Oxygen therapy has been the standard therapy for all sorts of conditions. If you’re feeling sick in any way, you’re very likely to get some oxygen supplement as soon as you get inside an ambulance or a hospital. And high flow oxygen to achieve high oxygen concentration in inspired air has been a central part of the standard treatment for suspected MI – the infamous MONA treatment. Is 100% oxygen good for you?
Lately, we’ve stopped giving pure oxygen to stroke patients, and giving 100% oxygen to post cardiac arrest patients is no longer recommended. Now, many are questioning the use of high flow oxygen in MI patients and other conditions where the patient’s oxygenation really isn’t compromised. The AHA guidelines seem to view room air and oxygen almost as equal in CPR settings, but recommending FiO2 > 40%. And the guidelines have started omitting the recommendation to use oxygen supplement for all ACS/MI’s. Still, I see patients with no oxygenation problem being put on high flow oxygen on face masks all the time.
It’s easy to understand why a patient with a sat of 82% could benefit from oxygen therapy. Less so with an MI patient with a sat of 99%. It goes against most things we know. Oxygen is transported by haemoglobin, and after all the haemoglobin has been saturated, excess oxygen is transported freely dissolved in the blood. But even though you can get impressively high PaO2 numbers on your ABGs, that extra oxygen dissolved in the blood is negligible for all clinical purposes. It doesn’t add anything to the blood’s actual oxygen delivery capacity to the tissues. If the oxygen sat is 100%, it’s 100%. You can’t get 110% saturation. The volume knob doesn’t go to 11.
But hyperoxygenation does have an effect. A negative effect. That was shown already back in the 50s. So why are we still giving high flow oxygen to patients with perfectly normal sats?
There is an underpowered systematic review in Heart from 2009 that concludes: “The limited evidence that does exist suggests that the routine use of high-flow oxygen in uncomplicated MI may result in a greater infarct size and possibly increase the risk of mortality”. And there are a few controlled trials on the way, among them, the AVOID trial: A randomized controlled trial of oxygen therapy in acute myocardial infarction Air Verses [sic] Oxygen In myocarDial infarction study (AVOID Study). I hope their research is better than their spelling.
But the best article I’ve read so far, is Oxygen therapy in myocardial infarction: an historical perspective. Entertaining and educational. Here are two images from that article:
The linked article goes on to list several studies and trials that shows either no advantage or a negative effect of high flow oxygen. And this isn’t news – the studies listed go all the way back to the 50’s, 60’s and 70’s. Go read it.
The authors of the article recommend that randomized controlled trials of the use of (high flow) oxygen therapy in myocardial infarction are urgently required. Another way of saying they don’t really believe in routine oxygen treatment in MI. So, why are we still routinely using it? If we have a treatment that seems to do harm in myocardial infarctions, and no good physiological explanations supporting it, shouldn’t we stop using it while we’re waiting for the RTCs?
*Just to point out the obivious: patients with hypoxemia should of course receive oxygen therapy as needed.