According to Aztec miniature, the chest was cut between ribs and the heart removed by hand. And the priest had only obsidian knife instead of steel.
Same could be plausible for north american indian.
Mongol shepherds use a different way : to avoid blood spilling, they cut the diaphragm and stop the heart by hand, without removing it.
So religious motivation/display of toughness could be more important than practicality in targeting the heart.