I’m a huge fan of oxygen. I breathe oxygen nearly every day, and without it, I would literally, moreso than figuratively, die.
But, oxygen is a highly reactive molecule with many adverse effects in the human body. Recognition of such seems to be in direct contrast to the otherwise reasonable hypothesis of increased oxygenation providing benefit in ischemic disease states. The most recognizable of these is acute myocardial infarction, where oxygen is enshrined in the classical (and outdated) MONA mnemonic.
This is the AVOID trial, randomizing patients in the field with prehospital diagnosis of STEMI to either 8L/min inhaled oxygen, or oxygen only as needed to maintain saturations >94%. All patients received aspirin from paramedics en route to the receiving facility, with further care as per local standards and protocols. The primary outcome was infarct size, as measured by peak troponin and creatine kinase levels.
Paramedics screened 836, randomized 638, an additional 50 were protocol non-compliant, and then 118 were declared not to be STEMI upon arrival at the receiving facility. The remaining 470 underwent angiography, and the final cohort for analysis was the 441 for whom STEMI was ultimately confirmed. Groups were generally similar between interventions, although there was an excess of 8 patients with LAD lesions in the oxygen arm and of 10 patients with circumflex lesions in the no-oxygen arm. There were 11 excess single-vessel patients in the oxygen arm and 17 excess multi-vessel disease patients in the no-oxygen arm.
The answer? Oxygen is probably bad. There was no statistically significant difference in mean peak troponin values, favoring the no-oxygen having a p-value of 0.18. The mean peak CK difference did, however, reach significance, with a p-value of 0.01. In the 127 patients for whom follow-up MRI imaging was available, measures of infarct size all favored the no-oxygen group, with p-values ranging between 0.04 and 0.08. However, clinical outcomes were all over the map. The no-oxygen arm had higher in-hospital mortality, but the oxygen arm had higher rates of recurrent myocardial infarction. Long-term, six-month outcomes were likewise similar, with trivial clinical differences.
So, oxygen application during routine pre-hospital transport for chest pain is certainly useless and wasteful – and most likely at least a little bit harmful.
“Air Versus Oxygen in ST-Segment Elevation Myocardial Infarction”
http://circ.ahajournals.org/content/early/2015/05/22/CIRCULATIONAHA.114.014494.abstract