I was thinking it had more to do with putrefaction setting in more quickly after prolonged illness. I suspect that heart beat was a first Modernist step toward where we’re at now with brain death. Traditional Thomistic theology would caution against using organ function as a criterion because it’s more about the presence of the animal soul, which is tied to putrefaction. I don’t buy heart beat or brain death. While they may be signs to a point, I don’t hold them to be definitive. We have so many examples even in natural science of people being brain dead or having had their hearts stopped and then later coming back around.