How to Use: I
1. The human whose name is written in this note shall die.
This is the most fundamental rule of the death note. If a person's name is in the death note, this person will die at the specified time or after the standard times. Like Ryuk says: Shinigamis don't normally use the DN in this or that uncommon situation or way. It was Ryuk who wrote the rules (at best copied a manual in the world of Shinigamis) so it is safe to say that the rules don't contain the whole truth.
A Shinigami normally kills human that have several years left - someone who has to use a pacemaker has low chances to live that long - so the Shinigamis just don't know what happens in this case (they are not interested in such things after all) and assume that heart attack is the ultimate fallback but in the end (at least in canon), the DN will never fail.