A medical device constructed according to the invention includes electrodes (12a, 12b), a measuring unit (24) for measuring a patient-dependent electrical parameter (e.g., impedance) of the patient, an electrotherapy generator (26) for delivering electrotherapy to the patient, and a processing unit (20) for controlling the delivery of electrotherapy to the patient. Electrotherapy is preferably delivered to the patient based on the measured patient-dependent electrical parameter and a predetermined response of a reference patient to a nominal electrotherapy. The actual electrotherapy delivered to the patient is controlled so that the electrotherapy has a probability of success for the patient that is equivalent to the probability of success of the nominal electrotherapy for the reference patient. A consistent shock efficacy across different patients is achieved.