There's a rule, that a death note entry can't lead to another's dead. So here is a bad example:
A has a bomb, which will explode, if his heart stops, (this bomb is extremely accurate, so if his heart even stops for one second, the bomb will explode.)
B has a death note, and wants to kill A.
A, has a hostage, who is close to him, and if the bomb will explode, the hostage will also die.
B writes A's name in its death note. Will A die, or will he survive, because if he dies, the hostage will also die?