1322 Exposure to static magnetic fields and disturbances of active implantable medical devices

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Abstract

Introduction

According to Directive 2013/35/EU for magnetic fields from 0 to 1 Hz, the exposure limit value (ELV) for sensory effects due to magnetic flux density is 2 T (Tesla) and 8 T for limbs. Interference with active implanted devices, e.g., cardiac pacemakers, starts at an action level (AL) of 0.5 mT. Interference can influence the functioning of medical devices. The aim is to describe some examples of static magnetic field exposure and calculations to evaluate the possible risk of disturbances to medical devices.

Methods

We measured and analysed field values on the surface of magnetic objects and how the exposure to static magnetic fields decreased when the distances of the source increased. The used metres were based on Hall effect sensors; Holaday Industries HI 3550 or Extech MF100. Then, we compared theoretically measured values to the abovementioned directive levels.

Results

Static magnetic flux density can be high (e.g., 20–200 mT) at the surfaces of magnets, and magnetising fields can be very high (0.5–3 T). However, the distance attenuation decreases the flux rapidly; for example, at a distance of 1 cm from the cable, the field can be 2.5 mT (shielded cable, current 400 A), but at a distance of 10 cm, 0.5 mT (AL for active implanted devices), and 30 µT at a distance of 1.0 m. Thus, the DC field very seldom exceed the action value. These attenuations can also be calculated theoretically.

Discussion

Our focus was on implantable medical devices. The distance to magnetic objects was found to be critical.

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