Interference in Implanted Cardiac Devices, Part I

IntroductionSensing intrinsic cardiac electrical activity is

essential for the function of pacemakers and im-plantable cardioverter defibrillators (ICDs). Exam-ples of undesired triggering or inhibition of pace-maker output by extraneous signals wereidentified early after the introduction of noncom-petitive, “demand” pacemakers. Hermetic shield-ing in metal cases, filtering, and interference re-jection circuits, together with a preference (muchmore marked in the United States1 than in Eu-rope2) for bipolar sensing, made contemporarypacemakers and ICDs relatively immune to elec-tromagnetic energy sources in homes and work-places. Sources of electromagnetic interference(EMI) remained ubiquitous in the medical envi-ronment. However, they were predictable andavoidable.

New technologies that use more of the elec-tromagnetic spectrum (i.e., wireless telephones,electronic article surveillance [EAS] devices) haverekindled interest in EMI risks for patients withimplanted cardiac devices. Although these tech-nologies do not constitute a major public healththreat, adverse interactions can occur. The coun-terpart to EMI is electromagnetic compatibility, ascience aimed at avoiding interference potentialby adding shielding or redesigning circuits againstspecific EMI sources. There are three essential el-ements to any electromagnetic compatibility prob-lem. There must be an electromagnetic source, areceptor or victim (in our case the implanted car-diac device) that cannot function properly due tothe electromagnetic phenomenon, and a path be-tween them that allows the source to interferewith the receptor. Each of these three elementsmust be present, although they may not be readilyidentified in every situation. Identifying at leasttwo of these elements and eliminating (or attenu-

ating) one of them generally solves electromag-netic compatibility problems. Collaborationamong industry, physicians, regulatory agencies,and consumer groups will hopefully achieve fullcompatibility between implanted devices andother technologies. This will require adoption ofinternational standards establishing the upperlimit of permissible field intensities for the wholeelectromagnetic spectrum. Implanted devicesshould not react to fields below this limit; moreintense fields will be prohibited.

This two-part review discusses EMI with im-planted cardiac devices. The first part of the re-view addresses general concepts and specificsources of EMI in everyday life and the workplace.The second part focuses on medical sources ofEMI, highlighting preventive measures.

Classification of Sources of EMISources of EMI can be classified according to

type and spectral frequency of energy emitted, andthe environment in which the source is encoun-tered (Table I). A detailed discussion of thephysics of electromagnetic fields is beyond thescope of this review.3,4 For clinical purposes, it isuseful to recognize radiated and conductedsources of EMI.

Radiated EMI can result from energy emittedfor communication purposes or as an unintendedeffect of other electrical activity (e.g., motor oper-ation in an electric razor). Electromagnetic fieldshave both an electric field measured in volts permeter and a magnetic field measured in amperes(A) per meter. Their sources can be broadly di-vided into radiofrequency waves with frequenciesfrom 0.1 Hz to 100 MHz (e.g., electric power, radioand television transmitter, electrocautery), andmicrowaves from 100 MHz to 12 GHz (e.g., radartransmitters, cellular telephones, microwaveovens) (Fig. 1). The frequency of EMI determinesthe efficiency of energy coupling to the device andthe resulting effect. The signal may be modulatedin amplitude or frequency, and it may occur inbursts or single long pulses. A radiofrequency car-rier with amplitude modulation induces voltagesin the signal processing and detection circuitry ofan implanted device that can be misinterpreted as

REVIEW

Interference in Implanted Cardiac Devices, Part ISERGIO L. PINSKI and RICHARD G. TROHMANFrom the Section of Cardiology, Rush Medical College and Rush-Presbyterian-St. Luke’s MedicalCenter, Chicago, Illinois

PACE, Vol. 25, No. 9 September 2002 1367

Address for reprints: Sergio L. Pinski, M.D., Cleveland ClinicFlorida, 2950 Cleveland Clinic Blvd., Weston, FL 33331. Fax: (954) 659-5292; e-mail: [email protected]

Received June 23, 2001; revised October 15, 2001; accepted De-cember 31, 2001.

Reprinted with permission fromJOURNAL OF PACING AND CLINICAL ELECTROPHYSIOLOGY , Volume 25, No. 9, September 2002

Copyright © 2002 by Futura Publishing Company, Inc., Armonk, NY 10504-0418.

intracardiac signals. The modulation on the car-rier is converted (demodulated) to a low frequencyvoltage waveform, allowing entry to the signalprocessing and detection circuitry. If the ampli-tude modulation has frequency components in thedevice’s physiological passband, significant inter-ference occurs. Although electromagnetic fieldscould also mimic radiofrequency telemetry andmodify programmable parameters in an implanteddevice, this is unlikely with current systems. Pro-gramming requires access codes to establish thetelemetry link, parity checks of transmitted mes-sages, and often simultaneous magnetic reedswitch closure by a steady magnetic field.

Directly conducted galvanic currents (mea-sured in A/m2) are most commonly introduced inthe body therapeutically (e.g., transcutaneouselectrical nerve stimulation), but they can also re-sult from physical contact with improperlygrounded electrical equipment. A wide range offrequencies may affect implanted devices, includ-ing the power frequencies of 50 Hz (Europe), 60Hz (United States), and 400 Hz (aircraft). Sensitivepacemakers or ICDs can react to galvanic currentsbelow the perception threshold (; 1 mA/cm2 formoist skin). Clinically, this will result in over-sensing in the channel where sensing is occurring.

Static magnetic fields are measured in units oftesla (T), which equals 10,000 gauss (G). The ion-izing radiation dose is the amount of energy ab-sorbed per unit mass of material, with units ofjoule per kilogram or gray (Gy). Radiation found inthe environment and medical imaging equipmenthas no effect on implanted electronic devices.Therapeutic radiation used in oncology can dam-

age the oxide layers of CMOS semiconductor cir-cuits in ICDs and pacemakers, and the effects arecumulative. Acoustic radiation from lithotripsymachines is used to disintegrate kidney and gall-bladder stones. About 1,500 discharges form a 20-kV spark gap generate pressure shock waves thatare typically 45 Mpa at the 12-mm diameter focalarea. If pressure waves of this magnitude are ap-plied directly to a pacemaker or ICD, the elec-tronic circuits could be damaged.

Sources of Knowledge Regarding EMIKnowledge of EMI effects on implanted de-

vices arises from three sources. Anecdotal reportshighlight the possibility of interactions but pro-vide little information regarding overall risk. Theinteraction may have depended on idiosyncraticprogramming or device malfunction. In the UnitedStates, the Food and Drug Administration’s (FDA)Center for Devices and Radiological Health main-tains a database of reported incidents of deleteri-ous interactions (Manufacturer and User FacilityDevice Experience [MAUDE]) that is searchableon-line.5 However, reporting is largely voluntaryand documentation uneven. Case reports pub-

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1368 September 2002 PACE, Vol. 25, No. 9

Table I.

Documented Sources of Electromagnetic Interference

Electromagnetic fieldsDaily life: Cellular telephones, electronic article

surveillance devices, metal detectors, some homeappliances (e.g., electric razor), toy remote controls,improperly grounded appliances held in closecontact to the body, slot machines

Work and industrial environment: High voltage powerlines, transformers, welders, electric motors,induction furnaces, degaussing coils

Medical environment: Magnetic resonance imagescanners, electrosurgery, defibrillation,neurostimulators, TENS units, radiofrequencycatheter ablation, therapeutic diathermy

Ionizing radiationMedical environment: Radiotheraphy

Acoustic radiationMedical environment: Lithotripsy

Figure 1. Electromagnetic spectrum. Frequencies usedfor communications in the radio and microwave range100 KHz–10 GHz (detailed in the lower bar) can interactwith implanted cardiac devices. (Adapted from MoulderJP. Cellular phone antennas and health. http://www.mcw.edu/gcrc/ cop/cell-phone-health-FAQ/toc.html A 2 with permission. Accessed August 16, 2002.)

lished in peer-reviewed journals (especially whenincluding a rechallenge in a controlled environ-ment) can be most valuable.

Prospective studies can be performed in vitro(i.e., bench testing) or in vivo, using laboratory an-imals or patient volunteers. In vitro studies areperformed with the implantable device sub-merged in a saline filled tank (to emulate electri-cal properties of tissue), with the source of radi-ated EMI in close proximity. These studies allowexpeditious study of interactions between variousEMI sources and devices. Multiple iterations ofthe experiment permit examination of the effectsof distance, position, field strength, and deviceprogramming on the frequency and severity of theinteraction. Although simulation studies predictinterference in vivo, they do not match clinical ex-posures identically. Discrepancies may be relatedto the inability to replicate the strength and pathof induced body fields, body position and move-ments, and shielding effects of the body. The ori-entation of the air gap between the source and thesaline tank (i.e., perpendicular versus parallel)can dramatically influence the distance thresholdfor interaction.6 More recently, the developmentof anatomically based electromagnetic models ofthe human body has allowed the use of numericalmodeling to quantify the relationship between anexternal electromagnetic field and the voltage in-duced in the leads of an implantable device.7 Suchmodeling can greatly strengthen the clinical rele-vance of in vitro simulation studies.

In limited, high risk circumstances (e.g., mag-netic resonance imaging [MRI]), in vivo testing hasbeen first conducted in laboratory animals im-planted with a pacemaker system. More com-monly, in vivo simulation studies require con-trolled patient exposure to potential sources ofEMI while the cardiac rhythm is monitored. Pa-tient exposure studies clarify the clinical signifi-cance of in vitro interactions. However, because ofthe time and effort involved, the number of as-sessed permutations is, by necessity, limited. It isimportant to recruit patients representative of thegeneral population with implanted devices toavoid inadvertent biases. The fact that manysources of EMI also interfere with real-time orHolter electrocardiographic (ECG) recordings alsocomplicate in vivo studies. Bipolar asynchronouspacing pulses that do not elicit a QRS complex areparticularly difficult to ascertain. Special record-ing techniques are often necessary. Furthermore,real-time telemetry between the implanted deviceand the programmer is often compromised byEMI, even when device function remains other-wise normal. Critical review of the literature sug-gests that many purported instances of EMI re-sulted from this inconsequential phenomenon.8

Furthermore, the programmer wand placed di-rectly over the device can act as an artificialshield. When available, analysis of annotatedstored electrograms is the ideal method to evalu-ate device behavior during exposure to potentialsources of EMI.

Pacemaker and ICD Responses to EMIThe most frequent responses to EMI are inap-

propriate inhibition or triggering of pacemakerstimuli, reversion to asynchronous pacing, andspurious ICD tachyarrhythmia detection. Repro-gramming of operating parameters and permanentdamage to the device circuitry or the electrode totissue interface are much less frequent.

Pacing Inhibition

Sustained pacing inhibition is potentiallycatastrophic in pacemaker dependent patients.Depending on the duration of inhibition andemergence of escape rhythms, lightheadedness,syncope, or death could result. Prolonged inhibi-tion is uncommon because of the protective algo-rithms available in pacemakers. Furthermore, themajority of patients currently undergoing pace-maker implantation are not completely depen-dent. Patients dependent on their ICD for brady-cardia pacing (e.g., after atrioventricular [AV]junction ablation to prevent spurious shocks forsupraventricular tachyarrhythmia) may be morevulnerable to catastrophic pacing inhibition fromEMI. In ICDs, automatic adjustment of the gain orsensing threshold according to the amplitude ofthe intrinsic R wave ensures sensing of low am-plitude (at times , 1 mV) ventricular depolariza-tion signals during ventricular fibrillation withoutoversensing of T waves and extracardiac sig-nals.9,10 In the absence of sensed complexes, twopotentially life-threatening diagnoses must beconsidered: asystole (requiring pacing) and fineventricular fibrillation (requiring amplifier gainadjustments for proper detection). To ensure de-tection of ventricular fibrillation, pacing onsettriggers an increase in sensitivity in most devices.These high sensitivity levels (; 0.2–0.3 mV) canpromote oversensing of extracardiac signals.Oversensing perpetuates because the absence ofspontaneous large amplitude escape beats main-tains the high operating sensitivity.11 Asyn-chronous pacing will not occur due to lack of reli-able ICD noise reversion modes. Therefore, EMIinduced prolonged inhibition and spurious tach-yarrhythmia detection become likely (see below).Simulation studies of the interactions betweensources of EMI and ICDs require recreation of a“worst-case scenario” (inducing maximum sensi-tivity during continuous pacing).

INTERFERENCE IN IMPLANTED CARDIAC DEVICES, PART I


Triggering of Rapid or Premature PacingOversensing of EMI by the atrial channel of a

pacemaker or ICD programmed to a tracking mode(DDD, VDD) can trigger ventricular pacing at ornear the upper tracking rate limit. Alternatively,automatic mode switching may occur if this func-tion is enabled. In some pacemakers, detection ofnoise in the atrial channel can trigger a noise re-version mode. Preferential detection of EMI is notuncommon because atrial sensitivity is usuallyprogrammed higher (more sensitive) than ventric-ular sensitivity. It is possible to observe rapid pac-ing due to atrial oversensing as the patient ap-proaches an electromagnetic field, followed by aperiod of ventricular oversensing (inhibition ormode reversion) as the field becomes stronger. Ifsustained, inappropriate pacemaker accelerationinduced by atrial oversensing may cause palpita-tion, hypotension, or angina.

Less commonly, EMI can induce rapid pacingvia other mechanisms. In QT sensing pacemakers,oversensing of EMI early in the QT window couldinduce the pacemaker to increase the pacing rate.EMI can also trigger rapid pacing (up to the sen-sor-triggered upper rate limit) by activating thesensor in minute ventilation pacemakers. The sig-nal emitted by acoustomagnetic EAS systems is atthe same frequency of the pulses used by someminute ventilation pacemakers to measuretransthoracic impedance. Minute ventilationpacemakers may also erroneously interpret thesignals generated by certain monitoring and diag-nostic equipment, including cardiac monitors,echocardiography equipment, apnea monitors,and respiration monitors, that also use bioelectricimpedance measurements.12,13 Pacing returns tonormal once the patient is disconnected from themonitors or the minute ventilation sensor in thepacemaker is deactivated.

Very strong electromagnetic fields could in-duce voltage in the lead(s) that may directly cap-ture the myocardium. For example, 58-kHz acous-tomagnetic EAS systems are capable of inducing3.7 V in pacemaker leads.14 Isolated prematurepaced beats (but no sustained rapid pacing) havebeen observed in patients. In vitro and in vivo an-imal studies15 have shown that application of 64MHz radiofrequency power, required to produceMRI scans, can result in rapid pacing at pulsingperiods between 200 and 1,000 ms. Rapid pacingrequires an intact lead connected to a pacemaker.Apparently, energy is coupled to the pacemakerdefibrillation protection diodes or the output cir-cuit, bypassing the runaway protection mecha-nisms. Very rapid pacing could induce ventricularfibrillation. Irregular rapid pacing at a rate ; 100beats/min, temporarily related to radiofrequencypulses during MRI, has been observed in a patient

with a VVI pacemaker programmed at subthresh-old output.16

Spurious Tachyarrhythmia Detection

EMI signals can satisfy ICD tachyarrhythmiadetection criteria and lead to spurious ICD dis-charges (with associated psychological morbidity,battery consumption, and occasional proarrhyth-mia17). As noted, pacemaker dependent patientscan suffer concomitant catastrophic inhibition ofpacing. In a follow-up study of 341 patients withcontemporaneous ICDs who received educationregarding avoidance of sources of EMI, spurioustachyarrhythmia due to EMI occurred five times infour patients.18 The incidence was of 0.75% perpatient-year of follow-up. Intermittent EMI can re-sult in shock delivery even in noncommitted de-vices. Many “noncommitted” devices will notabort two consecutive discharges during the same“episode” (i.e., sinus rhythm not redetected in be-tween), and therefore, will deliver shocks forrepetitive but self-limiting EMI. Biotronik (Berlin,Germany and Lake Oswego, OR, USA) and previ-ous generation Guidant (St. Paul, MN, USA) ICDsfunctioned “de facto” as committed in pacemakerdependent patients.19

In dual chamber ICDs that use atrial channelinformation to discriminate between atrial andventricular tachyarrhythmias, simultaneous over-sensing of EMI could result in varied and, for themost part, unpredictable arrhythmia detection.Pacemakers and defibrillators capable of detectingand treating atrial tachyarrhythmias have been re-cently introduced in clinical practice. Selectiveoversensing in the atrial channel could result inspurious pacing or shock interventions for atrialtachyarrhythmia. In turn, the spurious interven-tion could result in atrial, or more rarely, ventric-ular proarrhythmia. However, because the dura-tion of atrial tachyarrhythmia required fordetection is in general programmed longer thanfor ventricular arrhythmias,20 transient EMI is un-likely to satisfy atrial tachyarrhythmia detectioncriteria.

Noise Reversion Mode

Pacemakers incorporate protective algorithmsagainst prolonged inhibition from spurious sig-nals. A common response is transient reversion toasynchronous pacing.21 These algorithms arebased on the fact that rapid frequencies are un-likely to represent myocardial activation. In mostpacemakers, a noise sampling or noise interroga-tion window (also known as relative refractory pe-riod) occupies the second part of the ventricularrefractory period. Pacemakers do not respond tosignals during the initial portion of the ventricularrefractory period (i.e., ventricular blanking),

PINSKI, ET AL.


which is usually nonprogrammable and fixed oradjusted automatically by the generator based onthe strength and duration of the ventricular event.Signals recognized during the noise samplingwindow cannot reset the lower rate timer (there-fore preventing inhibition), but affect other timingintervals, most importantly, the ventricular refrac-tory period. In some models, a noise sampling pe-riod exists in the atrial and ventricular channels(Table II). The types of responses to signals sensedwithin the noise sampling period implemented bydifferent manufacturers include resetting of theentire (retriggerable) refractory period (e.g.,Medtronic, Minneapolis, MN, USA), resetting ofthe noise sampling period only (e.g., St. Jude, Min-neapolis, MN, USA), and reversion to asyn-chronous pacing for one full cycle (e.g., Inter-medics, Angleton, TX, USA). In the first two typesof responses, repetitive triggering of the noisesampling period eventually leads to asynchronouspacing.22

During simulation studies, a variable but nar-row window of inappropriate pacing or inhibitionis frequently observed at field or current strengthsimmediately below the reversion thresholds be-cause of intermittent oversensing. This phe-

nomenon appears relatively unimportant duringreal-life EMI exposure. Occasional inhibition overa range of external field strengths is possible be-cause EMI induced body currents can fluctuatewidely with changes in posture, respiratory phase,and other natural circumstances.23

Although generally safe, transient asyn-chronous pacing is not completely innocuous.Symptoms secondary to loss of AV synchrony andan irregular heart beat can occur. Competitionwith the spontaneous rhythm may induce ventric-ular tachyarrhythmias if the pacing stimulus cap-tures the ventricle during its vulnerable period.24

This is extremely uncommon in pacemaker pa-tients, as attested to by the routine use of a magnetduring clinic or transtelephonic pacemakerchecks. In patients with separate pacemaker anddefibrillator systems, pacemaker reversion due torepetitive sensing of ventricular fibrillation depo-larizations in the noise sampling window can leadto asynchronous pacing and interfere with ICD de-tection.25

Implementation of noise protection algo-rithms is much more difficult in ICDs (Table III).By design, these devices must be able to recognizethe rapid rates of ventricular tachycardia of fibril-



Table II.

Noise Reversion and Electrical Reset Responses of Contemporary Dual Chamber Pacemakers*

Detection of Detection ofNoise in the Noise in the

Manufacturer Model A Channel V Channel Partial Reset Full Reset

Biotronik Phylos/Actros Switch to DVI(R) Switch to DAD(R) None VDD, 11% decrease inprogrammed rate,programmed polarity

Guidant Discovery/Pulsar/ Switch to DVI Switch to DAT None VVI 65, detected polarityContak TR

Ela Medical Talent/Brio Switch to DVI Switch to DAD None VVI 70 beats/min, uni¶

Medtronic Sigma None Switch to DOO(R) None Programmed mode andpolarity, 65 beats/min†

Kappa 400 None Switch to DOO(R) Programmed VVI 65 beats/min,mode and detected polaritypolarity, 65beats/min

Kappa 600/700 None Switch to DOO(R) Programmed VVI 65 beats/min,mode, rate detected polarityand polarity

St. Jude Trilogy Switch to DVI(R) Switch to DOO(R) None VVI 70 beats/min,programmed polarity

Integrity Switch to DVI(R) Switch to DOO(R) None VVI 67 beats/min, uniVitatron‡ Diamond II Switch to DOO(R) Switch to DOO(R) None VVI 62.5 beats/min, uni

*Assumes programming in the DDD(R) mode. ¶ ”Dedicated bipolar” model (Brio DR222) reverts to bipolar. †In rare circumstancesventricular polarity could reset to unipolar. ‡Vitatron, Deren, the Netherlands. Other manufacturers are listed in text. (R) 5 pacing at thesensor-indicated rate if rate-responsive pacing enabled; uni = unipolar.

lation.10 Therefore, long refractory periods aftersensed events are not feasible. Asynchronous pac-ing is undesirable in patients vulnerable to reen-trant ventricular arrhythmias.17 Saeed et al.26

studied stored electrograms from 268 episodes ofmonomorphic VT among 52 patients, and foundthat 13 (5%) were induced by asynchronous ven-

tricular pacing after undersensing of the previousbeat.

Among current ICDs, those manufactured byMedtronic lack noise reversion capabilities.Guidant devices provide a programmable noise re-version mode (Off, VOO, DOO). However, theshort (40 ms) retriggerable noise sampling win-

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Table III.

Noise Reversion, Asynchronous Pacing and Electrical Reset Responses of Contemporary ICDs

AsynchronousManufacturer Model Noise Reversion Pacing Electrical Reset

Biotronik Belos VR VOO None VVI 70 beats/min, 7.5 V @ 1.5 msSingle zone at 270 ms,

30 J 3 8Tachos DR Asynchronous pacing Only AOO VVI 70 beats/min, 7.2 V @ 1 ms

in the chamber with Single zone at 150 beats/min,noise 30 J 3 6

Guidant Ventak AV III, Programmable: AOO, AOO(R)†, VVI 60 beats/min, 7.5 V @ 1 msContak CD DOO*, VOO, inhibit VOO(R)†, Single zone at 165 beats/min,

DOO(R)† maximum energy 3 5Prizm VR, Programmable: VOO*, VOO(R)† VVI 60 beats/min, 7.5 V @ 1 ms

Prizm II VR inhibit Single zone at 165 beats/min,maximum energy 3 5

Prizm DR, Programmable: AOO, AOO(R)†, Nonrate responsive mode (i.e.,Prizm II DR DOO*, VOO, inhibit VOO(R)†, DDDR to DDD) 60–120

DOO(R)† beats/min 7.5 V @ 1 msSingle zone at 165 beats/min,

maximum energy 3 5Ela Medical Defender IV Ventricular sensitivity None VVI 60 beats/min, 4.8 V, 0.37 ms

¯ until noise (i.e., Single zone at 297 ms, 33 J 3 4cycle , 63 ms) nolonger detected

Medtronic GEM III None None VVI 65 beats/min, 6 V, 1.6 msVR 7231

GEM III Single zone at 320 ms, 30 J 3 6DR 7275 High urgency alert sounds every

InSync 7272 20 hours until clearedGEM III AT None Programmable¶ VVI 65 beats/min, 6 V, 1.6 ms

7276 DOO, VOO Single zone at 320 ms, 30 J 3 6St. Jude Angstrom II, Programmable: VOO None VVI 50 beats/min, 5 V, 0.5 ms

Countour or OFF Defib Only: detection rate 146II, MD beats/min; 650 V 3 1, 705

V 3 5Photon Programmable: Programmable¶ VVI 60 beats/min, 5 V

VVI(R): VOO or AOO, VOO, Defib only: detection rate 146OFF* DDD(R), DOO beats/min; 800 V 3 3DDI(R): VOO,DOO, or OFF*,fixed rate of 50beats/min

*Nominal; ¶available only when tachyarrhythmia detection is disabled; †requires continuous telemetry link. (R) 5 pacingat the sensor-indicated rate if rate-responsive pacing enabled.

dow affords imperfect protection from inhibitionby exogenous interference. ICDs from Ela Medical(Montrouge, France and Plymouth, MN, USA) andSt. Jude also provide noise reversion modes, buttheir performance against common sources of EMIis not well documented. As ICDs are increasinglyimplanted in pacemaker dependent patients, thelack of reliable noise reversion modes may be-come clinically detrimental.

Electric (Power-On) Reset

Momentary strong EMI, by inducing veryhigh voltage within device circuits, or triggeringspecial microprocessor timers, may cause reset ofDDD and VVIR pacemakers to the VVI or VOOmode, a condition called power-on or electric re-set (Table II).27 Electric reset is less recognized inICDs, generally resulting in a “shock-box” config-uration with VVI pacing at 60 beats/min and max-imum energy shocks for rates . 145–170beats/min (Table III). Electrosurgery and externalor internal defibrillation are the most commoncauses of the reset phenomenon. In the resetmode, the pulse generator functions only with ba-sic factory preset instructions (pacing mode andparameters) stored in the nonvolatile read-onlymemory, as communication between the randomaccess memory (containing the programmable set-tings) and the microprocessor has been inter-rupted. In some pacemakers, the pacing mode andrate are similar during electrical reset and electivereplacement indicator. In devices with differentreplacement and reset parameters, strong EMI mayactivate either one. In some pacemakers, two lev-els of electrical reset (partial and full) exist. Partialreset tends to occur with less intense interference,generally preserving the programmed pacingmode and rates (Table II). In some pulse genera-tors, there will be no response to magnet applica-tion in the reset mode. The reset mode does not re-vert back when EMI is discontinued. A DDD(R)device reset to the VOO or VVI mode might causehypotension, particularly in patients with pace-maker syndrome. Resolution of the problem re-quires a specific programmer command. Acardiomyostimulator used in dynamic cardiomy-oplasty can also revert to asynchronous stimula-tion in response to EMI.28 Electric reset can be dif-ferentiated from battery depletion by telemetry ofbattery voltage and impedance. When reset is dueto EMI, the battery voltage should be normal (ap-proximately 2.8 V) and battery impedance normalor slightly raised according to battery age (Fig. 2).

Closure of the Reed Switch

Most pacemakers and ICDs contain a mag-netic reed switch that is closed by a ; 10-G mag-

netic field. This results in temporary asyn-chronous pacing in pacemakers and temporarysuspension of tachyarrhythmia detection andtherapy in most ICDs. Normal function returnswhen the magnetic field dissipates. Prior ICDmodels from CPI/Guidant were deactivated bycontinuous application of a magnetic field . 10 Gfor $ 30 seconds. Reactivation required reapplica-tion of the magnet for $ 30 seconds or a program-mer command. Several items that generate incon-spicuous strong magnetic fields, like magnetizedscrews,29 stereo speakers,29,30 and bingo wands31

have inadvertently deactivated Guidant ICDs. Incurrent models, this function is programmable(nominally off). Magnet application increasinglyis being used to trigger specific behaviors in newerdevices, including storage of electrograms andevent markers or replay of alert tones. Exposure to



Figure 2. Pacemaker interrogation after electrical reset(in this case triggered by a shock from an implantablecardioverter defibrillator). Although the initial screenreads “Replace Pacer,” the battery is not depleted (2.64V). Normal operating function was restored by aprogrammer command. (From Pinski SL, Trohman RG.Interference with cardiac pacing. Cardiol Clin 2000;18:219–239, with permission).

a strong magnetic field in patients who have thesefunctions activated can result in eccentric (butclinically inconsequential) device behavior.32

Static magnetic fields strong enough to closethe reed switch are unlikely to be present in in-dustrial environments. For example, in apetroleum refinery, peak fields close to 2 G weremeasured close to large compressors and in powerdistribution centers. However, the fields droppedoff to , 0.1 G at a distance of 4 feet.33 A variety ofso-called therapeutic magnets are commerciallyavailable for the treatment of arthritis and othermusculoskeletal ailments. Despite manufacturers’claims of strong magnetic field strengths (up to30,000 G), in vitro testing showed that the magnetswere able to close the reed switch only whenplaced at , 1 inch from the generator.34 Prostheticdental minimagnets can activate the reed switchonly when close (1 cm) to the pacemaker.35 There-fore, they do not represent a risk to pacemaker pa-tients.

Damage to the Generator or to the Electrode-Myocardial Interface

In the overwhelming majority of cases, the ef-fects of EMI are temporary, lasting only as long asthe device is within range of the source. However,strong EMI (e.g., electrosurgery and external de-fibrillation) can cause permanent damage to animplanted device. Circuitry damage, (resulting inoutput failure, pacemaker runaway, and othermalfunctions) can occur, requiring generator re-placement (at times emergent). Increases in pacingthresholds secondary to local heat related injury atthe myocardium lead interface are also possible.

Clinical Consequences of EMI

The effects of EMI on pacemakers and ICDsdepends on the intensity of the electromagneticfield, the frequency spectrum of the signal, thedistance and positioning (angle) of the device rel-ative to the source, the electrode configuration(unipolar or bipolar), nonprogrammable devicecharacteristics, programmed settings, and patientcharacteristics (Table IV).

Transient EMI producing 1-beat responses(e.g., inhibition of a single ventricular pacingpulse) is of no clinical significance. Symptomscan occur with longer exposure. The spatial prox-imity and orientation of the patient with an im-planted device to the potential source of EMI areimportant. Electric and magnetic fields decreaseinversely with the square of the distance from thesource. Some sources restrict emission of energyto a particular direction.4 It has been repeatedlydemonstrated that devices from different manu-facturers differ in susceptibility to various sourcesof EMI, depending on circuitry design. EMI from

digital cellular telephones, in particular, can besuppressed by incorporation of simple radiofre-quency feedthrough filters to the circuitry (Fig. 3).Manufacturers should supply information regard-ing EMI susceptibility. Implanters should selectdevices less susceptible to EMI.

A higher programmed sensitivity level in-creases device susceptibility to EMI. Unipolarpacemakers are more vulnerable to EMI fromsources in the lower range of the frequency spec-trum (e.g., power lines36). Left-sided unipolar im-plants are particularly susceptible because of thelarger loop for voltage induction between the leadand the generator. The impact of the sensing con-figuration decreases with a shorter radiationwavelength. For cellular telephones, for example,the greatest interaction occurs when the antennais placed over the device header. Neither the sens-ing electrodes near the distal tips of the leads, northe coated lead body, are susceptible. Ventricularoversensing appears more common with “inte-grated bipolar” than with “true bipolar” defibrilla-tor leads.37

The degree of pacemaker dependency is a cru-cial determinant of the clinical sequelae of EMI.Prolonged pacing inhibition will be asymptomaticin a patient with a good escape rhythm, but couldresult in catastrophic asystole in a pacemaker de-pendent patient.

Daily Life Sources of EMICellular Telephones and Other WirelessCommunication Devices

It has been estimated that by the year 2003,there will be 1 billion subscribers to wireless com-munication services worldwide. Although cellu-lar telephones will continue to be the most popu-

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Table IV.

Factors Influencing Electromagnetic Interference

Intensity of the fieldSignal spectrumDistance and position of the patientDuration of exposureNonprogrammable device characteristicsLead configurationProgrammed parametersSensitivityMode (baseline, noise reversion)Committed versus noncommitted (ICDs)Patient characteristics

Pacemaker dependencySusceptibility to asynchronous pacingSusceptibility to rapid pacing rates

lar wireless communication devices, personal dig-ital assistants, laptop computers, satellite tele-phones, and other appliances will be increasinglyused for wireless voice, data, and video transmis-sion. Assessment of the effects of cellular tele-phones on implanted cardiac devices has beencomplicated by the wide variety of technologies inuse.38 Analog cellular telephones predominate inthe United States, but a gradual shift toward digi-tal technology is occurring, due to the saturationof analog networks and the advantages afforded bydigital transmission in terms of privacy, clarity ofreception, and bandwidth for data transmission.In Europe, . 90% of cellular telephones in use areof the digital type. Digital technology in use in theUnited States include Time Division Multiple Ac-cess [TDMA]-11Hz, and TDMA-50Hz (also calledNorth American Digital Cellular, NADC), Code Di-vision Multiple Access (CDMA), and PersonalCommunication Services (PCS). It should be notedthat NADC and CDMA default to analog transmis-sion when a digital signal is not present. PCS tele-phones are incompatible with analog transmis-sion unless a dedicated “dual-mode” device isused. The Global System for Mobile Radio (GMS)is the digital modality predominant in Europe.Analog cellular telephones, as well as TDMA,CMDA, and GMS operate in the 820–960 MHzspectrum. PCS uses the 1.8–2.2 GHz band. As ofthe time of this writing, the spectrum for futurethird-generation multimedia networks has notbeen adjudicated in the United States. In theUnited States, the maximal power of hand heldtelephones is limited to 0.6 W. The power levelused by the telephone (and the consequent emit-ted electromagnetic field) fluctuates throughoutthe call, according to distance from the base sta-tion and the number of telephones being used onthe system at a time. Generally, the power gener-ated from European telephones is higher due to alower density of base stations. Vehicle mounted

units can transmit at higher power (up to 8 W), butare not in common use by the general public.

Although isolated case reports have suggestedthe potential for severe interactions,39 most re-search suggests that deleterious interactions areunlikely to happen with normal cellular tele-phone use. Large scale bench testing of the effectsof wireless telephones on pacemakers has beenconducted in Germany,40 at the FDA’s Center forDevices and Radiological Health,41 the MedicalDevices Bureau of Canada,42 and at the Universityof Oklahoma Wireless Electromagnetic Compati-bility Center.43 These studies encompassed sev-eral thousand runs of telephone and pacemakercombinations and provided consistent results.The internal filters in most implanted devices arehighly effective in rejecting the constant carrierfrequency of analog telephones. Interference isnonexistent or only occurs during the brief “shakehand” period before ringing. Although digital tele-phones transmit on the same carrier frequencies asthe analog telephones, the pulsed component ofthe transmission (in the 11–200-Hz range), can bedetected by the pacemaker sensing circuitry whenthe field is strong enough. PCS or similar tech-nologies produced interactions in , 1% of tests,while other digital technologies (GSM, NADC,TDMA-11) produced interference in 0–25% oftests. In all studies, a few models were responsiblefor a disproportionately large number of interac-tions, whereas others were largely immune. Theoverwhelming majority of interactions occurred atdistances , 10 cm. Pacemakers always reverted tonormal operation when the telephone was turnedoff.

Several investigators have systematicallystudied the effects of cellular telephones in pa-tients with pacemakers. Although there have beendiscrepancies in the reported frequencies of EMI(explained by differences in wireless technologiestested, exposure protocols, pacemaker models,pacemaker sensing polarity, programmed sensi-tivities, and definitions of interference), it can beconcluded that severe interactions are improbablewith most technologies during regular telephoneuse. In a comprehensive multicenter study, Hayeset al.44 tested 980 pacemaker patients for potentialinterference with five types of telephones (oneanalog and four digital: NADC, TDMA-11, PCS,and CDMA). Telephones were tested in a simu-lated worst-case scenario; in addition, NADC tele-phones were tested during transmission to simu-late actual use. Patients were monitored while thetelephones were held at the ipsilateral ear and ina series of maneuvers directly over the pacemaker.The incidence of any type of interference was 20%in the 5,533 tests. Tracking of interference sensedin the atrial channel, asynchronous pacing, and



Figure 3. Filtered four-wire feed through assemblyavailable in St. Jude pacemakers. Similar filters are nowalso present in pacemakers from Medtronic andGuidant. (From Selznick L, Mueller H, Chávez T. Cellulartelephone technology and its effect on implantablecardiac pacing systems. Sylmar, CA, Pacesetter, Inc,October, 1996, with permission).

ventricular inhibition were the most common re-actions observed (14%, 7%, and 6%, respec-tively). Interference was least frequent with analog(2.5%) and PCS (1.2%) systems. Clinically signif-icant EMI was observed in 7% of tests, and wasconsidered severe in 1.7 %. There was no clini-cally significant EMI when the telephone wasplaced in the normal position over the ear. Thepresence of feedthrough filters in the pacemakersalmost abolished the risk of EMI (from 29–56% to, 1%). In a study of 39 patients, Naegeli et al.45

demonstrated that EMI was more common withportable 8-W GSM telephones than with handheld 2-W models (7% vs 3% of tests) and thatatrial or ventricular oversensing were more fre-quent with pacemakers programmed at maximalsensitivity than at nominal sensitivity (6% vs 2%of tests). In a subset of 14 patients with VVIR pace-makers and programmable polarity, the same au-thors showed that pacing inhibition was morecommon in the unipolar mode. Additional studiesshowed that GSM telephones do not induce inap-propriate rapid pacing in patients with minuteventilation pacemakers,46 or atrial oversensing insingle-lead VDD pacemakers programmed at max-imum atrial sensitivity (0.1–0.25 mV).47

Potential interactions between ICDs and wire-less telephones have also been studied in vitroand in vivo. It should be noted that feedthroughfilters present in pacemakers from several manu-facturers are not as common in ICDs. Bassen etal.48 exposed ICDs from three manufacturers tomaximal power fields from analog and digital tele-phones (TDMA-11 and NADC). ICDs were pro-grammed to pace in the VVI mode at nominal sen-sitivity. No device reacted to the analog telephone,while all three models reacted to the TDMA-11telephone at minimal distances between 2.3 and5.8 cm. Interference from the NADC was observedonly when the most sensitive ICD was placed atclose distance (# 2.3 cm) from the source. Anotherin vitro study involved all (analog and digital)wireless telephone technologies in use in theUnited States and Europe and ICDs from fourmanufacturers.49 Interactions occurred (between asmall number of wireless telephones and someICDs) only at close proximity. No interactionswere noted between ICDs and telephones that op-erate in the 1,800–1,900-MHz bands. All observedinteractions involved two ICDs from one manu-facturer or the TDMA-11 technology. Subsequentrefinements in TDMA-11 technology (used onlyfor specialized business applications like truck-ing, delivery, and construction in the UnitedStates) that allowed reduction in maximum oper-ating power from 1 to 0.6 W should have reducedthe incidence of interaction. Jiménez et al.50

demonstrated that analog and GMS telephones

did not induce oversensing or interfere with thedetection of simulated ventricular tachyarrhyth-mias in Medtronic and Guidant ICDs.

Clinical “worst-case scenario” testing has notdisclosed significant interactions between ICDsand analog50 or digital NADC,51 GSM,50,52–54 orPCS53 telephones. In small studies, digital cellulartelephones did not interfere with the detection ofinduced ventricular fibrillation in the electro-physiology laboratory.54,55 Inconsequential inter-mittent loss of telemetered electrograms and sur-face ECGs and inscription of erroneous eventmarkers (i.e., “pseudo-oversensing”) recorded viathe programmer is common. Inductive hospitalpager systems may overlap with the carrier fre-quencies of some pacemaker programmers (32–37kHz) and also interfere with pacemaker teleme-try.56 This may result in inaccurate battery volt-age, current and impedance measurements, dis-turbances in intracardiac electrogram tracings, ortotal interruption of telemetric communications.

It can be concluded that cellular telephonescan potentially interfere with the function of im-planted cardiac devices. This interference doesnot pose a health risk when telephones are placedover the ear. Maintaining an activated cellulartelephone at least 6 inches (15 cm) from the deviceis key to avoid interactions. The FDA has issuedsimple recommendations to minimize the risks.Patients should avoid carrying their activated cel-lular telephone in a breast or shirt pocket overly-ing the implanted device. A wireless telephone inuse should be held to the ear opposite the sidewhere the device is implanted. A recent survey of1,567 Japanese pacemaker patients revealed thatalthough 94% were right-handed, 41% used theirleft hand preferentially to hold a wireless tele-phone.57 Not-so-obvious reasons for choosing onehand versus the other to hold the telephone in-cluded one side hard of hearing (10%) and use ofthe opposite hand for dialing or writing memos(22%). It appears that, at least in some patients,the hand preferentially used to hold the wirelesstelephone should also be considered when select-ing the site for pacemaker implant.

In the past, some investigators favored the useof analog telephones by patients with implanteddevices.38 With digital transmission modes be-coming dominant worldwide, such recommenda-tion is no longer practical. Current evidence sug-gests that as long as FDA recommendations arefollowed, the use of digital wireless telephones(especially those of PCS or similar technology) issafe. Individual patient testing in the clinic is notrecommended because of the likelihood of “false-positive” (e.g., interference with ECG monitoringequipment producing artifact) and “false nega-tive” (e.g., low signal intensity from the telephone,

PINSKI, ET AL.


which depends on several factors, including dis-tance to the nearest base station) results. It is morereliable to contact the device manufacturer to de-termine the results of formal testing with specificmodels.58 As other wireless communication de-vices become prevalent, their effects on implantedcardiac devices should be carefully scrutinized.For example, in third-generation handsets, thetransmission of high speed data will require in-creased power at the antenna. Limited in vitro andin vivo testing suggests that 3-W GSM telephonesdo not interfere with the function of an im-plantable ECG loop recorder.59

EAS DevicesEAS devices (also known as antitheft devices

or antishoplifting gates) are ubiquitous in retailstores and libraries. The transmitter in these de-vices emits an electromagnetic field designed tointeract with a “tag” in a store item. As a result ofthe interaction, the tag emits back a signal that isthen detected by the receiver. Customers are ex-posed to an electromagnetic field as they walkthrough the gate that consists of a pair of transmit-ter and receiver pedestals. EAS systems differgreatly in the frequency and strength of emittedfields. High frequency swept radiofrequency (e.g.,Checkpoint QS 2000, 8.2 MHz; SensormaticSaver, 8.4 MHz), low frequency acoustomagneticsystems (e.g., Sensormatic Ultramax, 58 kHz), andextremely low frequency electromagnetic systems(e.g. Knogo MM-85, 218 Hz; Sensormatic AisleKeeper, 534 Hz) constitute the leading technolo-gies worldwide. The technologies serve differentretailers’ needs in terms of area covered, cost, de-tection and “false alarm” rate, and are not strictlyinterchangeable. The general consumer cannotdifferentiate them by their external appearance.Electromagnetic fields from these devices have thepotential to induce interference signals in thesensing circuit of implanted cardiac devices. Sev-eral case reports have indicated the possibility ofclinically important interactions between EASsystems and pacemakers or ICDs.60,61 There hasbeen controversy over the frequency and severityof those interactions.62 Prospective studies haveclarified the incidence, severity, and risk factorsfor EMI from EAS systems.

An in vitro study from the Canadian MedicalDevices Bureau63 showed that 20 of 21 pacemakermodels reacted to the field of an acoustomagneticEAS system, while 10 reacted to an electromag-netic system. Responses included inhibition andnoise reversion. Interference occurred when thesimulator was within 33 cm of the transmissionpanel for the acoustomagnetic system, and 18 cmfor the electromagnetic system. Dodinot et al.64 ex-posed 32 patients with 26 different pacemaker

models to the fields of radiofrequency (7.4–9MHz) and magnetic (300 Hz and 10 KHz) EAS sys-tems. No interactions were observed with the ra-diofrequency system, while 50% of dual chamberpacemakers (all unipolar) exhibited significantpacing inhibition when exposed to the fields fromthe magnetic system. Mugica et al.65 exposed 204pacemaker patients to two different EAS systems(acoustomagnetic at 58 kHz and electromagneticat 73 Hz) for up to 30 seconds, unless undesirableinterference occurred earlier. At least one type ofinteraction occurred in 17% of patients. Interfer-ence was twice as likely with the acoustomagneticsystem. Atrial tracking, asynchronous pacing, andsingle beat inhibition were observed. All the in-teractions were transient and deemed benign.McIvor et al.14 studied the effects of six EAS sys-tems in 50 patients with pacemakers from sevendifferent manufacturers. One exposure protocolmimicked the most common real-life situation,walking at a normal pace midway between thegates. A “worst-case scenario” protocol requiredthe patients to lean against the transmitter gatewith the body parallel and then perpendicular tothe transmitter. Interactions occurred with 48pacemakers, almost exclusively with acoustomag-netic systems. No pacemaker reacted to the sweptradiofrequency systems. Only two patients pre-sented transient asynchronous pacing while ex-posed to an electromagnetic system. The fre-quency of interactions with the acoustomagneticsystem increased with the duration and closenessof the exposure. It was 16% when walkingthrough the gates and 96% when leaning againstthe pedestal. Transient asynchronous pacing wasthe most common response, followed by atrialoversensing with tracking, ventricular oversens-ing with inhibition, and “voltage-induced” pacedbeats. Changing the sensing configuration fromunipolar to bipolar or programming a lower sensi-tivity setting did not abolish the interactions, butlimited them to closer distances from the center ofthe gate..

EAS systems can trigger spurious ICD shocks.Particularly concerning is a report by Santucci etal.66 A patient with complete heart block and aVentak AV ICD in an abdominal pocket developedmultiple shocks and near-fatal inhibition of pac-ing on exposure to an acoustomagnetic EAS sys-tem. Provocative testing with similar equipmentin a controlled environment reproduced the inter-action. The maximum distance at which ventricu-lar oversensing occurred was 30 cm. When sensi-tivity was reprogrammed from “nominal” to “leastsensitive,” the interaction only occurred at closerproximity. McIvor et al.14 did not find instances offalse tachyarrhythmia detection in 25 patientswith ICDs exposed to different types of antitheft



devices, but the ICDs were not programmed topace during the testing. Groh et al.67 studied theinteraction between ICDs and two electromagneticand one acoustomagnetic EAS devices in 169 pa-tients. No spurious detections occurred during a10–15-second walk through the gates. False ven-tricular fibrillation detection occurred in three pa-tients (one Medtronic 7219 and two Guidant 1746)during a 2-minute exposure to the acoustomag-netic system. When the 2-minute exposure was re-peated during continuous pacing in 126 patients,oversensing was observed in 19 (15%). Oversens-ing was severe (complete or prolonged pacing in-hibition) in 7 (6%), including the same three pa-tients who had spurious tachyarrhythmiadetection at baseline and four additional patientswith Ventritex (Sunnyvale, CA, USA) ICDs duringexposure to an Aislekeeper electromagnetic sys-tem (programmability precludes determination ofspurious detections in Ventritex devices.) In 12(9%) patients, intermittent delayed pacing (com-patible with noise augmented T wave oversens-ing) was seen. All the patients with serious inter-actions had an abdominal implant, but bymultivariate analysis, diminished R wave ampli-tude and a Ventritex ICD were the only predictorsof interactions. The effect of antitheft devices ondetection of ventricular tachyarrhythmias has notbeen studied.

Available evidence suggests that although se-vere interactions between EAS systems and im-planted cardiac devices can occur, they are un-likely when patients walk through the gates at anormal pace. However, interactions are likelywith prolonged, close exposure to acoustomag-netic or electromagnetic systems. Patients shouldbe instructed not to linger in proximity or leanagainst theft deterrent gates. Retailers shouldavoid placing systems where people are requiredto linger, like checkout counters. Merchandise orinformation (e.g., store floor plans) should not bedisplayed in close proximity to antitheft systems.The FDA recommends that all manufacturers ofelectronic antitheft systems develop labeling orsignage to post on or near all new and currently in-stalled systems, indicating that an electronic an-titheft system is in use. The labeling or signageshould be positioned so that it is visible before anindividual enters the monitored area.68

Metal DetectorsHandheld and walkthrough metal detectors

are used for security applications. They functionby sensing disturbances in electromagnetic fields.Handheld metal detectors typically operate at afrequency of 10–100 kHz and produce weak fields(# 4 A/m at a distance of 1 inch). Weapons are de-tected only within 1–4 inches. Walkthrough metal

detectors have coils on one or both sides of theequipment. They operate in a continuous wave(5–10 kHz) or pulsed mode (200–400 Hz). Mag-netic fields measured at the chest level inside thearch are , 2 G.69 Typically, a person walkingthrough will be exposed for 3 seconds. Copper-man et al.70 monitored 103 patients with a varietyof pacemakers (mostly nonprogrammable VVIunits) crossing an airport metal detector gate andfound no interactions.

The FDA has received one report of a spuri-ous ICD shock triggered by a handheld metal de-tector in an airport. In several other instances,ICDs from Guidant/CPI reverted to “monitoronly” mode after being exposed to metal detec-tors.5 Current FDA recommendations state that itis safe for patients with implanted cardiac de-vices to walk through a metal detector gate, al-though the alarm may be triggered by the genera-tor case. If scanning with a hand held metaldetector is needed, the patients should ask the se-curity personnel not to hold the detector close tothe implanted device longer than absolutely nec-essary. An alternate form of personal search canalso be requested.71

Electric PowerEMI from electric power can occur if patients

come in proximity to high voltage overhead powerlines (accidentally or by occupation) or it may becaused by electrical appliances held close or in di-rect contact with the chest. Implanted devices aresusceptible to interference signals of 50–60 Hz,frequencies that lie within the bandwidth sam-pled for detection of intracardiac signals.

Detrimental effects from incidental exposureto high voltage lines are unlikely. For example,even at 40-m distance from a 400-kV line, the elec-tric field strength is low (, 1 kV/m). Only fieldstrengths . 5 kV/m influence pacemaker behav-ior. Patients working in close proximity to highvoltage sources (e.g., lines, distribution transform-ers) can suffer EMI.72 In vivo studies disclosedthat all kind of responses (inhibition, triggering,noise reversion) could occur, depending on thestrength of the field, the generator model, the sens-ing configuration, and the programmed sensitiv-ity.36,73,74 Bipolar sensing protects from EMI in allbut the most extreme environmental conditions,like power generating stations, while with unipo-lar sensing inappropriate pacemaker behavior canoccur during routine daily exposures. Mehdirad etal.75 reported a television cable line installer withan ICD, who, while kneeling on damp ground in autility tunnel, accidentally grasped a 60 V/30 A al-ternating current power line with his bare lefthand, causing immediate electrocution. He re-mained conscious but was unable to release the

PINSKI, ET AL.


power line. He then received a shock from his ICD,causing him to be thrown back and as a result, thepower line was released from his hand. The storedintracardiac electrogram revealed normal sinusrhythm followed by detection of 60-Hz electricalnoise, detected with shortest intervals of 120 ms(the blanking period of the device). After confir-mation, a 12-J shock was delivered. Upon releaseof the power line from his hand, the electricalnoise detection was no longer present.

EMI from household appliances is more likelywith improper grounding. Anecdotal reports haveincriminated toy remote controls,76 electric ra-zors,77 current leak from a water boiler (manifest-ing when opening the hot water faucet),78 and vi-brators.18 A report by Seifert et al.77 on a patientwith spurious ICD discharges while using an elec-tric razor is especially illuminating. Provocativetesting confirmed oversensing of 50-Hz powerwith the patient’s razor and a brand-new similarunit. Further evaluation suggested an insulationbreak, which was located (at operative revision)by the ventricular coil of the “integrated” bipolarEndotak (Guidant Inc.) lead. The system was op-erating “de facto” in a unipolar mode.

Slot MachinesA report by Madrid et al.79 suggests that slot

machines represent another source of EMI. Over a2-year period, they encountered four ICD patientswho received shocks while playing with slot ma-chines. Stored electrograms or RR interval histo-ries were compatible with electrical noise. Char-acteristics of the culprit slot machines were notreported. These observations have not been repro-duced, nor were simulation studies undertaken.Until the issue is clarified, it is prudent to warnICD patients of this potential interaction.

Working Environment Sources of EMIIndustrial Equipment

Although in general desirable, the return of thepatient with an implanted cardiac device to a workenvironment suspected of high level EMI can bechallenging. Among the myriad potential EMIsources, arc or spot welders, industrial weldingmachines, degaussing coils, and electric motors arefrequent cause of concern. These sources do notonly emit energy in the radiofrequency spectrum,their associated magnetic fields could potentially



Figure 4. Spurious implantable cardioverter defibrillator shock due to electromagneticinterference (EMI) from electric power. Stored far-field (F, coil-to-can) and near-field (N, tip-to-coil) electrograms plus annotated event markers (M) in a patient who received a shock while usinga power drill in a flooded basement. The maximal ventricular sensitivity was programmed to 0.3mV; 60-Hz interference is clearly visible. Premature paced beats (arrowheads) represent operationof the ventricular rate stabilization algorithm. The patient released the drill immediately after theshock. EMI in the sensing channel disappears promptly.

close the reed switch in pacemakers and ICDs.Each patient should be evaluated individually, buta few generalizations can be made. Bipolar sensingsystems with close-coupled (# 1 cm) electrodesshould be used preferentially in patients who maybe exposed to high levels of EMI at work. Newerdual coil defibrillation leads with dedicated bipo-lar sensing may be useful in this setting, as theymake the prior trade-off between defibrillation effi-ciency and susceptibility to interference no longernecessary. The sensitivity should not be pro-grammed very high in relation to the intrinsic elec-trogram amplitude. Implant testing of ventricularfibrillation detection at the least sensitive setting(e.g., 1.2 mV with Medtronic ICDs) allows estima-tion of the sensing “safety margin” and appropriatereduction in the chronically programmed sensitiv-ity. It is useful to ask a technical consultant fromthe device manufacturer to conduct a comprehen-sive EMI test at the patient’s work site. However,this service may not be generally available due toliability issues. There is no professional reimburse-ment provided for an on-site visit by clinic staff.Testing should include measuring of magneticfields at different distances from the source plus re-

view of telemetered and stored electrograms andevent markers while the patient is operating theequipment (ICDs should be programmed “monitor-only” to avoid spurious shocks).80 In pacemakerdependent patients, testing a device identical tothe one implanted coupled to a heart simulator rep-resents a safe, sensitive preliminary step.81 In pa-tients with Guidant ICDs, a simple screening strat-egy that included listening to QRS synchronousbeep tones (a programmable feature) after extend-ing the detection duration while the patient rou-tinely operates the equipment is safe and effec-tive.82 In patients with ICDs from St. Jude andGuidant that are exposed to intense magnetic fieldsat work, inhibition of tachyarrhythmia therapy inresponse to magnet application can be disabled.Additional general precautions include ensuringappropriate grounding of the equipment andavoiding close contact with the EMI source. Arcwelders, for example, should not carry the cableson their shoulder. If they experience light-headed-ness or an ICD shock (Fig. 4), patients should be in-structed to stop operating the equipment and tocontact their physician. Many patients can safelyreturn to work with these precautions.83

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Interference in Implanted Cardiac Devices, Part I