Determination of the interference voltage in implantable medical devices with bipolar electrodes

This study investigates the coupling-model between external magnetic fields and induced voltages at the sensing input of the implantable medical devices with unipolar and bipolar electrodes. Therefore an analytical model has been developed allowing the determination of induced voltage into a pacemaker in a magnetic field with low frequencies between 10 Hz and 30 kHz. A distinction between left pectoral unipolar and bipolar pacemaker electrodes has been made here. Furthermore the different positions of a bipolar electrode were taken into consideration. It was found that depending on the position of the bipolar electrodes in a homogeneous magnetic field, major differences of induced voltage occur. The main task of this project is to ascertain the influence of electric conductivity on interference voltage in bipolar electrodes. The measurement circuit has been placed into an original pacemaker case and therefore allowed the comparison between bipolar and unipolar electrodes by only mode switching. The magnetic field has been generated by a Helmholtz coil. Part of the Helmholtz coil system was a model of a human torso filled with saline solution to simulate human tissue. The result of the measurements showed a dependency between electric conductivity and induced voltage. The extensively discussed security factor of bipolar electrodes could be calculated from the above-mentioned measurements. The assumed transition in interference voltage from bipolar to unipolar electrodes at about 100 kHz has been significantly lower in these experiments.