Line Stimulation for Cardiac Electrical Therapy

Figure 1: Photomicrographs of sections approximately parallel to and 0.1-0.2 mm below anterior ventricular epicardial surface of the left ventricle (panel a) and right ventricle (panel b). The white bar corresponds to 0.5 mm. Both ventricles of the heart contain fibers.

Description:

Ventricular fibrillation is lethal if it is not halted within minutes. Electrical defibrillation shocks are the only effective means to halt fibrillation. Patients with recurrent fibrillation frequently receive an implantable defibrillator that senses fibrillation and quickly applies a shock from electrodes in or near the heart. Our research is exploring the use of elongated, or line, electrodes to deliver the shock to the heart. Line electrodes are advantageous because they can be fabricated as catheters and implanted in the heart by threading the catheter through veins. This eliminates the need for traumatic open-chest surgery during implantation. We have sought ways to optimize shock delivery from line electrodes. We use optical dyes and a laser scanner to measure how the shock interacts with the fibers that make up the heart. We have found that a line electrode produces a more homogeneous response in the heart if the electrode is oriented parallel to the fibers compared with other directions. Ongoing work is exploring how we may incorporate line electrodes parallel to fibers into a therapy that takes advantage of the homogeneous response. Also we are attempting to further increase the homogeneity of the response by adjusting the distribution of current that emanates from the line electrode. We have measured the distribution of current and performed finite element model calculations to determine the distribution in detail. We plan to fabricate a modified electrode in which we will introduce resistance near the ends of the line electrode to lessen the rise in current that emanates from the ends. We will then determine with the laser scanner whether such a modified electrode increases the homogeneity of the response in the heart.

Figure 2: Distribution of current emanating from a line electrode calculated with a finite element model (top graph) and measured in discrete terminals of a segmented line electrode in 4 rabbit hearts (center graph, mean±one sd), and effect of introducing series resistors of 4 Kohm, 2 Kohm, 1 Kohm and 0.5 Kohm, respectively, in the leads for terminals at or near the left end of a line electrode (bottom graph). Distances of 0 and 10 mm correspond to electrode ends. With no resistors, elevated current occurs near electrode ends in the model and in hearts during stimulation. Introducing the series resistors near the left end of the electrode, such that the end terminal has highest resistance and consecutive terminals have decreasing resistance, eliminated the rise in current at the electrode end.

Selected publications about this project:

Baynham TC, Knisley SB, "Roles of Line Stimulation-Induced Virtual Electrodes and Action Potential Prolongation in Arrhythmic Propagation," (manuscript in press in Journal Of Cardiovascular Electrophysiology, 21 typed pages): publication in February or March 2001.Baynham TC, Knisley SB, "Effective Epicardial Resistance of Rabbit Ventricles,"Annals of Biomedical Engineering, 27;96-102:1998Knisley SB, Baynham TC, "Line stimulation parallel to myofibers enhances regional uniformity of transmembrane voltage changes in rabbit hearts," Circulation Research 81;229-241: 1997

Date last updated: 1/17/2001