Entropy Index as a Measure of Heartbeat Irregularity
  A method is proposed to analyze the heartbeat waveform that can yield a reliable characterization of the structure after only a few pulses. The measure suggested is entropy index that is related to the one found effective in describing chaotic behaviors in a wide variety of physical systems. When applied to the ECG data that include ventricular fibrillation, the index is shown to change drastically within a few pulses. Wavelet analysis is used to exhibit different scaling behaviors in different phases. 
Rudolph C. Hwa (University of Oregon)