The invention relates to an implantable electrical tissue stimulation system (10), comprising: one or more electrical terminals (58) configured for being coupled to one or more stimulation leads (12)； output stimulation circuitry (50) configured for outputting electrical stimulation energy to the one or more electrical terminals (58) in accordance with a defined waveform； monitoring circuitry (70) configured for measuring one or more electrical characteristics of the tissue； and control circuitry (74) configured for modifying a pulse shape of the defined waveform based on the measured one or more electrical characteristics, thereby changing the characteristics of the electrical stimulation energy outputted to the one or more electrical terminals (58).
