Testing immunity of active implantable medical devices to industrial magnetic field environments

The aim of this work is to develop a new method for testing the immunity of active implantable medical devices to low frequency industrial magnetic fields (between 50 Hz and 50 kHz) up to the high occupational exposure limits. It is based on an experimental approach using a specific test bench with the capability of reproducing real exposure situations and testing multiple orientations between the magnetic field and the device. The solution adopted was to combined 3 concentric Helmholtz coils on 3 perpendicular axes. The test bench was designed using a numerical simulation software. A good agreement between the numerical model and the test bench characterisation was observed. The determination of the interference threshold of a pacemaker as a function of the frequency (50 Hz to 50 kHz) is given as an example. The experimental results show between 50 Hz and 400 Hz an interference threshold between the public and the occupational exposure limits. Between 400 Hz and 50 kHz, no malfunction below the occupational high exposure limits was observed. The encountered malfunctions are completely reversible. It also appears that the occurrence of interferences strongly depends on the magnetic field orientation. This new test method will be applied to several pacemakers and could be used or adapted to other active medical implants such as cardioverter defibrillators or neurostimulators.