Immunity of cardiac pacemakers to
low frequency magnetic fieldS

A. Hedjiedj, M. Nadi
Laboratoire d’Instrumentation Electronique de Nancy, Faculté des Sciences
Université H. Poincaré de Nancy I – BP 239 – 54506 Vandoeuvre les Nancy – France

Abstract: This paper describes the behavior of single and double chamber pacemakers subjected to a low frequency magnetic field. The results allow to define the detection levels of the stimulators and to show the reactions of the latter while the interfering signal is applied.

INTRODUCTION

The important development of systems containing electromagnetic sources, in everyday life or in industry (e.g. electrical appliances or induction heating), has been accompanied by projected standards or regulations aiming at avoiding the risks to patients with medical implants. In the case of cardiac  stimulators, studies dealing with interferences between low frequency electromagnetic fields and these implants have been published. These studies originally concerned interferences caused by the power distribution network [1,2]. Since then, studies about the interferences between electronic article surveillance systems and stimulators have also been published [3]. The main difficulty of these studies is essentially linked to the great diversity of possible situations, which makes comparing results difficult.

This article describes an in vitro study of the behavior of cardiac stimulators submitted to low frequency electromagnetic disturbances. The method developed is based upon the principle specific to electromagnetic compatibility which recommends starting from the target (the cardiac stimulator), identifying and quantifying the disturbances (the source), and then introducing secondary influencing parameter in stepwise fashion. The tests are carried out on single and double chamber stimulators, and the magnetic field is sinusoidal at 50 Hz, 60 Hz, 10 kHz and 25 kHz. These tests allow to define the detection levels of the stimulators and to reveal the reactions of the latter while the interfering signal is being applied.

MATERIALS AND METHODS

The source of the disturbances is a Helmholtz-type coil. It consists of two 1.5 m loops separated by a 0.75 m gap. The loops are formed of six jointed turns of 16 mm2 wire. This structure, powered by a programmable power generator, is used to subject a cardiac stimulator to a perfectly controlled magnetic field, both in amplitude and direction, in order to simulate any real-world exposure situation.

The stimulators tested are configured in inhibited mode and both the stimulation and the detection are programmed in unipolar mode. The device under test is made of a stimulator equipped with one or two probes which, via the gel medium, forms one or two surface loops S (Fig. 1). Each probe is completed by a rectangular loop made of 30 jointed turns, whose surface S’ is 165 cm2 (11 cm*15 cm). Using this loop allows to increase the apparent surface of the loop formed by the stimulator and the probe. These loops are connected in series with each probe (atrial and ventricular); the combination is placed in a tank containing a gel that models the conductivity of tissues (for these tests, the conductivity is 1mS/cm). Figure 2 shows the device under test in the case of a single-chamber stimulator. The resistance R represents the load on the stimulator.

Figure 1. The device under test

Figure 2. Increase of the apparent surface loop

The tank containing the stimulator is placed at the center of the Helmholtz coil. The coupling between the magnetic field and the loop formed by the device under test is maximal. The stimulator is submitted to the magnetic field for 10 minutes. After each test, the use of telemetry allows to measure the behavior of the stimulator during the application of the interfering signal.

RESULTS

The results shown below illustrate how the two types of stimulator work. Figure 3 represents a single chamber stimulator submitted to a 25 kHz magnetic field; the detection sensitivity (S) is programmed to be 0.7 mV. The x-axis represents the V_I voltage proportional to the current flowing through the source. The vertical axes have a double reading direction corresponding to the percentage stimulation (Stim) and detection (Det), respectively. The tests highlight a window effect of the stimulator detection circuits, within which numerous detections are noticed. There is an 80% maximum detection ratio for V_I=24.7 mV.

Figure 3. Behavior of a single chamber pacemaker

Figures 4 and 5 show the reactions of a double chamber stimulator to 50 Hz and 25 kHz magnetic fields, respectively. The detection sensitivity of the atrial and ventricular chambers are, respectively, S_A=0.4 mV and S_V=2.2 mV. The vertical axis on the left represents the percentage of ventricular (VD) and atrial (AD) detections, and the periods in asynchronous (As) and safety (Sec) modes. The axis on the right represents the percentage of ventricular and atrial stimulations (V/A stim.). A window effect of the detection circuits is again noticed. The reactions of the stimulator vary over the detection range. Atrial and ventricular detections are noticed together with periods in asynchronous and safety modes.

Figure 4. Behavior of a double chamber pacemaker

Figure 5. Behavior of a double chamber pacemaker

Discussion

The results reveal the global behavior of the stimulators during the application of the magnetic field. For the single chamber pacemaker a detection corresponds to the inhibition of a pulse, which is translated into a variation of the instantaneous frequency of the stimulator. For the double chamber stimulator the problem is more complex and a thorough analysis is needed to determine if the system works correctly.

It is possible to determine the equivalent magnetic field corresponding to the detection levels without the additional(s) loop(s). Tables 1 and 2 show the equivalent magnetic field corresponding to the detection ranges for the two types of stimulators under study, for the four studied frequencies. By comparison with the recommended values of the European directive relative to the exposure of the public to electromagnetic fields [4], these detection levels remain in all cases superior to the maximum values.

Table 1 : Detection ranges of a double chamber pacemaker

	Detection ranges
Sensitivity	50 Hz	60 Hz	10 kHz	25 kHz
SV=2.2 mV SA=0.4 mV	153 mT – 174 mT	142 mT - 161 mT	108 mT - 121 mT	47 mT -    56 mT

Table 2 : Detection ranges of a single chamber pacemaker

	Detection ranges
Sensitivity	10 kHz	25 kHz
0.7 mV	41 mT-51 mT	34 mT-43 mT
2 mV	108 mT-125 mT	79 mT- 86 mT
3 mV	-	109 mT-124 mT

References

[1] A. Scholten and J. Silny, 2001,The interference threshold of cardiac pacemakers in electric 50 Hz fields, Journal of Medical Engineering & Technology, Volume 25, Number 1, pages 1-11.

[2] Toivonen L., Valjus J., Hongisto M., Metso R., 1991, The influence of elevated 50 Hz electric and magnetic fields on implanted cardiac pacemakers : the role of lead configuration and programming of the sensitivity, Pace, Vol. 14, N°12, pp. 2114-2122.

[3] Lucas E.H., Johnson D., McElroy B.P., 1994, The effects of electronic article surveillance systems on permanent cardiac pacemakers : an in vitro study (Part II), PACE, Vol. 17, pp. 2021-2026.

[4] Journal officiel des communautés européennes (7/07/1999) Recommandation du conseil du 12 juillet 1999 relative à la limitation de l’exposition du public aux champs électromagnétiques (de 0 Hz à 300 GHz).

Acknowledgements: This work is supported by EDF R&D.