[goodells]

It would be quite distressing because of the accumulation of CO2 in the blood, even with completely adequate oxygenation delivered intrarectally. The slight change in acid-base balance is what makes a person feel the need to breathe, and CO2 is an acidic byproduct of metabolism. This is why people with metabolic acidosis (e.g. in diabetic ketoacidosis or sepsis) have an increased respiratory rate.

[eszed]

Would CO2 still build up if someone isn't breathing at all? I'm guessing so, since you say CO2 is a byproduct of metabolism. Alternatively, could respiration exhaust enough CO2 even in a situation where the lungs are too damaged to take in sufficient oxygen?

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?

[goodells]

Yes, CO2 still builds up.

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.

[dillydogg]

In my thoracic surgery rotations in med school I was taught that the strongest stimulus for increasing the respiratory drive was the acidification of cerebrospinal fluid. Which, of course, correlates with the blood pH. This information comes from some studies in the 60s with goats, and the old guard are happy to hang their hat on it.

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.