Interference of active implants caused by electrical and magnetic fields on hand-held electric tools

Status:

completed 10/2012

Aims:

Each year, over 100,000 active implants such as pacemakers and defibrillators are fitted to patients in Germany. Around 10% of these are fitted to persons of working age who must be re-integrated into working life following implantation. Before the wearers of active implants are able to return to their workplaces, investigations are necessary into whether the function of the implants could be impaired by electric, magnetic or electromagnetic fields in the working environment. For many tasks, this is not considered an issue. It is however critical for work involving manually guided electric tools such as power drills, circular saws, jig saws and manual routers. These tools are generally held very close to the body during work. Fields emitted by these tools are coupled almost directly into the implant through the surface of its electrode. Should the field exceed a certain value, the function of the implant may be disrupted, resulting in a threat to the health of its wearer. The wearer of the implant is therefore advised to maintain a safety distance of up to 40cm from the equipment. Experience has shown the maintenance of such a great distance to be unrealistic in practice.

Activities/Methods:

To date, safety distances have generally been based upon the results of field measurements performed in companies on tools and comparison of them with the permissible values of the current DIN VDE 0848 Part 3-1 draft standard, "Safety in electrical, magnetic and electromagnetic fields - Part 3-1: Protection of persons with active implants in the frequency range 0 Hz to 300 GHz". This standard is based upon simplified mathematical models, which in turn are based for example upon simple geometric and homogeneous tissue structures. Studies performed at the IFA in recent years have shown that giving consideration to such models leads to the distance being estimated conservatively. The present project is to examine systematically the levels of the fields emitted by manually guided electric tools as a function of the power supply (mains, battery) and the potential influence of these levels upon implants in the body. A mathematical model was to be used to calculate the disruptive voltages induced at the input of an implant. This model employs a body model in which the arrangement of the tissue structures corresponds to the human anatomy. The objective was to use the results to formulate practical protective measures for wearers of implants during work involving manually guided electric tools.

Results:

The fields emitted by manually guided electric tools during operation are generally magnetic in nature. Owing to the (low) levels of the electrical voltages used, the electric fields are negligible.

The fields coupled into the electrodes of an active implanted device result in electrical voltages being induced in a body model. The level of the induced voltage is dependent upon:

• the design of the motor, particularly the laminations of the iron core (for example ring or U-shaped)
• the motor power absorbed during use of the tool
• the electrodes (e.g. unipolar or bipolar)
• the distance between the tool and the electrodes
• the size of the induction surface (surface area of the electrodes)
• the type of implant
• the orientation of the electrode surface with respect to the field

Motors with U-shaped laminations emit higher magnetic flux densities than motors with ring laminations. The distribution of the magnetic flux density on the surface of the motor is inhomogeneous on all motors.

High motor ratings lead to high motor currents and therefore to high emitted magnetic fields and induced voltages. Under any given field conditions, higher induced electrical voltages arise at the input of an implant with unipolar electrodes than at the input of an implant with bipolar electrodes.

In the body model, the values permissible to EN 50527-2-1, Annex E for a pacemaker are exceeded in an unipolar electrode (effective electrode area 155 cm²) implanted from a medical perspective in the left pectoral region under optimum field coupling and at maximum motor power when the distance between the pacemaker and the electrode area is less than eight cm. In a right-pectoral implantation, this distance is reduced according to the ratio between the electrode areas for right-pectoral and left-pectoral implants.

The induction areas arising when bipolar electrodes are used are substantially smaller than those arising with unipolar electrodes. The tip-to-ring distance is decisive for the magnitude of the area. The location of the implant has virtually no influence upon the level of induced voltage at the input of the implant. At tip-to-ring distances of less than two cm, the permissible values are exceeded only if the electric tool comes into contact with the body model.

The results indicate that the safety distances specified at present for active implants with unipolar electrodes are sensible and should be retained. For active implants with bipolar electrodes, these distances need not be observed. For standard manually guided electric hand tools, it is sufficient for the tool to be prevented from touching the body of the implant wearer in the proximity of the implant.

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