Low energy magnetic stimulation of the phrenic nerve - a simulation study.

Peripheral magnetic stimulation is a promising assistive technique for rehabilitation. Today's magnetic stimulation devices, designed for transcranial stimulation, operate at currents of 6 kA and higher. This makes them expensive and bulky. Many motor neurons in peripheral nerves are more accessible, have large diameters and require significantly lower field strengths for stimulation. In this work we present a simulation environment to determine the threshold current required to trigger an action potential in phrenic nerve motor neurons for different coil geometries. An anatomical model was used for coil placement and realistic field calculations. The field distribution was calculated using the finite integration technique and then applied to a neuronal model to simulate the axon membrane dynamics. For general applicability, the coil-nerve distance and the axon diameter were varied. We show that the required current was approximately 1.3 kA for a nerve-coil distance of 35 mm, which corresponds to 20 % of the available power of a commercial TMS device. By including the nearby vagus nerve in the simulations, we showed that accidental stimulation of this nerve is highly unlikely. Our results pave the way for the development of smaller, less complex and more affordable stimulators and promise to increase the use of peripheral magnetic stimulators in clinical settings.