A computationally efficient method of determining the duration of ventricular fibrillation and the probability of successful defibrillation by analyzing an electrocardiogram using the logarithm of the absolute value of the autocorrelation function over a range of lags is disclosed. The method is particularly well suited for use in currently available defibrillators. The method is used in conjunction with frequency based measures, such as the angular velocity, to provide markedly improved accuracy in determining ventricular fibrillation duration, to indicate appropriate therapies to be delivered, and to assess the quality of ventricular fibrillation.