Activation times at each electrode are determined by identifying the moments of maximum negative slope of the ventricular electrograms. If the electrograms show two or more deflections, then the steepest slope of the activation complex is assigned as the local activation time. The mean of the consecutive activation intervals during ventricular fibrillation and the 5th percentile of the histogram of intervals between successive activations is determined for each channel of the multielectrode. We use the 5th percentile (shortest VV intervals) as an approximation of the functional refractory period during ventricular fibrillation.