Abstract: An implantable device for the treatment of malfunctions of a patient's heart is disclosed. The device includes a first modality for sensing an electrical function in the patient's heart and providing a first signal representative of such electrical function, and a second modality for sensing an haemodynamic function in the patient's heart and providing a second signal representative of such haemodynamic function. The device is also provided with first circuitry that is primarily responsive to the signal from one of the sensing modalities for analyzing the state of the heart's function and providing any required electrical shock therapy to the heart. The device further includes second circuitry that is responsive to uncertainty in regard to the therapy to be provided by the first circuitry and is operative to change the responsiveness of the first circuitry to the signal provided by the other of the sensing modalities for controlling the therapy that is to be provided to the heart.