In an implantable cardioversion device, the condition of the patient's heart is determined from an intrinsic ventricular parameter, such as the ventricular rate, and therapy is provided with a shock or pulse generator, including, for instance, antitachycardia pacing therapy or defibrillation shocks. Initially, the conditions, i.e., ventricular tachycardia or fibrillation/flutter, are determined using predetermined values for a set of thresholds defining the various conditions. Thereafter, the thresholds are changed by increasing or decreasing the therapy thresholds from the predetermined values based on the success rate of the corresponding therapy.