Now the thing about the drug that the Haitians use that cause a catatonic state is that the person who is buried is almost fully aware of everything that is going on but cannot respond autonomically. This is why, when the victim witnesses the voodoo witch pulling them out the grave they are eternally grateful and enslave themselves.
It wouldn't be a hard stretch to think about adrenochrome from catatonic people, it would actually be highly plausible for the fear factor and getting that adrenalin pumping.