Verification of the finite element method to model subthreshold electrical current density in saline.

Therapeutic electrical stimulation (TES) is a rehabilitative stimulation therapy for individuals with neuromuscular disorders. TES uses subthreshold transcutaneous electrical stimulation to cause muscle growth and neuron sprouting through increased blood flow. To optimize TES therapy, a new method was required to determine the electrical current density in the target muscle. This is because TES's subthreshold stimulation intensity does not produce any measurable physiological effect. The finite element method (FEM) was the numerical modeling method chosen. FEM provides the ability to the model complex properties found in living beings, such as different tissue types and anisotropic tissue properties. TES uses a 35 Hz biphasic waveform. At this low frequency, the quasi-static approximation was used. This allows the heat transfer model solution to be used to find the steady state electric field solution. The goal of this paper is to show that the heat transfer results are the same as the electric field measurements made in a biological phantom. Laboratory measurements were made in a one liter cylinder of normal saline using bipolar stimulation. Electric field measurements were made using silver chloride electrodes at various points in the stimulated solution. This allowed the mapping of the electric potentials. The empirical data compared favorably to the results of the FEM thus validating in the method and assumptions. Finite element modeling provides an accurate method to visualize the electrical current densities resulting from bipolar stimulation. The FEM can now be used to determine the optimal electrode configuration for TES and other electrical stimulation protocols. This computer modeling method will ultimately lead improved patient results.