If this was enough to temporarily replace breathing I wonder how that would feel if you were otherwise healthy. I imagine not breathing would instinctively feel quite strange and even distressing.
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There are also chemoreceptors for oxygen concentration in the circulatory system as well.
I think everything you have said is correct, I just wanted to add a few more details for anyone who is interested.
All that apart, I'm guessing this would be used in emergency situations, where a patient is likely already unconscious and could be kept under sedation until transferred to ECMO. Is CO2 buildup dangerous on its own? If so, in what kind of time-frame? What's the upper limit on the additional minutes this therapy could buy?
In an acute situation where oxygenation isn't sufficient, the imminent threat of anoxic brain injury and end-organ dysfunction is the concern. Measures would obviously be taken to correct that, up to and including rapidly sedating and paralyzing a patient in order to mechanically ventilate them with an increased fraction of inhaled oxygen and/or additional pressure (PEEP) to increase the surface area in the alveoli available for gas exchange.
Respiratory acidosis (i.e. the accumulation of CO2 and acidification of the blood due to inadequate breathing) is generally not harmful on its own, the concern there is just adequate oxygenation. However there are metabolic causes of acidosis, usually due to lactic acid accumulation, which lead to end-organ dysfunction because lots of enzymatic reactions in the body expect a very narrow pH range to work effectively. This occurs over a period of days, though.
So, it sounds like if this works (big if, of course, at this point), sedation + an enema could be a better "bridge" to mechanical ventilation than CPR. That would be amazing (if it works); science fiction stuff.
The main determinant of successful CPR is maintaining coronary perfusion pressure with unrelenting chest compressions so that the heart has a fighting chance at starting to beat normally again. Moving the blood so that it has enough pressure at the aorta where the coronaries branch off of is way way way more important than keeping it oxygenated, which we're already pretty good at. In fact, over-oxygenation in CPR has been shown to be detrimental to outcomes because it causes oxidative stress at the cellular level. Oxygen is nasty, it's amazing that life evolved to harness it.
I do agree that modern medicine (especially emergency medicine) is really cool, that's why I switched careers after working in software engineering. We have lots of tools at our disposal, it's already science fiction. Modern resuscitation involves drugs that manipulate the ion channels of the heart in various ways, we can shift fluids around by changing the osmolarity of IV fluids (and we can pump them into you through your bones after drilling into them if needed...), cardiac monitors and AEDs will time a shock just right depending on the dysrhythmia to increase the odds of success, we can even just repeatedly shock a heart to make it beat in some situations like an AV block. And that's just the stuff that they let paramedics do (i.e. trained monkeys, I am one).