I don't understand what you mean, like which issues? Pain at the last minute?
Even if there isn't anxiety, if suddenly all your organs are deprived of oxygen, I would imagine that would be painful, possibly suddenly painful if they go from having normal oxygen to suddenly none really quickly. I am not sure when, if ever, unconsciousness happens. I haven't read the PPH closely, however.
You're researching everything in such a thorough and careful way, you probably know better than I would whether my concerns could be valid. Personally, I feel like if I were to injest something that would stop my oxygen completely by blocking oxygenation in my blood, suddenly simultaneously depriving all organs of oxygen, I would want that heroin or some drug to block any possible extreme pain. Pain is awful.
But perhaps pain is inevitable, I don't know.