An implantable cardioverter defibrillator system for determining and delivering an optimal programmed first-shock strength, comprising: (a) a plurality of implantable electrodes; (b) a shock delivery subsystem, connected to the electrodes; and (c) a ULV subsystem comprising; i) a sensor, connected to the electrodes, for sensing the electrical activity of the heart, including a change with respect to time of the T-wave of a cardiac signal and including the presence of fibrillation; ii) a timer connected to the sensor for providing a series of shock times, iii) a test-shock driver, connected to the timer, for transmitting timing and amplitude information regarding T-wave test shocks; iv) a memory unit, connected to the test shock driver and the shock subsystem, for storing programmable values such as pacing cycle length, timing intervals, an initial shock strength, and values for incrementing and decrementing shock strength; and v) a controller, connected to the sensor, test-shock driver, and shock subsystem for incrementally varying shock strength and the shock times; whereby the system provides a test shock having a shock strength and shock time selected by the controller; (d) whereby: (i) the shock delivery subsystem delivers a first test shock to the heart by: delivering a test shock at a test shock strength and at a test shock time; and sensing for cardiac fibrillation; and (ii) if fibrillation is not sensed, repeating step (i) at the test shock strength and at a different shock time; (iii) if fibrillation is sensed, setting the cardiac shock strength of an implantable cardioverter defibrillator system as a function of the test shock strength, characterized by sensing a change with respect to time in a T-wave of an electrical cardiac signal prior to delivering said first test shock, whereby the shock times provided by said timer are timed relative to the maximum derivative of the T-Wave and both test shocks are timed in relation to said maximum derivative of the T-wave. (a) a plurality of implantable electrodes; (b) a shock delivery subsystem, connected to the electrodes; and (c) a ULV subsystem comprising; i) a sensor, connected to the electrodes, for sensing the electrical activity of the heart, including a change with respect to time of the T-wave of a cardiac signal and including the presence of fibrillation; ii) a timer connected to the sensor for providing a series of shock times, iii) a test-shock driver, connected to the timer, for transmitting timing and amplitude information regarding T-wave test shocks; iv) a memory unit, connected to the test shock driver and the shock subsystem, for storing programmable values such as pacing cycle length, timing intervals, an initial shock strength, and values for incrementing and decrementing shock strength; and v) a controller, connected to the sensor, test-shock driver, and shock subsystem for incrementally varying shock strength and the shock times; whereby the system provides a test shock having a shock strength and shock time selected by the controller; i) a sensor, connected to the electrodes, for sensing the electrical activity of the heart, including a change with respect to time of the T-wave of a cardiac signal and including the presence of fibrillation; ii) a timer connected to the sensor for providing a series of shock times, iii) a test-shock driver, connected to the timer, for transmitting timing and amplitude information regarding T-wave test shocks; iv) a memory unit, connected to the test shock driver and the shock subsystem, for storing programmable values such as pacing cycle length, timing intervals, an initial shock strength, and values for incrementing and decrementing shock strength; and v) a controller, connected to the sensor, test-shock driver, and shock subsystem for incrementally varying shock strength and the shock times; whereby the system provides a test shock having a shock strength and shock time selected by the controller; (d) whereby: (i) the shock delivery subsystem delivers a first test shock to the heart by: delivering a test shock at a test shock strength and at a test shock time; and sensing for cardiac fibrillation; and (ii) if fibrillation is not sensed, repeating step (i) at the test shock strength and at a different shock time; (iii) if fibrillation is sensed, setting the cardiac shock strength of an implantable cardioverter defibrillator system as a function of the test shock strength, characterized by sensing a change with respect to time in a T-wave of an electrical cardiac signal prior to delivering said first test shock, whereby the shock times provided by said timer are timed relative to the maximum derivative of the T-Wave and both test shocks are timed in relation to said maximum derivative of the T-wave. (i) the shock delivery subsystem delivers a first test shock to the heart by: delivering a test shock at a test shock strength and at a test shock time; and sensing for cardiac fibrillation; and delivering a test shock at a test shock strength and at a test shock time; and sensing for cardiac fibrillation; and (ii) if fibrillation is not sensed, repeating step (i) at the test shock strength and at a different shock time; (iii) if fibrillation is sensed, setting the cardiac shock strength of an implantable cardioverter defibrillator system as a function of the test shock strength, characterized by sensing a change with respect to time in a T-wave of an electrical cardiac signal prior to delivering said first test shock, whereby the shock times provided by said timer are timed relative to the maximum derivative of the T-Wave and both test shocks are timed in relation to said maximum derivative of the T-wave.