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You are not a 100% efficient oxygen extraction machine. There’s still a lot of oxygen in your breath. Less than pure air would have, to be sure, but not 0%. (It’s actually about 16%-18% oxygen.) Your blood *is* pretty much saturated, but your breath is not depleted of oxygen in the doing.
Lungs that aren’t breathing are bringing in 0% new oxygen. *Some* is better than *none.*
1. You exhale about 75% of the oxygen you inhale
2. mouth to mouth isn’t beneficial. They don’t teach it anymore.
3. Chest compressions, chest compressions, chest compressions!
Your inhales are about 20% O2, and your exhales are about 15% O2. That 15% O2 is still worth it.
Edit for more exact numbers: atmosphere (the general air) is listed at 21% oxygen, and most studies of exhales is at 16% oxygen. And it seems that what *really* matters is how much pressure the oxygen has, so as long as there’s at least 5% of oxygen at sea level pressures it will still be useful to have in your lungs (this is why space suits can be filled with pure oxygen, which is normally unhealthy, but at about 20% of the pressure). I was reading the [Breathing wiki page](https://en.wikipedia.org/wiki/Breathing).
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It’s more about getting any kind of air into their lungs to keep oxygen flowing to vital organs. The chest compressions are the crucial part, but mouth-to-mouth can help too. Better to have some CO2 mixed with O2 than no air at all.
Actually, the air you exhale during CPR still contains about 16-17% oxygen, which is enough to help the person. The idea is to keep oxygenated blood flowing to the brain and other vital organs to buy time until professional help arrives. Plus, chest compressions are the real MVPs in CPR, they move blood around even if the person isn’t breathing.
Your lungs do not absorb the majority of the oxygen in the air you breathe. The air you inhale is about 21% oxygen and the air you exhale is about 16% oxygen. When performing CPR it would be preferable to have a bag valve, mask, and even better if you have oxygen to connect to it. But if you don’t have either administering rescue breaths, while performing high-quality compression will help keep the brain and organs perfused.
Common misunderstanding!
Air you breathe in: 78% nitrogen, 21% oxygen, ~1% argon.
Air you breathe out: 78% nitrogen, 17% oxygen, 4% carbon dioxide, 1% argon.
So what you exhale is still 17% oxygen, only down 4% from normal air (!), and it’s only 4% carbon dioxide.
**TLDR:** **Blowing in 17% oxygen with 4% CO2 is better than them getting no oxygen at all.**
“**IF** you’re blowing CO2” well, it wouldn’t be.
Fortunately for our non-responsive patient, humans only extract about 1/4 of the O2 in each breath! Plenty left to help others with!
Also: CPR doesn’t require you to breathe for the patient anymore. (If you do, it’s 2breaths/30compressions.) the compressions push on the lungs a little, and the most important aspect BY FAR is to keep the blood circulating. Also avoids a decent chunk of the danger to the rescuer.
Human lungs are not all that efficient. The air you exhale has less oxygen and more CO2, but it’s actually still breathable. You can even repeat that for a few cycles before it becomes a problem, though it does eventually. Some scuba divers take advantage of this, using machines called *rebreathers* to make their air supply last longer.
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