During Ventricular Fibrillation

Ventricular fibrillation (VF) is the principal cardiac rhythm disorder responsible for sudden cardiac death in humans. The accurate determination of local cardiac activation during VF is essential for its mechanistic elucidation. This has been hampered by the rapidly changing and markedly heterogeneous electrophysiological nature of VF. These difficulties are manifested when attempting to differentiate true propagating electrical activity from electrotonic signals and when identifying local activation from complex and possibly fractionated electrograms. The purpose of this investigation was to test the hypothesis that the presence of a balanced inwardly and outwardly directed transmembrane charge, obtained from the ratio of the inward to outward area under the cardiac transmembrane current curve (-/+ Tm area), could reliably differentiate propagating from electrotonic deflections during VF. To test this hypothesis, we applied a recently described technique for the in vivo estimation of the transmembrane current (Im) during cardiac activation. A 17-eleTNhe determination of the spatial distribution of activation states in cardiac muscle is essential for the correct interpretation of electrical activation maps and the concepts they support.1 It is widely acknowledged that this interpretation may be difficult2-S whether one uses unipolar or bipolar extracellular recording techniques. A recent and thorough examination of several strategies for gauging the reliability of activation maps obtained from epicardial arrays has clearly emphasized that the classification of what constitutes local electrical activation must be clearly predetermined and must precede any conclusions about the mechanisms of arrhythmia.1 In an attempt to study ventricular fibrillation (VF), we6 and other investigators57-9 have resorted to the use of automated-detection algorithms. However, a degree of uncertainty has always been present, which hampers definitive mechanistic conclusions6 and has led to controversies in the interpretation of results.10 This uncertainty in the interpretation of extracellular recordings during VF motivated us to seek a new approach to their understanding. Received April 28, 1993; accepted November 30, 1993. From the University of Alberta School of Medicine (F.X.W., P.A.P., K.M.K.), Edmonton, Alberta, Canada, and the Department of Biomedical Engineering (R.P.), Duke University, Durham, NC. Correspondence to Francis X. Witkowski, MD, 2C2.39 Walter MacKenzie Center, Division of Cardiology, University of Alberta, Edmonton, Alberta, Canada T6G 2R7. ment orthogonal epicardial electrode array was combined with an immediately adjacent optical fiber array to record electrical and optically coupled transmembrane potential signals during VF. Recordings were obtained during electrically induced VF in six dogs to determine the lm associated with activation and the time course of repolarization, as well as unipolar electrograms and bipolar electrograms recorded at multiple centerto-center interelectrode distances from 0.2 to 3 mm. Propagating local activations were associated with the presence of an easily identified inwardly directed Im, with a balanced inward and outward charge (-/+ Tm aream1.0). Electrotonic waveforms lacked this inward Tm (-/+ Im area-0.0). Normal Na'-mediated inward currents were directly demonstrated to be responsible for some activations during VF. (Circ Res. 1994;74:507-524.)