For those flying in Europe (as Jeppesen Route Manual says):
SPECIAL PROCEDURES FOR IN-FLIGHT CONTINGENCIES EUROPE
Ref Regional Supplementary Procedures ICAO DOC7030 - EUR
EMERGENCY DESCENT PROCEDURES
ACTION BY PILOT-IN-COMMAND
1. When an aircraft operated as a controlled flight experiences sudden decompression or a (similar) malfunction requiring an emergency descent, the aircraft shall, if able:
a. Initiate a turn away from the assigned route or track before commencing the emergency descent;
b. advise the appropriate air traffic control unit as soon as possible of the emergency descent;
c. Set transponder Code to 7700 and select the Emergency Mode on the automatic dependent surveillance / controller-pilot data link communications (ADS/CPDLC) system, if applicable;
d. turn on exterior lights;
e. watch for conflicting traffic both visually and by reference to ACAS (if equipped), and
f. coordinate its further intentions with the appropriate ATC unit.
2. The aircraft shall not descend below the lowest published minimum altitude which will provide a minimum vertical clearance of 300m (1000ft) or in designated mountainous terrain 600m (2000ft) above all obstacles located in the area specified.