Embodiments of the invention present methods and systems for determining an optimal defibrillation shock waveform for application to the heart of a patient to stop a rhythm abnormality such as ventricular fibrillation or ventricular tachycardia. Such methods and systems may include measuring and/or collecting information for a cardiac waveform of a patient, produced as a result of either an electrical stimulus applied to a heart of the patient, which may be a pacing shock/stimulus and/or a defibrillation shock waveform, or as the result of intrinsic cardiac activation, determining a characteristic of the cardiac waveform, comparing the determined characteristic of the cardiac waveform to a plurality of values for the characteristic with optional reference to the defibrillator system impedance, wherein each value of the characteristic is associated with a predetermined value for a parameter of an optimal defibrillation shock waveform, and selecting the predetermined value for the parameter of the optimal defibrillation shock waveform based on the comparison.