The effect of ischemia on ventricular fibrillation as measured by fractal dimension and frequency measures

Introduction: Most animal studies of ventricular fibrillation (VF) waveform characteristics involve healthy animals with VF initiated by electric shock. However, clinical VF is usually the result of ischemia. The waveform characteristics in these two types of VF may differ. The angular velocity (AV), frequency ratio (FR) and median frequency (MF) are three frequency-based measures of VIE. The scaling exponent (ScE), the Logarithm of the absolute correlations (LAC) and the Hurst exponent (HE) are three measures of the fractal dimension of VF. Hypothesis: We hypothesized that these quantitative measures would differ between ischemic and electrically initiated VF. Methods: VF was induced in 14 swine by electric shock and in 12 swine by ischemia. For ischemia induced VF animals, an angioplasty catheter was positioned in the mid-LAD and the balloon inflated. A mean of 891 +/- 608 (S.D.) s later, VF occurred. For electrically induced animals, an AC current was passed through a catheter in the RV. Following initiation by either method, VF was recorded for 7 min. Sequential 5 s epochs were analyzed for AV, FIR, MF and fractal dimension measures. Results: Ischemic VF demonstrated a significantly higher fractal. dimension as estimated by the ScE for the first 0-90s (p = 0.021) and for 90-180s (p = 0.016). The Hurst exponent was significantly higher for ischemic VF for both 0-90s (p < 0.0001) and 90-180s (p < 0.0001). The fractal dimension as estimated by the LAC method was not significantly different for 0-90 s (p = 0.056) but was highly significant for 90-180 s (p = 0.001). During the initial 90 s the groups did differ in all measures of frequency as follows: AV (p < 0.001), FR (p < 0.001), MF (p < 0.001). These differences did not persist beyond 90 s except for a mild elevation of the FR after 270 s (p < 0.02). Conclusion: Fractal based measures indicate an increase in the fractal dimension of ischemia induced VF for the first 180s when compared to electrically induced VF. Frequency-based measures uniformly demonstrate a pattern of higher frequencies for electrically induced VF for the first 90s. The increased fractal dimension and decreased frequencies associated with ischemia induced VF may reflect changes in the underlying myocardial physiology that can be used to guide therapies. (C) 2007 Elsevier Ireland Ltd. All rights reserved.