Filling Implantable Medical Devices for Leak Checking

A method for introducing an analyte gas into a cavity of an implantable medical device includes analyzing gas that exits the cavity through an outlet in communication with the cavity and introducing an analyte gas into the cavity via an inlet in communication with the cavity until the composition of the gas exiting the cavity comprises a predetermined percent or amount of the analyte gas. The inlet and outlet may be sealed, and the device may be leak tested.