Effects on Infarct
Following the first report in 1900 of supplemental oxygen relieving angina, pre- and in-hospital oxygen "has really been a fundamental component of first-aid management of patients with suspected acute myocardial infarction, and this is done the world over," Stub said. International guidelines differ on who should be given oxygen, he noted, "but all guidelines recognize that this fundamental of practice has very limited evidence behind it in a randomized clinical-trial fashion."
However, recent physiologic data suggest that even as little as 15 minutes of oxygen can cause hyperoxia, leading to a reduction in coronary blood flow, increased coronary vascular resistance, increased oxygen free radicals, and disturbed microcirculation, he said, "and this all may contribute to increased reperfusion injury, myocardial injury during acute coronary syndromes."
AVOID was an investigator-initiated randomized, controlled, multicenter trial with the aim of comparing supplemental oxygen therapy with no oxygen in STEMI patients with oxygen saturation in the normal range.
It was a "pragmatic" trial coordinated by the research division of Ambulance Victoria, in conjunction with nine tertiary-care centers in Melbourne, Australia. "A key component of the trial is that all patients were randomized by the paramedics prehospital," he noted.
Patients were included if they had symptoms suspicious of MI for less than 12 hours, normal oxygen levels, defined as O2sat>94% measured by pulse oximeter, and a diagnostic prehospital ECG with ST-elevation on two or more contiguous leads. Patients were excluded if oxygen saturation was below 94%, they were in an altered conscious state, they received oxygen prior to randomization, or there was planned transport to a nonstudy hospital.
Patients with confirmed STEMI randomized to the oxygen arm (n=218) received 8 L/min of O2 given prehospital right through to admission to the coronary cath lab for primary angioplasty and until they were stable on the ward. Patients in the no-oxygen arm (n=223) were given no oxygen unless they became hypoxic (O2sat