Scaling structure of electrocardiographic waveform during prolonged ventricular fibrillation in swine

Ventricular fibrillation (VF) is the most common arrhythmia causing sudden cardiac death. Hort ever, the likelihood of successful defibrillation decli nes with increasing duration of VF. Because the morphology of the electroca rdiogram (ECG) waveform during VF also changes with time, this study examin ed a new measure that describes the VF waveform and distinguishes between e arly and late VF. Surface ECG recordings were digitized at 200 samples/s fr om nine swine with induced VF. A new measure called the scaling exponent na s calculated by examining the power-law relationship between the summation of amplitudes of a 1,024-point (5.12 second) waveform segment and the time scale of measurement. The scaling exponent is a local estimate of the fract al dimension of the ECG waveform. A consistent power-law relationship was o bserved for measurement time scales of 0.005-0.040 seconds. Calculation of the scaling exponent produced similar results between subjects, and disting uished early VF (< 4-minute duration) from late VF (greater than or equal t o 4-minute duration). The scaling exponent was dependent on the order of th e data, supporting the hypothesis that the surface ECG during VF is a deter ministic rather than a random signal. The waveform of VF results from the i nteraction of multiple fronts of depolarization within the heart, and may b e described using the tools of nonlinear dynamics. As a quantitative descri ptor of waveform structure, the scaling exponent characterizes the time dep endent organization of VF.