Significance of inwardly directed transmembrane current in determination of local myocardial electrical activation during ventricular fibrillation.

Abstract

Ventricular fibrillation (VF) is the principle 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 (-/+ Im 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-element 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 Im associated with activation and the time course of repolarization, as well as unipolar electrograms and bipolar electrograms recorded at multiple center-to-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 (-/+ Im area approximately 1.0). Electrotonic wave-forms lacked this inward Im (-/+ Im area approximately 0.0). Normal Na(+)-mediated inward currents were directly demonstrated to be responsible for some activations during VF.

Optical Mapping Approaches to Cardiac Electrophysiological Functions.

“…On the other hand, the "fiber-optic pickup" probe comprises a single optical fiber to deliver excitation light from the laser to the myocardium and six surrounding optical fibers to return the dye fluorescence from the myocardium to the photodetector [73,74]. The pickup probe is placed close to the myocardium stained with a voltage-sensitive fluorescent dye.…”

Using an Artificial Neural Network to Detect Activations during Ventricular Fibrillation

“…Witkowski and Kavanagh and their colleagues developed the Transmembrane Current Method (TCM) for differentiating between local activations and distant activity. TCM is used to estimate a value proportional to the transmembrane current (8). Blanchard and her co-workers showed that the Rule-Based Method (RBM) in combination with TCM can be useful for determining the locations of electrical activations during VF (9).…”

Design, characterization and testing of a thin-film microelectrode array and signal conditioning microchip for high spatial resolution surface laplacian measurement.

“…The ability to measure potentials at this resolution can aid in measuring or eliminating the potential gradient between the interstitium and the extracellular volume conductive medium. This may further improve the SL measurement technique, defined as the second spatial derivative of the field potential on the heart surface, which was reported to be superior to the unipolar electrogram (potential difference between the exploring electrode to the reference electrode) for the detection of local activation during ventricular fibrillation [19,20]. The in-vivo transmembrance current density (1m) calculated using the Surface Laplacian technique may be improved as suggested in previous simulation studies [21].…”

“…It has been reported that the Surface Laplacian electrogram is superior to the unipolar electrogram in detecting local activation time during VF [20,22,23]. Also, SL morphologies can determine the activation block and wavefront initiation, and improve the transmembrance current density calculation [23].…”

“…Specifically, a single Surface Laplacian electrogram requires that the electrodes be as small as possible (~25 microns in diameter), close together (~75 microns apart) [24], and recorded at a higher sampling rate than unipolar electrode recordings (16 kHz) [22]. In addition, the current conventional technology being implemented utilizes long wires to transmit the signal from the electrodes to the first stage amplifier which results in a marked decrease in the SNR.…”