A method of determining a state of ventricular fibrillation from a waveform, includes: performing calculations on the Fourier transform of the waveform voltage values by using the quotient of the power in a high frequency band and of the power in a low frequency band; and determining the state of ventricular fibrillation by relating this ratio to the state of ventricular fibrillation. A defibrillation system for use in treatment of ventricular fibrillation includes at least one sensor to measure heart rhythm and at least one applicator to apply a defibrillation shock to a patient. The system further includes at least one processor which is adapted to calculate the ratio and also to obtain user input regarding survival of subgroups and average response times in the user's domain which allows calculation of an estimated maximum overall survival and selection of an optimum threshold for the ratio measure based on this maximum. The system further includes a user interface system in operative connection with the processor to provide information related to the ratio to a user.
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