An implantable medical device, such as a heart pacemaker, or defibrillator, has a welded capsule housing which is filled with a gas mixture containing helium, the helium permitting the detection of leaks in the capsule housing. To improve the electrical insulating properties, the remaining gas of the mixture is selected from a group of gases having electrically insulating properties, such as nitrogen, sulfur hexafluoride, carbon dioxide, or halogenated hydrocarbons.