A cardiac stimulator for measuring pacing impedance includes a tank capacitor for delivering charge to the heart via leads, a shunt resistor, and high-impedance buffers for measuring pacing current through the shunt resistor. During the stimulation pulse, the voltage across the shunt resistor, as sampled by a high-impedance buffer, variously indicates lead and cardiac tissue resistance or capacitance. A high-impedance buffer measures the voltage between the tank capacitor and ground immediately following the stimulation pulse to allow estimation of the lead/heart tissue capacitance. A lead/heart tissue capacitance estimate is determined by look-up table is in memory or successive approximation. When the lead/heart tissue capacitance and lead resistance have been determined, a plurality of parameters for analyzing and optimizing a cardiac stimulation system may be calculated, such as the instantaneous current, the average current, the charge, and the energy delivered to the cardiac tissue.