A method of determining a state of ventricular fibrillation, includes: measuring the rhythm of the heart during ventricular fibrillation for a period of time; creating a lagged phase space reconstruction of the measured rhythm; determining a first value related to the rate of change of the leading edge of the phase space reconstruction over the period of time; and determining the state of ventricular fibrillation by relating the first value to the state of ventricular fibrillation. A defibrillation system includes at least one processor in communication with a sensor to measure heart rhythm and an applicator to apply a defibrillation pulse. The processor is adapted to create a lagged phase space reconstruction of ventricular fibrillation heart rhythm and to determine a first value related to the rate of change of the leading edge of the phase space reconstruction over a period of time.