An improved apparatus and method for evaluating the need for, and performing as applicable, defibrillation. In one aspect, an improved defibrillation apparatus utilizing cardiographic impedance waveforms for determining cardiac output and accurately correlating this output to shockable or non-shockable cardiac conditions is disclosed. One exemplary embodiment uses electrodes having optimal spacing to enhance the accuracy of the impedance measurement. Another exemplary embodiment uses time-scale processing of the waveforms to identify fiducial points therein. Yet another embodiment uses advanced decision logic (such as fuzzy logic) to perform the aforementioned evaluation. The use of pacing spike detection and beat parsing based thereon is also disclosed.