An electromagnetic compatibility study of cardiac pacemaker to low frequency interferences A. Babouri, A. Hedjiedj, J.P Andretzko, L.Guendouz, M. Nadi Laboratoire d’Instrumentation Electronique de Nancy, Faculté des Sciences et Techniques, Université Henri Poincaré Nancy 1, BP 239, 54506 Vandoeuvre – les – Nancy, France E-mail:
[email protected] Tel: (0033) 383684156 Fax (0033) 383684153
Abstract: This paper presents an in vitro study of the behavior of cardiac pacemakers submitted to low frequency electromagnetic interferences. The developed method is based upon the principle specific to electromagnetic compatibility, which recommends starting from the target, identifying the disturbances and then introducing secondary influencing parameters in stepwise fashion. The presented results are concerned with the evaluation of pacemaker’s immunity to led and beaming interferences. In that last case, the interference source is magnetic field. Tests are carried out on 6 single chamber pacemakers and 5 dual chamber pacemakers. The studied interfering signal frequencies are 50 Hz, 60 Hz, 10 kHz and 25 kHz. 1. Introduction This article presents a study of the immunity of pacemakers to low frequency electromagnetic interferences. The heart pacemaker through its heart-generated signal listening function is likely to be targeted by these electromagnetic interferences. Its particular function implies to have an accurate knowledge of potentially interfering situations a carrier may be exposed to. Numerous, studies dealing with interferences between low frequency electromagnetic fields and these implants have been published [1-3]. These studies originally concerned interferences caused by the power distribution network [4-9]. More recently, studies about the interferences between electronic article surveillance systems and stimulators have also been published [10-14]. This ground, a.k.a. intermediate frequencies, is made more complicated by the diversity of technical data. In the mobile telecommunication ground[15,16], a common denominator exists between the different manufacturers (frequencies, sources strengths, beaming elements etc.). On the contrary, for systems implementing low frequency electromagnetic sources that a carrier may be exposed to, in everyday life or in industry (e.g. electrical appliances or induction heating, electronic article surveillance systems), the main difficulty essentially linked with the great diversity of situations which makes comparing results difficult. This paper presents an in vitro approach dealing with the implementation of a metrological set up dedicated to the characterization of cardiac pacemaker immunity to low frequency electromagnetic disruptions. The general problematic consists in checking this immunity in relation with led disruptions (the electrical voltage signals or current) and in relation with beaming disruptions (magnetic fields).
2. Material and method Tests have been carried out for 10 cardiac pacemakers. The studied frequencies are 50 Hz, 60 Hz, 10 kHz and 25 kHz. In this study, the devices have all been configured in inhibited stimulation (VVI and DDD for cardiac single chamber and dual chamber pacemakers according to the international codification), this configuration being the most widespread. 2.1 Galvanic coupling Preliminary tests consist in a galvanic coupling (fig.1a) between the disruptive source and the pacemaker. An in vitro study (fig.1b) completes this first approach. In this case the pacemaker is inserted in a tank containing a gelatine-based-model mimicking the electrical conductivity of tissues[17]. The interfering signal is applied through two plane electrodes located at each side of the tank (figure 1b). Generator Atrial or ventricular lead
Veff
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Generator (interfering signal)
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(a) In Air tests
(b) In vitro tests Figure 1 : Galvanic coupling
2.2 Magnetic coupling The second step concerns the behaviour of pacemakers submitted to a magnetic field. Figure 2 is a schematic representation of the measuring set up. The source of 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 pacemaker to a controlled magnetic field. The current crossing the Helmholtz structure is measured via a current probe which sensitivity is 100 mV/A. Considering the limits of the source amplifier, the maximal values of the field vary from 157 µT for a 50 Hz frequency signal to about 8 µT for 25 kHz frequency.
Faraday cage
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Gen erator 0-15 M Hz
A mplifier 40 Hz-100 kHz
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Figure 2: instrumentation of measure in the case of a magnetic coupling The device under test is composed by the lead-equipped pacemaker and form a first loop ; this last is completed by a coplanar rectangular loop made of 30 jointed turns. The areas are respectively A≈180 cm2 and Aapp ≈160 cm2; The total effective area is about 4980 cm2. Using this additional loop allows to increase the effective loop area formed by the pacer and the lead. For dual chamber pacemaker, each leads are completed by an additional loop. This device is studied in two configurations. For the first one, consisting in the “free space” attempts, the device is completed by a resistance R which represents the load of the pacemaker (fig. 3a) and is placed on a directional support at the centre of the Helmholtz source. The second case corresponds to in vitro attempts ; the probe-equipped pacemaker and the additional loop are inserted in the gelatine based model (fig. 3b), describe above. N=30 spires
B (boîtier) R = 700
E (électrode)
(a) The device under tests in Air
(b) The device under tests in Vitro
Figure 3 : Magnetic coupling
For all the configurations, tracking of the pacer is achieved with a device telemetry system. The collected data are given by counters that supply the number and percentage of stimulation and detection. The collected data results in how the pacer detects the interfering signal. 3. Results For each configuration tested, an initialisation of the pacemaker statistical counters is done before each test. The disturbing signal (conducted or radiated) is then applied for 15 minutes.
During the application of the disturbing signal, the telemetry is deactivated to avoid any interference with the pacemaker under test. At the end of this period, the values given by the different statistical counters are collected. The presented results illustrate the different steps of this study. Each test is carried out for the 4 studied frequencies and for different detection sensitivities. 3.1 Conducted disruption a- Galvanic coupling Figure 4 summarises the behaviour of 6 single chamber pacemakers submitted to a conducted interfering signal. This figure displays the detection thresholds of tested pacemakers for the different studied frequencies. For all tested pacemakers, no detection occurs for 50 Hz and 60 Hz interfering signals. For 10 kHz and 25 kHz frequencies, detection occurs. In this graph the x-axis represents the amplitude of the applied interfering signal. The y-axis represents the different pacers called A, B, C, D, E and F. Grey lines and black lines display the detection thresholds for 10 KHz and 25 KHz interfering signals respectively. They are variable according to the housing and the frequency signal, and are also related to the pre-programmed detection sensitivities.
Detection thresholds for a10 kHz interfering signal Detection thresholds for a25 kHz interfering signal
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Figure 4 : Detection levels for a single chamber pacemakers In the detection ranges, the behaviour of the pacer vary according to the amplitude of the interfering signal Veff, resulting in variations of the statistical counter values. Figure 5 illustrate the reaction of a single chamber pacemaker subjected to a conducted 10 kHz
interfering signal. On this graph, the x-axis represents the effective value Veff of the tension applied between pacemaker terminals (fig. 1a). The y-axis is double and with double reading direction. On the scale of right-hand side, which is read upwards, the bars indicate the values of the statistical counter indicating the percentage of stimulation recorded by the pacemaker. The scale on the left, which is read from top to bottom, indicates the different types of over sensing. This figure shows pacemaker’s over sensing from a voltage threshold of 2770 mV. From this voltage, the pacemaker exhibits detection and extra systole resulting in a reduction in the number of stimulations (< 10%). single-chamber, OPUS G4624 , S=1mv, F=10KHz ES
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Figure 5: Behaviour of dual chamber pacemaker according to the applied signal Vgene – in air study b- In vitro tests As for single chamber pacer, any detection occurs with 50 Hz or 60 Hz interfering signals. Figure 6 illustrates the performances of a dual chamber pacemaker subjected to a 25 KHz interfering signal. For these tests, the detection sensitivities of the atrial and ventricular chambers are set to SA=0.4 mV and SV =2.2 mV. Interaction between the pacer and the induced interfering signal are noted first in the atrial chamber, starting from a signal of approximately 110 mV. The pacemaker reaction are atrial detection and atrial extra systoles resulting in a reduction in the number of atrial stimulations (< 10%). From a 1670 mV interfering signal, ventricular over-sensing are noted with a decrease of ventricular stimulation (< 10 %).
Atrial Detection (%) Ventricular Detection (%) Atrial Stimulation (%)
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Figure 6: Behaviour of dual chamber pacemaker according to the applied signal Vgene – in vitro study
3.2 Radiated coupling For this coupling mode, 50 Hz and 60 Hz magnetic field interact with only one dual chamber pacemaker device while for 10 kHz and 25 kHz magnetic field, interactions occurs with all the tested devices. Figures 7 and 8 show the reactions of a single chamber pacemaker submitted to a 25 kHz magnetic field according to figure 3a and 3b respectively ; for both configuration, the detection sensitivity of the pacer is programmed to be 0.7 mV. On these figures, the x-axis represents the magnetic field express in micro Tesla. The vertical axis on the right corresponds to the percentage of stimulation, the left-hand one represents the percentage of detection (Det). Single chamber pacemaker - S=0.7 mV - f=25 kHz Stimulation 0 10 20 30 40 50 60 70 80 90 100
100 90 80 70 60 50 40 30 20 10 0 1,09
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Figure 7 : Behaviour of single chamber pacemaker submitted to a magnetic field (free space)
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Figure 8: Behaviour of single chamber pacemaker submitted to a magnetic field (I n vitro) Because of the limits of the Helmholtz source due to material factor, we have increase the coupling area formed by the pacemaker and the associated elements so as to increase the Veb signal amplitude induced at the pacemaker terminals. So the values of the magnetic field corresponding to the detection threshold were measured on devices presenting a effective area Aeff about 4980 cm2. However, from the measured detection thresholds serving as reference, one can estimate the equivalent magnetic fields corresponding to the detection levels of a device presenting a loop area A representative of a real life situation. Neglecting the influence of the additional loop on the field distribution, these values are deduced from the measured ones by multiplying the latter’s by a factor equal to the area loop ratio
A . For example, for A eff
the case related to figure 7, if one consider a coupling area of 200 cm2 which represents the coupling area in case of an average patient, the corresponding detection ranges are [27.1 µT – 37.6 µT]. Assuming the same coupling area, we deduce the equivalent detection thresholds related to figure 8 which are [17.2 µT – 25.9 µT]. By comparison with the recommended values of the European directive relative to the exposure of the public to electromagnetic fields [19], these detection levels remain, as for all the tested pacemakers, superior to the maximum values. The decrease in the in vitro detection thresholds is essentially attribute to the conducting coupling medium in case of in vitro tests. Taking account of the complexity of the problem, the effect of induced eddy-currents on the electromotive force induced at the pacemaker terminals has to be evaluate using an numerical calculation. 4. Conclusion This study enabled to set up a sequential approach to evaluate the behaviour of pacemaker devices submitted to a 50 Hz, 60 Hz, 10 kHz and 25 kHz disruptive sinusoidal signal is applied. The different tested configurations leads to different reactions of the pacers, according to the frequency of the interfering signal and the devices. The data collected by the
statistical counters show precisely how the pacer interfere with the disruptive signal. For most pacemaker, it results into a switching to asynchronous mode. In this mode the pacer is no more sensing any signal and it will pace at fixed frequency, but since this switching may be intermittent, this results in a complex behaviour of the pacer. This behaviour is related to the pacemaker sensing system which includes hardware and detection algorithm. In this study ten pacemakers have been tested. The first tested configuration corresponding to a galvanic coupling between the interfering source and the victim, do not relate to a real-life situation, but just constitutes a starting point for a first model validation. The in-vitro approach which is more closed to a real exposition situation has shown different types of interaction. For 50 Hz and 60 Hz interfering signal frequencies, only one device has been disrupted. For 10 kHz and 25 kHz frequencies, all the pacers exhibited different types of interaction which shows the complexity of the pacemaker sensing system. The set-up proposed in this paper is quite simple and can not hardly modelling any reasonable entry point of an external voltage source to the body, however, it’s a good starting point for a first model validation. The attempts were made for sinusoidal interfering signals and for 50 Hz, 60 Hz, 10 kHz and 25 kHz frequencies. This choice, limited as it may be, enables a sequential approach to the problem. Other frequencies or complex wave forms can be applied in the same way. 5. Acknowledgement This work is supported by EDF(Direction des Etudes et Recherche d’Electricité de France). References [1] COMAR Reports1998: Radio frequency interference with medical devices: A Technical Information Statement. IEEE Engineering in Medicine and Biology Magazine,17(3):111114. [2] Hayes DL, Wang PJ, Reynolds DW, Estes III NAM, Griffith JL, Stefens RA, Carlo GL, Findlay GK, Johnson CM 1997 Interference with cardiac pacemakers by cellular telephones. New Eng. J. Med. (New England Journal of Medic ) 336, 1473-1479. [3] KS Tan and I Hinberg 1998 Can Wireless Communication Systems Affect Implantable Cardiac Pacemakers? An In Vitro Laboratory Study ,Biomedical Instrumentation Technology 32, no. 1, 18–24. [4] Scholten A, Joosten S, Silny J 2005 Unipolar cardiac pacemakers in electromagnetic fields of high voltage overhead lines. med./biol. , J Med Eng Technol. 29 (4): 170 - 175 [5] A. Scholten and J. Silny2001, The interference threshold of cardiac pacemakers in electric 50 Hz fields, Journal of Medical Engineering & Technology. 25, Number 1, pages 1-11. [6] Trigano A, Blandeau O, Souques M, Gernez JP, Magne I 2005, Clinical study of interference with cardiac pacemakers by a magnetic field at power line frequencies. med./biol., J Am Coll Cardiol 45 (6): 896 – 900 [7] 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. 14, N°12, pp. 2114-2122. [8] Butrous G.S., Bexton R.S., Barton D.G., Male J.C., Camm A.J. 1983 Interference with the pacemakers of two workers at electricity substations, British Journal of industrial Medicine 40, pp. 462-465. [9] Kaye G.C, Butrous G.S., Allen A., Meldrum SJ, Male JC, Camm AJ.1988 The effect of 50 Hz external electrical interference on implanted cardiac pacemakers Pace. 11, 999-1008.
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