An implantable medical device communication unit bi-directionally communicates with an external device. The communication unit includes a receiver arranged to receive communication signals, having a signal strength, from the external device. The receiver includes a discriminator that is provided with a discriminator threshold such that the receiver means only accepts received signals having a signal strength higher than said discriminator threshold to activate the medical device. The discriminator threshold is adapted to be adjusted in response to control signals from a control unit in the medical device.