The Influence of Elevated 50 Hz Electric and Magnetic Fields on Implanted Cardiac Pacemakers: The Role of the Lead Configuration and Programming of the Sensitivity LAURI TOIVONEN, JORMA VALJUS,* MIKKO HONGISTO,* and RIITTA METSO From the First Department of Medicine, Helsinki University Central Hospital, and the *Research and Development Division, Imatran Voima Oy, Helsinki, Finland
TOIVONEN. L., ET AL.: The Influence of Elevated 50 Hz Electric and Magnetic Fields on Implanted Cardiac Pacemakers: The Role of the Lead Configuration and Programming of the Sensitivity. The influence of the electromagnetic interference fEMJJ on performance of 15 implanted cardiac pacemakers (12 generator models) was tested during exposure at a high voltage substation. All patients had an adequate spontaneous heart rate during fhe study, Tests were performed in the ventricular inhibited mode with unipoiar sensing in ail pacemakers and repeated with bipolar sensing in four pacemakers. The sensitivity was set to a regular, functionally proper level and then to the highest available level. Exposure was done to moderate (1.2-1.7 kV/m) and strong (7.0-8.0 kV/m) electric fields, which correspond to the immediate vicinity of 110 and 400 kV power lines, respectively. In moderate electric fields the output was inhibited in one pacemaker at regular sensitivity (1.7-3.0 mV) and in five pacemakers at the highest sensitivity (0.5-1.25 mV). In strong electric fields the output was inhibited in five pacemakers at regular sensitivity and several pacemakers converted to noise reversion mode ai the highest sensitivity, in bipolar mode only one of/our pacemakers at high sensitivity fO.5-1.0 mV) was inhibited in the strongest electric field. whereas all four did so in the unipolar mode. One pacemaker with unipolar sensitivity at 0.5 mV was interfered by 63 |xT magnetic field. The results confirm thai the programmed sensitivity level and the lead configuration markedly influence pacemakers' vulnerability to EMI. Bipolar sensing mode is rather safe in the presence of EMI, which is encountered in public environments. The programmable features of today's pacemakers permit individualized, less stringent safety measures to avoid electromagnetic hazards. (PACE, Vol. 14, December 1991) pacemaker, lead configuration, sensitivity, electromagnetic interference
Introduction The electromagnetic interference (EMI) is a well recognized hazard for cardiac pacing. It may temporarily inhibit pacemaker's output or induce asynchronous pacing.^"^ High voltage power lines
Address for reprints; Lauri Toivonen, M.D., First Department of Medicine. Helsinki University Central Hospital. Haartmaninkatu 4, SF-00290 Helsinki. Finland. Fax: (358) 0-4714574. Received February 6. 1991; revision April 17, 1991; revision Augnst 14. 1991; accepted August 22. 1991.
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are a common source of strong EMI to which pacemaker patients may be exposed incidentally or by occupational reasons. Even stronger electric fields during diagnostic and therapeutic medical procedures may lead to reprogramming of the pacemaker,'^"'^ damage to the generator/ elevation in the pacing threshold,'^ and serious cardiac arrhythmias.**'^ Susceptibility of various generator models to EMI has been studied previously.^•^•''"" Among other factors, the sensitivity circuit determines the vulnerability of a pacemaker to EMI. Previous re-
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ports have referred to lowering the sensitivity setting in order to avoid interference.'^ Theoretically, bipolar pacemakers are better protected from EMI due to a smaller receiver length and provide undisturbed recognition of spontaneous heartbeats better than unipolar pacemakers. Exposure tests with implanted pacemakers^'' and other clinical experience^ have suggested a benefit from bipolarity. In the present study we examined the influence of EMI on implanted cardiac pacemakers with emphasis on the programmed level of sensitivity and the lead configuration. The pacemaker patients, who all had an adequate spontaneous heart rate during the study, were exposed to electric and magnetic fields in a high voltage substation and monitored with on-line electrocardiography.
Methods Eifteen patients with an implanted cardiac pacemaker participated in the study. Only patients with a satisfactory underlying spontaneous heart rhythm were included. Patients with a history of coronary artery disease, heart failure, or significant cardiac arrhythmias were excluded. Ambulatory electrocardiography was performed with the pacemaker programmed to a low rate back-up ventricular pacing to ascertain the presence of an adequate heart rhythm in case the pacemaker would be inhibited in the study. Clinical characteristics of the patients are indicated in Table I. Each patient gave informed consent. The study was approved by the ethical review board of the Helsinki University Central Hospital. Eight patients had a single chamber and seven
Table 1. Patient Ctiaracteristics Spontaneous Rhythm During the Study
Patient Number
Age/Sex
Heart Disease
8 9 10
45/M 23/F 43/M 48/F 42/F 46/M 61/M 65/M 26/F 57/M
None None None None VSD, operated None None Valvular None None
11
51/M
None
12
23/F
None
13
49/M
14
48/M 47/M
Valvular, operated None None
1 2 3 4 5
6 7
15
Indication for Pacing 2° AVB 2° AVB on exercise 2° AVB 2° AVB 3° AVB, resolved Sick sinus syndrome 2° AVB Sick sinus syndrome iHypervagotonia Carotid sinus hypersensitivity Carotid sinus hypersensitivity 3° AVB occasionaiiy at rest 3° AVB, resolved Sick sinus syndrome Hypervagotonia
Heart Rate*
Atrioventricular Conduction
Pacing Mode*' DDD DDD DDD VVIR VVI WI DDD DDD
95
2° AVB Normal Normal 2° AVB r AVB Normal 2° AVB r AVB Normal Normal
95
Normal
WI
110
Normal
WI
110
Normal
WI
70
Normal Normal
WI
(min"^) 45 90 85 70 105 50 100
90 90
80
DDI DDI
W!
'The lowest spontaneous ventricular rate during the exposure; " t h e patients' clinical pacing mode; AVB = atrioventricular block; VSD = ventricular septal defect.
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a dual chamber pacemaker. There were 12 different generator models produced by four manufacturers (Table II), introduced mainly during the late 1980s. Tbe generators were located in the rigbt prepectoral region and the tested leads were endocardial and positioned in tbe right ventricular apex. The atrial leads, if present, were positioned in the rigbt atria! appendix. Four pacemakers could be programmed to unipolar and bipolar modes; all others were unipolar. Twelve generators provided telemetric readout of tbe programmed values.
Programming the Pacemakers for the Test Tbe sensing of normal ventricular beats was examined. Artificial inhibition of ventricular pacing by myopotentials was examined during vigorous contraction of the pectoral and shoulder muscles. Sensitivity levels that detected normal heartbeats and the levels tbat permitted artificial inhibition of pacing are indicated in Table II. The sensitivity was kept on a previously used level, usually nominal, unless undersensing or myopotential inhibition required the sensitivity to be in-
Tabie ii. The Pacemaker Models, the Sensing Performance, and the Sensitivities Programmed for the Test
Sensing Performance (mV) Patient Number
Pacemaker Manufacturer and Model
1
Intermedlcs 284-05 Cosmos II
2
284-05 Cosmos II
7
Medtronic 7005 Symbios 8416 Legend 8403 Activitrax 8341 Minix Pacesetter 2010T Paragon
8
2010T Paragon
3 4
5 6
9 10 11 12 13 14 15
285 Genesis 285 Genesis 241 Programalith Siemens 703 Sensolog 748 Dialog 688 Prolog MP 686 Prolog P
Tested Lead Configuration*
Successful Sensing**
Programmed Sensitivitytt
Myopotential
inhibltiont
Reguiar
Highest
uni bi uni bi
> 7 > 7 > 7 4
1.5
2.0
< 0.5 1.0 < 0.5
1.0 1.0
2.5
1.0
uni uni uni uni
> > > >
5 5 5 5
1.25 1.25 < 1.25 1.25
2.5 2.5 2.5 2.5
1.25 0.62 1.25 0.62
uni bi
3 5 >10 > 10 4 > 14 4
1.0 < 0.5 1.0 1.0
2.0
0.5
1.2 2.5 2.0
2.0 3.0 2.0
< 0.9 0.9 1.6 < 2.1
2.4 1.7 2.1 2.1
uni bi uni uni uni uni uni uni uni
4.8 6.8 >2.7 > 2.1
1.0
0.5
3.0
0.5 0.5 0.5 0.5
0.9 0.9
* uni = unipolar, bi = bipolar; '* lowest sensitivity level (in mV) at which spontaneous ventricuiar beats were detected (> = less sensitive level not available); f lowest sensitivity level (in mV) that permitted inhibition of the ventricular output during pectoral muscle contraction; t t sensitivity levels (in mV) that were used in the study. Intermedics, Inc.. Freeport, TX, USA; Medtronic, Inc., Minneapolis, MN, USA; Pacesetter Systems, Inc., Sylmar, CA, USA; Siemens Medical Systems, Inc., Iselin, NJ, USA.
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creased or decreased. The test procedures were also performed using the highest sensitivity level of each generator. Four pacemakers were tested in the unipolar and hipolar sensing mode. The exposure tests were performed in the VVI mode, since interpretation of pacing abnormalities in dual chamher pacing would have heen uncertain with the used telemetry method. The pacing rate was programmed lower than the patient's spontaneous heart rate [to 35-80 min~^) during slow walking. The exposure test was then performed, after which the rate was programmed higher than the spontaneous heart rate (to 55-120 min"^) and the test was repeated. The Exposure Fields Four areas were selected along the maintenance roads of a 400 kV outdoor substation. Two sites (areas I and II) were selected to cover the electric field strengths in the immediate vicinity of 110 kV power lines with portal towers having conductors horizontally. One site (area III) corresponded to the vicinity of the 400 kV power lines, and in the last site (area IV) the electric field strength was slightly higher than in the area III. The field strengths are indicated in Table III. The magnetic flux density that simulated the conditions prevailing under the 110 and 400 kV portal tower lines varied from a mean of 2.6-14 (xT (Tahle III). One site (area I) had a strong magnetic field (range 45-86 |xT) generated by a pair of compensator reactors. The patients walked slowly around a spot in each area first with their arms hanging on the sides
and then with their arms elevated horizontally. Both maneuvers were performed for 1 minute at each test site and with each combination of pacemaker settings. The same maneuvers were performed in a control area outside the high voltage substation with the pacemakers set to the highest sensitivity. The unperturhed electric field strength, i.e., the field strength without the modifying effect of a conducting object, was measured with a spherical dipole electrode 1 m above the ground level. The magnetic flux density was measured similarly with a root mean square voltmeter from the outlets of a coil antenna. In area IV the arrangement of live conductors permitted mathematical evaluation of the electric and magnetic fields. This was carried out for redundancy and the results were in harmony with the measured fields. The electric field strength and the magnetic flux density inside the exposure areas remained relatively constant throughout a single test day. Between separate days the magnetic flux density varied more than the electric field strength, depending on the consumption of electricity. Documentation of the Heart Rhythm and Pacing Heart rhythm was monitored continuously using a single channel electrocardiogram (ECG) obtained via a Hewlett-Packard 78100A telemetry transmitter (Hewlett-Packard, Waltham, MA, USA] and recorded on paper at a speed of 25 mm/ sec (Mingolog 7, Siemens, Solna, Sweden). Radio telephone contact was used to synchronize the recording with the exposure maneuvers. It also as-
Table III. The Electric Field Strengths and Magnetic Fiux Densities in the Test Areas Test Areas
Electric Fieid (kV/m) Mean zt SD Magnetic Field ((xT) Mean :t SD
1*
II*'
lilt
IV*
1.2 ± 0.1
1.7 ± 0 .2
7.0 ± 0.3
8.0 ± 0.5
63 ± 21
2.6 ± 1 .0
14 ± 4
7.4 ± 2.5
* An area vt/ith a strong magnetic field; " an area corresponding to the vicinity of 110-kV power iines; f an area corresponding to the vicinity of 400-kV power lines; % an area with the strongest electric field.
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sured quick intervention if any abnormalities in the heart rhythm or pacing would have required cessation of the exposure. The ECG was also documented with a two-channel amhulatory electrocardiographic recorder (Marquette, Milwaukee, WI, USA) for later analysis. A delay in the pacing impulse longer than the programmed interval was taken as artificial inhibition. A pacing impulse following a QRS complex at an interval shorter than the programmed pacing interval was taken as premature impulse (Fig. 1). Even a single abnormal pacing event during a 1minute test period was taken as indicative of true interference, since ahnormal events were never observed in the control area. However, the cases where the event occurred only once are indicated. A pacemaker was considered to be in the noise reversion mode when it exhibited asynchronous pacing continuously at a rate consistent with this mode.
The pacing rate was measured and a telemetry report of the programmed variables was obtained after each exposure to the strongest electric field.
Results Abnormalities in Pacing Regular Unipolar Sensitivity During regular sensitivity [1.7-3.0 mV) in unipolar mode only one pacemaker showed any pacingabnormality in the moderate (mean 1.2-1.7 kV/m) electric field strengths (Table IV). In the stronger (7.0-8.0 kV/m) electric fields the output was occasionally inhibited in five pacemakers and premature pacing impulses were observed in six pacemakers. Abnormalities were more frequent with the arms in the elevated position than on the sides. There was a tendency to an increase in ab-
Figure 1. The upper panel shows sporadic premature pacing impulses during eJectromagnetic interference. The programmed rate is loiver than (he sponfanoous heart rate. The middle panel demonstrafes continuous asynchronous pacing in the noise reversion mode under persistent strong electric field. The capturing and noncapturing pacing impulses folJovv at regular intervals. The Jower paneJ indicates artificial inhibition of the pacemaker output by electromagnetic interference. Some intervals between adjacent pacing impulses and between the pacing impulse and the succeeding spontaneous heartbeat are longer than the programmed pacing interval.
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Table IV. Number of Pacemakers with Abnormal Pacing During Unipolar Sensing at Regular Sensitivity Test Areas Programmed Sensitivity (mV)
N
3
2
2.5
5
2.1-2.4
3
1.7-2.0
5
All pacemakers
15
Pacing Abnormality Inhibited Premature Inhibited Premature Inhibited Premature Inhibited Premature Inhibited Premature
1 Side/Elev**
II Side/Elev
III Side/Elev
IV Side/Elev
Any Area Side/Elev
0/0 0/0
0/0 0/0 0/0 0/0
0/0 0/0
0/1 0/1
0/0
0/0
0/0 1/2 1/2 0/0 1/2 1/2 2/4
0/0 271* 2/2 0/2 1/3 2/4
0/1 0/1 0/0 0/0 2/2 2/2 0/2 1/3 2/5
3/6
3/6
0/0
0/0 0/0 0/0
0/r
0/0
0/0 0/0
0/0
0/0
0/0
0/1 0/0
0/0
* Event only once during the exposure; " side = arms hanging on the sides, elev pacemakers.
normalities when the sensitivity was programmed higher (Table IV). Highest Unipolar Sensitivity During the highest available sensitivity (0.5-1.25 mV) in unipolar mode also the moderate
arms elevated horizontally; N = number of
electric fields frequently provoked inhibition of the impulse and premature pacing (Table V). On the other hand, not every pacemaker was inhibited in the strongest electric field, suggesting differences between the generator models. Pacemakers at a sensitivity of 0.5 mV were inhibited in the moderate electric fields, but not in the stronger
Table V. Number of Pacemakers with Abnormal Pacing During Unipolar Sensing at the Highest Sensitivity Test Areas Programmed Sensitivity (mV) 1.25 1.0
0.9-0.62 0.5 All pacemakers
N
2 2 4
4 12
1
11
Side/Etev**
Side/Elev
Inhibited
0/0
Premature Inhibited Premature Inhibited Premature Inhibited Premature Inhibited Premature
0/0
0/0 0/0
0/0 0/0 1/1 0/1
0/0
r/1*
0/0
0/0 3/3 1/3 5/5 1/4
Pacing Abnormality
3/4
2/2 3/4
2/2
Ml Side/Elev
!V Side/Elev
Any Area Side/Elev
0/0 0/0 1/0 2/2 1/1 2/2 0/0 4/4 2/1 8/6
0/0 0/0
0/0
r/0
2/1 2/2 1/1 2/3 4/4 4/4 7/6 8/9
2/2 1/1 2/3 0/0 4/4 2/1 8/9
0/0
* Event only once during the exposure; '* side = arms hanging on the sides, elev = arms elevated horizontally; N = number of pacemakers.
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Table VI. Number of Pacemakers with Abnormal Pacing During Bipolar Sensing at the Highest Sensitivity Test Areas Programmed Sensitivity (mV)
N
1.0
2
0.5
2
All pacemaiters
4
Pacing Abnormality Inhibited Premature Inhibited Premature Inhibited Premature
1 Side/Elev*
II Side/Elev
III Side/Elev
IV Side/Elev
Any Area Side/Elev
0/0 0/0 0/0 0/0
0/0 0/0 0/0 0/0 0/0 0/0
0/0 0/0 0/1 0/0 0/1 0/0
0/0 0/0 1/1 0/0 1/1 0/0
0/0 0/0 1/1 0/0 1/1 0/0
0/0 0/0
' Side = arms hanging on the sides, elev = arms elevated horizontally; N = number of pacemakers.
fields due to conversion to the noise reversion mode that produced continuous asynchronous pacing. Altogether nine of the 12 pacemakers were adversely influenced with high unipolar sensitivity. One pacemaker at a sensitivity of 0.5 mV was probably affected hy magnetic interference, since a slightly stronger electric field (area II) did not produce inhibition or premature pacing, as was seen in a weaker electric field [area I) where the magnetic field was strong.
pacing with variable impulse intervals. The programmed parameters evaluated via telemetry remained unchanged. Inhibition of the pacing impulses caused short cardiac pauses that ranged from 0.8-1.9 seconds in length. Their duration was determined either by resumption of pacing or appearance of a spontaneous heartbeat [Fig. 1]. Some patients had isolated, but none had successive, ventricular premature beats during the exposure.
Discussion
Highest Bipolar Sensitivity During the highest sensitivity (0.5-1.0 mV) in bipolar mode oniy one of the four pacemakers showed any abnormality and that occurred in the strongest fields [Table VI]. This pacemaker was occasionally inhibited hut did not exhibit asynchronous pacing. The observation contrasted to unipolar sensing with the same programmed level on which all these pacemakers behaved abnormally: two of them already in the field strength of 1.2 ± 0.2 kV/m. None of thp pacemakers showed artificially inhibited or asynchronous pacing during similar maneuvers in the control area either in unipolar or hipolar mode.
Our study on implanted pacemakers indicates that the programmed sensitivity level and the lead configuration critically influence the susceptibility of pacemakers to EMI, Many of the tested, recently introduced generator models behaved normally in the vicinity of electric power lines when the sensitivity was programmed according to clinical requirements in unipolar mode, but paced erroneously when set very sensitive. In contrast, the same units in bipolar mode were rather insensitive to EMI. The present generator models continue to be vulnerable to EMI, but their programmable features help to overcome the hazards to cardiac pacing.
Other Pacemaker and Rhythm Abnormalities
Electric Currents Within the Body
No temporary change was observed in the pacing rate except for intermittent inhibition of
Electric fields create currents inside the body that have been estimated and calculated previ-
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PACEMAKER LEAD CONFIGURATION, SENSITIVITY, AND EMI
ously. Current strengths of 10-337 |xA have heen ohserved in the upper chest region in 1-20 kV/m electric fields,^ The minimum currents that convert pacemakers to the noise reversion mode have ranged from 29 to more than 320 |xA.^'^ Toleration of EMI has differed widely, especially between older generator models. Pacemakers are particularly vulnerahle to 50 Hz power frequency, since the spectrum of cardiac signals requires sensing within this frequency band/'* Simulation studies and real exposures do not match exactly.'" The discrepancy may be related to inability to determine the strength and path of the induced fields within the body, dependence of the currents on the direction of the interfering field, influence of the position and movements of the body,^ and the shielding effect of the body. Pacemakers' Sensitivity to Electric and Magnetic Fields We observed that in unipolar mode, only one pacemaker was inhibited temporarily in a moderate field strength of 1.7 ± 0.2 kV/m when using regular, functionally adequate sensitivity, but five of 12 pacemakers did so with the highest available sensitivity. Inhibition of pacing was as common as premature pacing, except in the stronger fields where the pacemakers tended to remain in the noise reversion mode when set very sensitive. Direct comparison of the same pacemaker units with both lead configurations indicated that bipolar sensing compared to unipolar is markedly more tolerable to EMI, e.g., at a sensitivity of 0.5 mV rare abnormalities were seen in 7.0-8.0 kV/m fields in the bipolar mode, whereas every pacemaker at the same sensitivity level was already disturbed in a field of 1.7 kV/m in the unipolar mode. Inhibition of pacing occurs in a narrow zone between the field strength that does not interfere pacing and one that converts the pacemaker to the noise reverson mode.^'^ The induced body currents fluctuate widely in natural circumstances, and occasional inhibition is, therefore, possihle over a wide range of external field strengths. The magnetic flux densities that cause pacing abnormalities in nonimplanted cardiac pacemakers have ranged from 40-70 \i.T.^^ In our study the pacemakers were not interfered by magnetic fields
PACE, Vol. 14
that ranged from 48-90 (xT with regular sensitivity, but with high sensitivity in unipolar mode vulnerability seemed possible. To estimate the influence of a 50 Hz magnetic field on pacemakers it is essential to know the coil area, determined by the course of the pacemaker lead. This area is largest when the pulse generator is placed on the left side of the chest, which makes the left-sided generator location markedly more sensitive. The present observations on magnetic interference apply, therefore, only to pacemakers with the generator on the right upper chest quadrant. Extrapolation of the Environmental Risk The tested electric field strengths are encountered locally below the high voltage lines that operate at 110-400 kV voltage.^^-^^ At a distance of 40 m even from a 400 kV line the electric field strength is rarely above 1 kV/m. The 400 kV lines are common in energy transmission, though are mainly met in rural areas. Low voltage operated household and office appliances produce markedly lower electric fields. Exposure to magnetic fields of the tested flux density could occur, e.g., near industrial AC-machines, distribution transformers, and also while using electric drills and mixers if they are brought close to the chest.'""^''These devices are localized sources of EMI compared with the high voltage lines. Ventricular sensitivity can usually be set to at least 2.5-3 mV without undersensing, which in bipolar mode seems safe. It is often necessary to program atrial sensitivity high, due to a low signal amplitude that makes atrial and dual chamber pacemakers more vulnerable. Generators are built to revert from universal pacing mode to ventricular inhibited or asynchronous mode, if interference is sensed through the atrial lead. A pause in ventricular pacing does not, therefore, follow from interference via the atrial lead. Limitations of the Study The exact relationship between the sensitivity level and the vulnerability of the pacemaker to EMI was not the aim of the study. This relationship is confounded by different susceptibility of the generator models to EMI and the variation in the induced body currents between the patients. Eor
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TOIVONEN, ET AL.
the latter reason, and because the tested sensitivity levels were not standardized, the performance of the different generator models could not be compared either. Whether extended pauses in pacing would have occurred could not be established since spontaneous heart rhythm always overtook soon. Clinical Implications The overall guidelines for the pacemaker patients in general and the regulations for safe elec-
trical environment in community planning have to be maintained in a similar manner as has been the case during the past decade in order to cover also the most vulnerable cases. The patients having a bipolar pacemaker set to low sensitivity are reasonably well protected from asystole, even in circumstances that have recently not been recommended for pacemaker patients. Tailoring the pacemaker features to individual needs diminishes the necessity to restrict the patients' activities in surroundings with elevated electromagnetic fields.
References 1. Butrous GS, Bexton RS, Barton DG, et al. Interference with the pacemakers of two workers at electricity substations. Br I Ind Med 1983; 40:462-465. 2. Butrous GS, Male JG, Webber RS. et al. The effect of power frequency high intensity electric fields on implanted cardiac pacemakers. PAGE 1983; 6:1282-1292. 3. Kaye GG, Butrous GS, Allen A, et al. The effect of 50 Hz electrical interference on implanted cardiac pacemakers. PACE 1988; 11:999-1008. 4. Belott PH, Sands S, Warren J. Resetting of DDD pacemakers due to EMI. PAGE 1984: 7:169-172. 5. Hayes DL, Trusty J, Ghristiansen J, et al. A prospective study of electrocautery's effect on pacemaker function, (abstract] PAGE 1987: 10:442. 6. Levine PA, Balady GJ, Lazar HL, et al. Electrocautery and pacemakers: Management of the paced subject to electrocautery. Ann Thorac Surg 1986; 41:313-317. 7. Gould L, Patel G, Betzu R, et al. Pacemaker failure following electrocautery. Glin Prog Electrophysiol Pacing 1986; 4:53-55. 8. Warnowicz-Papp MA. The pacemaker patient and the electromagnetic environment. Glin Prog Pacing Electrophys 1983; 1:166-176. 9. Bridges JE, Frazier MI, Hauser RG. The effect of 60 Hertz electric fields and currents on implanted cardiac pacemakers. Proc Inter Symp Electromagnetic Gompatibility, New York, NY, IEEE. 1978. pp. 258-265. 10. Deller AG. Toff WD, Hobbs RA. et al. The development of a system for the evaluation of electromagnetic interference with pacemaker function: Haz-
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11.
12.
13.
14.
15.
16. 17. 18.
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ards in the aircraft environment. J Med Engl Technol 1989; 13:161-165. Salmi I, Eskola HJ, Pitkanen MA. et al. The influence of electromagnetic interference and ionizing radiation on cardiac pacemakers. Strahlenther Onkol 1990; 166:153-156. Ghen D. Philip M, Philip PA, et al. Gardiac pacemaker inhibition hy transcutaneous electrical nerve stimulation. Arch Phys Med Rehabil 1990; 71:27-30. Electric Power Research Institute. Evaluation of the effects of electric fields on implanted cardiac pacemakers. Project Report EA-3917. Palo Alto. GA, Electric Power Research Institute, University of Rochester, 1985. Adams T. The electrode-biointerface: Sensing. In SS Barold, I Mugica (eds.J: New Perspectives in Gardiac Pacing. Mount Kisco, NY. Futura Publishing Gompany, Inc., 1988. pp. 17-25. Valjus J. Physiological effects of low-frequency electric and magnetic fields (in Finnish). Research report IVO-A-04/87. Helsinki, Finland, Imatran Voima Oy, 1987. pp. 1-154. Stuchly MA. Human exposure to static and timevarying magnetic fields. Health Physics 1986; 512:215-225. Kaune WT, Stevens RG, Gallahan Nl, et al. Residental magnetic and electric fields. Bioelectromagnetics 1987; 8:315-335. luutilainen J, Saali K, Eskelinen I. et al. Measurements of 50 Hz magnetic fields in Finnish homes. Research report IVO-A-02/89. Helsinki, Finland, Imatran Voima Oy, 1989, pp. 1-31.
PAGE. Vol. 14
TOIVONEN. L., ET AL.: The Influence of Elevated 50 Hz Electric and Magnetic Fields on Implanted Cardiac Pacemakers: The Role of the Lead Configuration and Programming of the Sensitivity. The influence of the electromagnetic interference fEMJJ on performance of 15 implanted cardiac pacemakers (12 generator models) was tested during exposure at a high voltage substation. All patients had an adequate spontaneous heart rate during fhe study, Tests were performed in the ventricular inhibited mode with unipoiar sensing in ail pacemakers and repeated with bipolar sensing in four pacemakers. The sensitivity was set to a regular, functionally proper level and then to the highest available level. Exposure was done to moderate (1.2-1.7 kV/m) and strong (7.0-8.0 kV/m) electric fields, which correspond to the immediate vicinity of 110 and 400 kV power lines, respectively. In moderate electric fields the output was inhibited in one pacemaker at regular sensitivity (1.7-3.0 mV) and in five pacemakers at the highest sensitivity (0.5-1.25 mV). In strong electric fields the output was inhibited in five pacemakers at regular sensitivity and several pacemakers converted to noise reversion mode ai the highest sensitivity, in bipolar mode only one of/our pacemakers at high sensitivity fO.5-1.0 mV) was inhibited in the strongest electric field. whereas all four did so in the unipolar mode. One pacemaker with unipolar sensitivity at 0.5 mV was interfered by 63 |xT magnetic field. The results confirm thai the programmed sensitivity level and the lead configuration markedly influence pacemakers' vulnerability to EMI. Bipolar sensing mode is rather safe in the presence of EMI, which is encountered in public environments. The programmable features of today's pacemakers permit individualized, less stringent safety measures to avoid electromagnetic hazards. (PACE, Vol. 14, December 1991) pacemaker, lead configuration, sensitivity, electromagnetic interference
Introduction The electromagnetic interference (EMI) is a well recognized hazard for cardiac pacing. It may temporarily inhibit pacemaker's output or induce asynchronous pacing.^"^ High voltage power lines
Address for reprints; Lauri Toivonen, M.D., First Department of Medicine. Helsinki University Central Hospital. Haartmaninkatu 4, SF-00290 Helsinki. Finland. Fax: (358) 0-4714574. Received February 6. 1991; revision April 17, 1991; revision Augnst 14. 1991; accepted August 22. 1991.
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are a common source of strong EMI to which pacemaker patients may be exposed incidentally or by occupational reasons. Even stronger electric fields during diagnostic and therapeutic medical procedures may lead to reprogramming of the pacemaker,'^"'^ damage to the generator/ elevation in the pacing threshold,'^ and serious cardiac arrhythmias.**'^ Susceptibility of various generator models to EMI has been studied previously.^•^•''"" Among other factors, the sensitivity circuit determines the vulnerability of a pacemaker to EMI. Previous re-
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PACEMAKER LEAD CONFIGURATION, SENSITIVITY, AND EMI
ports have referred to lowering the sensitivity setting in order to avoid interference.'^ Theoretically, bipolar pacemakers are better protected from EMI due to a smaller receiver length and provide undisturbed recognition of spontaneous heartbeats better than unipolar pacemakers. Exposure tests with implanted pacemakers^'' and other clinical experience^ have suggested a benefit from bipolarity. In the present study we examined the influence of EMI on implanted cardiac pacemakers with emphasis on the programmed level of sensitivity and the lead configuration. The pacemaker patients, who all had an adequate spontaneous heart rate during the study, were exposed to electric and magnetic fields in a high voltage substation and monitored with on-line electrocardiography.
Methods Eifteen patients with an implanted cardiac pacemaker participated in the study. Only patients with a satisfactory underlying spontaneous heart rhythm were included. Patients with a history of coronary artery disease, heart failure, or significant cardiac arrhythmias were excluded. Ambulatory electrocardiography was performed with the pacemaker programmed to a low rate back-up ventricular pacing to ascertain the presence of an adequate heart rhythm in case the pacemaker would be inhibited in the study. Clinical characteristics of the patients are indicated in Table I. Each patient gave informed consent. The study was approved by the ethical review board of the Helsinki University Central Hospital. Eight patients had a single chamber and seven
Table 1. Patient Ctiaracteristics Spontaneous Rhythm During the Study
Patient Number
Age/Sex
Heart Disease
8 9 10
45/M 23/F 43/M 48/F 42/F 46/M 61/M 65/M 26/F 57/M
None None None None VSD, operated None None Valvular None None
11
51/M
None
12
23/F
None
13
49/M
14
48/M 47/M
Valvular, operated None None
1 2 3 4 5
6 7
15
Indication for Pacing 2° AVB 2° AVB on exercise 2° AVB 2° AVB 3° AVB, resolved Sick sinus syndrome 2° AVB Sick sinus syndrome iHypervagotonia Carotid sinus hypersensitivity Carotid sinus hypersensitivity 3° AVB occasionaiiy at rest 3° AVB, resolved Sick sinus syndrome Hypervagotonia
Heart Rate*
Atrioventricular Conduction
Pacing Mode*' DDD DDD DDD VVIR VVI WI DDD DDD
95
2° AVB Normal Normal 2° AVB r AVB Normal 2° AVB r AVB Normal Normal
95
Normal
WI
110
Normal
WI
110
Normal
WI
70
Normal Normal
WI
(min"^) 45 90 85 70 105 50 100
90 90
80
DDI DDI
W!
'The lowest spontaneous ventricular rate during the exposure; " t h e patients' clinical pacing mode; AVB = atrioventricular block; VSD = ventricular septal defect.
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a dual chamber pacemaker. There were 12 different generator models produced by four manufacturers (Table II), introduced mainly during the late 1980s. Tbe generators were located in the rigbt prepectoral region and the tested leads were endocardial and positioned in tbe right ventricular apex. The atrial leads, if present, were positioned in the rigbt atria! appendix. Four pacemakers could be programmed to unipolar and bipolar modes; all others were unipolar. Twelve generators provided telemetric readout of tbe programmed values.
Programming the Pacemakers for the Test Tbe sensing of normal ventricular beats was examined. Artificial inhibition of ventricular pacing by myopotentials was examined during vigorous contraction of the pectoral and shoulder muscles. Sensitivity levels that detected normal heartbeats and the levels tbat permitted artificial inhibition of pacing are indicated in Table II. The sensitivity was kept on a previously used level, usually nominal, unless undersensing or myopotential inhibition required the sensitivity to be in-
Tabie ii. The Pacemaker Models, the Sensing Performance, and the Sensitivities Programmed for the Test
Sensing Performance (mV) Patient Number
Pacemaker Manufacturer and Model
1
Intermedlcs 284-05 Cosmos II
2
284-05 Cosmos II
7
Medtronic 7005 Symbios 8416 Legend 8403 Activitrax 8341 Minix Pacesetter 2010T Paragon
8
2010T Paragon
3 4
5 6
9 10 11 12 13 14 15
285 Genesis 285 Genesis 241 Programalith Siemens 703 Sensolog 748 Dialog 688 Prolog MP 686 Prolog P
Tested Lead Configuration*
Successful Sensing**
Programmed Sensitivitytt
Myopotential
inhibltiont
Reguiar
Highest
uni bi uni bi
> 7 > 7 > 7 4
1.5
2.0
< 0.5 1.0 < 0.5
1.0 1.0
2.5
1.0
uni uni uni uni
> > > >
5 5 5 5
1.25 1.25 < 1.25 1.25
2.5 2.5 2.5 2.5
1.25 0.62 1.25 0.62
uni bi
3 5 >10 > 10 4 > 14 4
1.0 < 0.5 1.0 1.0
2.0
0.5
1.2 2.5 2.0
2.0 3.0 2.0
< 0.9 0.9 1.6 < 2.1
2.4 1.7 2.1 2.1
uni bi uni uni uni uni uni uni uni
4.8 6.8 >2.7 > 2.1
1.0
0.5
3.0
0.5 0.5 0.5 0.5
0.9 0.9
* uni = unipolar, bi = bipolar; '* lowest sensitivity level (in mV) at which spontaneous ventricuiar beats were detected (> = less sensitive level not available); f lowest sensitivity level (in mV) that permitted inhibition of the ventricular output during pectoral muscle contraction; t t sensitivity levels (in mV) that were used in the study. Intermedics, Inc.. Freeport, TX, USA; Medtronic, Inc., Minneapolis, MN, USA; Pacesetter Systems, Inc., Sylmar, CA, USA; Siemens Medical Systems, Inc., Iselin, NJ, USA.
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PACEMAKER LEAD CONFIGURATION, SENSITIVITY. AND EMI
creased or decreased. The test procedures were also performed using the highest sensitivity level of each generator. Four pacemakers were tested in the unipolar and hipolar sensing mode. The exposure tests were performed in the VVI mode, since interpretation of pacing abnormalities in dual chamher pacing would have heen uncertain with the used telemetry method. The pacing rate was programmed lower than the patient's spontaneous heart rate [to 35-80 min~^) during slow walking. The exposure test was then performed, after which the rate was programmed higher than the spontaneous heart rate (to 55-120 min"^) and the test was repeated. The Exposure Fields Four areas were selected along the maintenance roads of a 400 kV outdoor substation. Two sites (areas I and II) were selected to cover the electric field strengths in the immediate vicinity of 110 kV power lines with portal towers having conductors horizontally. One site (area III) corresponded to the vicinity of the 400 kV power lines, and in the last site (area IV) the electric field strength was slightly higher than in the area III. The field strengths are indicated in Table III. The magnetic flux density that simulated the conditions prevailing under the 110 and 400 kV portal tower lines varied from a mean of 2.6-14 (xT (Tahle III). One site (area I) had a strong magnetic field (range 45-86 |xT) generated by a pair of compensator reactors. The patients walked slowly around a spot in each area first with their arms hanging on the sides
and then with their arms elevated horizontally. Both maneuvers were performed for 1 minute at each test site and with each combination of pacemaker settings. The same maneuvers were performed in a control area outside the high voltage substation with the pacemakers set to the highest sensitivity. The unperturhed electric field strength, i.e., the field strength without the modifying effect of a conducting object, was measured with a spherical dipole electrode 1 m above the ground level. The magnetic flux density was measured similarly with a root mean square voltmeter from the outlets of a coil antenna. In area IV the arrangement of live conductors permitted mathematical evaluation of the electric and magnetic fields. This was carried out for redundancy and the results were in harmony with the measured fields. The electric field strength and the magnetic flux density inside the exposure areas remained relatively constant throughout a single test day. Between separate days the magnetic flux density varied more than the electric field strength, depending on the consumption of electricity. Documentation of the Heart Rhythm and Pacing Heart rhythm was monitored continuously using a single channel electrocardiogram (ECG) obtained via a Hewlett-Packard 78100A telemetry transmitter (Hewlett-Packard, Waltham, MA, USA] and recorded on paper at a speed of 25 mm/ sec (Mingolog 7, Siemens, Solna, Sweden). Radio telephone contact was used to synchronize the recording with the exposure maneuvers. It also as-
Table III. The Electric Field Strengths and Magnetic Fiux Densities in the Test Areas Test Areas
Electric Fieid (kV/m) Mean zt SD Magnetic Field ((xT) Mean :t SD
1*
II*'
lilt
IV*
1.2 ± 0.1
1.7 ± 0 .2
7.0 ± 0.3
8.0 ± 0.5
63 ± 21
2.6 ± 1 .0
14 ± 4
7.4 ± 2.5
* An area vt/ith a strong magnetic field; " an area corresponding to the vicinity of 110-kV power iines; f an area corresponding to the vicinity of 400-kV power lines; % an area with the strongest electric field.
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sured quick intervention if any abnormalities in the heart rhythm or pacing would have required cessation of the exposure. The ECG was also documented with a two-channel amhulatory electrocardiographic recorder (Marquette, Milwaukee, WI, USA) for later analysis. A delay in the pacing impulse longer than the programmed interval was taken as artificial inhibition. A pacing impulse following a QRS complex at an interval shorter than the programmed pacing interval was taken as premature impulse (Fig. 1). Even a single abnormal pacing event during a 1minute test period was taken as indicative of true interference, since ahnormal events were never observed in the control area. However, the cases where the event occurred only once are indicated. A pacemaker was considered to be in the noise reversion mode when it exhibited asynchronous pacing continuously at a rate consistent with this mode.
The pacing rate was measured and a telemetry report of the programmed variables was obtained after each exposure to the strongest electric field.
Results Abnormalities in Pacing Regular Unipolar Sensitivity During regular sensitivity [1.7-3.0 mV) in unipolar mode only one pacemaker showed any pacingabnormality in the moderate (mean 1.2-1.7 kV/m) electric field strengths (Table IV). In the stronger (7.0-8.0 kV/m) electric fields the output was occasionally inhibited in five pacemakers and premature pacing impulses were observed in six pacemakers. Abnormalities were more frequent with the arms in the elevated position than on the sides. There was a tendency to an increase in ab-
Figure 1. The upper panel shows sporadic premature pacing impulses during eJectromagnetic interference. The programmed rate is loiver than (he sponfanoous heart rate. The middle panel demonstrafes continuous asynchronous pacing in the noise reversion mode under persistent strong electric field. The capturing and noncapturing pacing impulses folJovv at regular intervals. The Jower paneJ indicates artificial inhibition of the pacemaker output by electromagnetic interference. Some intervals between adjacent pacing impulses and between the pacing impulse and the succeeding spontaneous heartbeat are longer than the programmed pacing interval.
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Table IV. Number of Pacemakers with Abnormal Pacing During Unipolar Sensing at Regular Sensitivity Test Areas Programmed Sensitivity (mV)
N
3
2
2.5
5
2.1-2.4
3
1.7-2.0
5
All pacemakers
15
Pacing Abnormality Inhibited Premature Inhibited Premature Inhibited Premature Inhibited Premature Inhibited Premature
1 Side/Elev**
II Side/Elev
III Side/Elev
IV Side/Elev
Any Area Side/Elev
0/0 0/0
0/0 0/0 0/0 0/0
0/0 0/0
0/1 0/1
0/0
0/0
0/0 1/2 1/2 0/0 1/2 1/2 2/4
0/0 271* 2/2 0/2 1/3 2/4
0/1 0/1 0/0 0/0 2/2 2/2 0/2 1/3 2/5
3/6
3/6
0/0
0/0 0/0 0/0
0/r
0/0
0/0 0/0
0/0
0/0
0/0
0/1 0/0
0/0
* Event only once during the exposure; " side = arms hanging on the sides, elev pacemakers.
normalities when the sensitivity was programmed higher (Table IV). Highest Unipolar Sensitivity During the highest available sensitivity (0.5-1.25 mV) in unipolar mode also the moderate
arms elevated horizontally; N = number of
electric fields frequently provoked inhibition of the impulse and premature pacing (Table V). On the other hand, not every pacemaker was inhibited in the strongest electric field, suggesting differences between the generator models. Pacemakers at a sensitivity of 0.5 mV were inhibited in the moderate electric fields, but not in the stronger
Table V. Number of Pacemakers with Abnormal Pacing During Unipolar Sensing at the Highest Sensitivity Test Areas Programmed Sensitivity (mV) 1.25 1.0
0.9-0.62 0.5 All pacemakers
N
2 2 4
4 12
1
11
Side/Etev**
Side/Elev
Inhibited
0/0
Premature Inhibited Premature Inhibited Premature Inhibited Premature Inhibited Premature
0/0
0/0 0/0
0/0 0/0 1/1 0/1
0/0
r/1*
0/0
0/0 3/3 1/3 5/5 1/4
Pacing Abnormality
3/4
2/2 3/4
2/2
Ml Side/Elev
!V Side/Elev
Any Area Side/Elev
0/0 0/0 1/0 2/2 1/1 2/2 0/0 4/4 2/1 8/6
0/0 0/0
0/0
r/0
2/1 2/2 1/1 2/3 4/4 4/4 7/6 8/9
2/2 1/1 2/3 0/0 4/4 2/1 8/9
0/0
* Event only once during the exposure; '* side = arms hanging on the sides, elev = arms elevated horizontally; N = number of pacemakers.
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TOIVONEN. ET AL.
Table VI. Number of Pacemakers with Abnormal Pacing During Bipolar Sensing at the Highest Sensitivity Test Areas Programmed Sensitivity (mV)
N
1.0
2
0.5
2
All pacemaiters
4
Pacing Abnormality Inhibited Premature Inhibited Premature Inhibited Premature
1 Side/Elev*
II Side/Elev
III Side/Elev
IV Side/Elev
Any Area Side/Elev
0/0 0/0 0/0 0/0
0/0 0/0 0/0 0/0 0/0 0/0
0/0 0/0 0/1 0/0 0/1 0/0
0/0 0/0 1/1 0/0 1/1 0/0
0/0 0/0 1/1 0/0 1/1 0/0
0/0 0/0
' Side = arms hanging on the sides, elev = arms elevated horizontally; N = number of pacemakers.
fields due to conversion to the noise reversion mode that produced continuous asynchronous pacing. Altogether nine of the 12 pacemakers were adversely influenced with high unipolar sensitivity. One pacemaker at a sensitivity of 0.5 mV was probably affected hy magnetic interference, since a slightly stronger electric field (area II) did not produce inhibition or premature pacing, as was seen in a weaker electric field [area I) where the magnetic field was strong.
pacing with variable impulse intervals. The programmed parameters evaluated via telemetry remained unchanged. Inhibition of the pacing impulses caused short cardiac pauses that ranged from 0.8-1.9 seconds in length. Their duration was determined either by resumption of pacing or appearance of a spontaneous heartbeat [Fig. 1]. Some patients had isolated, but none had successive, ventricular premature beats during the exposure.
Discussion
Highest Bipolar Sensitivity During the highest sensitivity (0.5-1.0 mV) in bipolar mode oniy one of the four pacemakers showed any abnormality and that occurred in the strongest fields [Table VI]. This pacemaker was occasionally inhibited hut did not exhibit asynchronous pacing. The observation contrasted to unipolar sensing with the same programmed level on which all these pacemakers behaved abnormally: two of them already in the field strength of 1.2 ± 0.2 kV/m. None of thp pacemakers showed artificially inhibited or asynchronous pacing during similar maneuvers in the control area either in unipolar or hipolar mode.
Our study on implanted pacemakers indicates that the programmed sensitivity level and the lead configuration critically influence the susceptibility of pacemakers to EMI, Many of the tested, recently introduced generator models behaved normally in the vicinity of electric power lines when the sensitivity was programmed according to clinical requirements in unipolar mode, but paced erroneously when set very sensitive. In contrast, the same units in bipolar mode were rather insensitive to EMI. The present generator models continue to be vulnerable to EMI, but their programmable features help to overcome the hazards to cardiac pacing.
Other Pacemaker and Rhythm Abnormalities
Electric Currents Within the Body
No temporary change was observed in the pacing rate except for intermittent inhibition of
Electric fields create currents inside the body that have been estimated and calculated previ-
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PACEMAKER LEAD CONFIGURATION, SENSITIVITY, AND EMI
ously. Current strengths of 10-337 |xA have heen ohserved in the upper chest region in 1-20 kV/m electric fields,^ The minimum currents that convert pacemakers to the noise reversion mode have ranged from 29 to more than 320 |xA.^'^ Toleration of EMI has differed widely, especially between older generator models. Pacemakers are particularly vulnerahle to 50 Hz power frequency, since the spectrum of cardiac signals requires sensing within this frequency band/'* Simulation studies and real exposures do not match exactly.'" The discrepancy may be related to inability to determine the strength and path of the induced fields within the body, dependence of the currents on the direction of the interfering field, influence of the position and movements of the body,^ and the shielding effect of the body. Pacemakers' Sensitivity to Electric and Magnetic Fields We observed that in unipolar mode, only one pacemaker was inhibited temporarily in a moderate field strength of 1.7 ± 0.2 kV/m when using regular, functionally adequate sensitivity, but five of 12 pacemakers did so with the highest available sensitivity. Inhibition of pacing was as common as premature pacing, except in the stronger fields where the pacemakers tended to remain in the noise reversion mode when set very sensitive. Direct comparison of the same pacemaker units with both lead configurations indicated that bipolar sensing compared to unipolar is markedly more tolerable to EMI, e.g., at a sensitivity of 0.5 mV rare abnormalities were seen in 7.0-8.0 kV/m fields in the bipolar mode, whereas every pacemaker at the same sensitivity level was already disturbed in a field of 1.7 kV/m in the unipolar mode. Inhibition of pacing occurs in a narrow zone between the field strength that does not interfere pacing and one that converts the pacemaker to the noise reverson mode.^'^ The induced body currents fluctuate widely in natural circumstances, and occasional inhibition is, therefore, possihle over a wide range of external field strengths. The magnetic flux densities that cause pacing abnormalities in nonimplanted cardiac pacemakers have ranged from 40-70 \i.T.^^ In our study the pacemakers were not interfered by magnetic fields
PACE, Vol. 14
that ranged from 48-90 (xT with regular sensitivity, but with high sensitivity in unipolar mode vulnerability seemed possible. To estimate the influence of a 50 Hz magnetic field on pacemakers it is essential to know the coil area, determined by the course of the pacemaker lead. This area is largest when the pulse generator is placed on the left side of the chest, which makes the left-sided generator location markedly more sensitive. The present observations on magnetic interference apply, therefore, only to pacemakers with the generator on the right upper chest quadrant. Extrapolation of the Environmental Risk The tested electric field strengths are encountered locally below the high voltage lines that operate at 110-400 kV voltage.^^-^^ At a distance of 40 m even from a 400 kV line the electric field strength is rarely above 1 kV/m. The 400 kV lines are common in energy transmission, though are mainly met in rural areas. Low voltage operated household and office appliances produce markedly lower electric fields. Exposure to magnetic fields of the tested flux density could occur, e.g., near industrial AC-machines, distribution transformers, and also while using electric drills and mixers if they are brought close to the chest.'""^''These devices are localized sources of EMI compared with the high voltage lines. Ventricular sensitivity can usually be set to at least 2.5-3 mV without undersensing, which in bipolar mode seems safe. It is often necessary to program atrial sensitivity high, due to a low signal amplitude that makes atrial and dual chamber pacemakers more vulnerable. Generators are built to revert from universal pacing mode to ventricular inhibited or asynchronous mode, if interference is sensed through the atrial lead. A pause in ventricular pacing does not, therefore, follow from interference via the atrial lead. Limitations of the Study The exact relationship between the sensitivity level and the vulnerability of the pacemaker to EMI was not the aim of the study. This relationship is confounded by different susceptibility of the generator models to EMI and the variation in the induced body currents between the patients. Eor
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TOIVONEN, ET AL.
the latter reason, and because the tested sensitivity levels were not standardized, the performance of the different generator models could not be compared either. Whether extended pauses in pacing would have occurred could not be established since spontaneous heart rhythm always overtook soon. Clinical Implications The overall guidelines for the pacemaker patients in general and the regulations for safe elec-
trical environment in community planning have to be maintained in a similar manner as has been the case during the past decade in order to cover also the most vulnerable cases. The patients having a bipolar pacemaker set to low sensitivity are reasonably well protected from asystole, even in circumstances that have recently not been recommended for pacemaker patients. Tailoring the pacemaker features to individual needs diminishes the necessity to restrict the patients' activities in surroundings with elevated electromagnetic fields.
References 1. Butrous GS, Bexton RS, Barton DG, et al. Interference with the pacemakers of two workers at electricity substations. Br I Ind Med 1983; 40:462-465. 2. Butrous GS, Male JG, Webber RS. et al. The effect of power frequency high intensity electric fields on implanted cardiac pacemakers. PAGE 1983; 6:1282-1292. 3. Kaye GG, Butrous GS, Allen A, et al. The effect of 50 Hz electrical interference on implanted cardiac pacemakers. PACE 1988; 11:999-1008. 4. Belott PH, Sands S, Warren J. Resetting of DDD pacemakers due to EMI. PAGE 1984: 7:169-172. 5. Hayes DL, Trusty J, Ghristiansen J, et al. A prospective study of electrocautery's effect on pacemaker function, (abstract] PAGE 1987: 10:442. 6. Levine PA, Balady GJ, Lazar HL, et al. Electrocautery and pacemakers: Management of the paced subject to electrocautery. Ann Thorac Surg 1986; 41:313-317. 7. Gould L, Patel G, Betzu R, et al. Pacemaker failure following electrocautery. Glin Prog Electrophysiol Pacing 1986; 4:53-55. 8. Warnowicz-Papp MA. The pacemaker patient and the electromagnetic environment. Glin Prog Pacing Electrophys 1983; 1:166-176. 9. Bridges JE, Frazier MI, Hauser RG. The effect of 60 Hertz electric fields and currents on implanted cardiac pacemakers. Proc Inter Symp Electromagnetic Gompatibility, New York, NY, IEEE. 1978. pp. 258-265. 10. Deller AG. Toff WD, Hobbs RA. et al. The development of a system for the evaluation of electromagnetic interference with pacemaker function: Haz-
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ards in the aircraft environment. J Med Engl Technol 1989; 13:161-165. Salmi I, Eskola HJ, Pitkanen MA. et al. The influence of electromagnetic interference and ionizing radiation on cardiac pacemakers. Strahlenther Onkol 1990; 166:153-156. Ghen D. Philip M, Philip PA, et al. Gardiac pacemaker inhibition hy transcutaneous electrical nerve stimulation. Arch Phys Med Rehabil 1990; 71:27-30. Electric Power Research Institute. Evaluation of the effects of electric fields on implanted cardiac pacemakers. Project Report EA-3917. Palo Alto. GA, Electric Power Research Institute, University of Rochester, 1985. Adams T. The electrode-biointerface: Sensing. In SS Barold, I Mugica (eds.J: New Perspectives in Gardiac Pacing. Mount Kisco, NY. Futura Publishing Gompany, Inc., 1988. pp. 17-25. Valjus J. Physiological effects of low-frequency electric and magnetic fields (in Finnish). Research report IVO-A-04/87. Helsinki, Finland, Imatran Voima Oy, 1987. pp. 1-154. Stuchly MA. Human exposure to static and timevarying magnetic fields. Health Physics 1986; 512:215-225. Kaune WT, Stevens RG, Gallahan Nl, et al. Residental magnetic and electric fields. Bioelectromagnetics 1987; 8:315-335. luutilainen J, Saali K, Eskelinen I. et al. Measurements of 50 Hz magnetic fields in Finnish homes. Research report IVO-A-02/89. Helsinki, Finland, Imatran Voima Oy, 1989, pp. 1-31.
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