kern commented that the optimal dose and application of oxygen needs to be further investigated.
“Perhaps the arrhythmias could be explained by further ischemic damage from oxygen super radicals, but I’m not sure about the recurrent MIs, which typically come from further plaque rupture,” Dr. Kern observed.
But most important to know, he continued, would be the optimal oxygen blood levels, not purely saturation. “We know that if you’re 99% or 99.5% saturated, there is a wide range of oxygen that you may have in your system all the way up from 100 to 300 torr,” Dr. Kern said. “In the cardiac arrest literature, it’s become apparent that the very high levels, greater than 300 torr, do look to be harmful. Could the same thing be important here?”
In terms of what to do with MONA, he concluded, “I’m not sure I’ll break up, but I’m pretty sure I’ll date MONA less.”
Stub D. A randomized controlled trial of oxygen therapy in acute ST-segment elevation myocardial infarction: the Air Versus Oxygen in Myocardial Infarction (AVOID) study. Presented at: American Heart Association Scientific Sessions; November 19, 2014; Chicago, IL.
- Dr. Stub reports no relevant conflicts of interest.
- Dr. Kern reports serving on the science advisory boards of PhysioControl and Zoll Medical.