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Both premortem and postmortem processes involve a surgery to access the circulatory system, and perfusion (pumping) of fluid chemicals through it.
In the BPF experiments, the premortem process involved cannulating the carotid arteries of a sedated and unconscious, but still living and breathing animal. In a MAiD postmortem process, lethal drugs are taken by the patient, and only after their heart stops and they are declared dead does any surgery begin. That’s why the rat experiment with Critch waited five minutes: to simulate the time it takes to do the surgery on a large mammal.
From a scientific perspective, the important difference is ischemia: in a premortem procedure, the heart and lungs are still active, and the brain is still receiving oxygen up until the time you start pumping chemicals through it. In a postmortem procedure, the heart/lungs have stopped, the brain is deprived of oxygen, and the clock is ticking on the cascade of failures that happen after death.
(Minor science point: euthasol wouldn’t be administered in a premortem procedure; it’s a euthanizing drug, so it’s only for the postmortem procedures. Anesthesia/sedation chemicals like ketamine/xylazine would be used for a premortem procedure. Otherwise, you got it all right.)