When talking to ATCOs over here in germany one thing becomes quite clear: In case of an emergency descent even more important than telling them that you do that is squawking 7700. The reason is the very densely used airspace, which in turn means very small ATC sectors both laterally and vertically. Additionally there are different agencies controlling different altitude bands. And usually aircraft that fly below or above the sector an ATCO works are filtered out so he can't see them, based on either Mode S ID or squawk.
However, 7700 is displayed on all sectors regardless of other filters and they can start much sooner to clear the path.