Title: Effects of External Electrical and Magnetic Fields on Pacemakers and Defibrillators
Abstract: HomeCirculationVol. 128, No. 25Effects of External Electrical and Magnetic Fields on Pacemakers and Defibrillators Free AccessResearch ArticlePDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissions ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toFree AccessResearch ArticlePDF/EPUBEffects of External Electrical and Magnetic Fields on Pacemakers and DefibrillatorsFrom Engineering Principles to Clinical Practice Roy Beinart, MD and Saman Nazarian, MD, PhD Roy BeinartRoy Beinart From the Section for Cardiac Electrophysiology, Division of Cardiology, Johns Hopkins University School of Medicine, Baltimore, MD. (R.B., S.N.); and Davidai Arrhythmia Center, Leviev Heart Center, Sheba Medical Center, Tel Aviv University, Tel Hashomer, Israel (R.B.) Search for more papers by this author and Saman NazarianSaman Nazarian From the Section for Cardiac Electrophysiology, Division of Cardiology, Johns Hopkins University School of Medicine, Baltimore, MD. (R.B., S.N.); and Davidai Arrhythmia Center, Leviev Heart Center, Sheba Medical Center, Tel Aviv University, Tel Hashomer, Israel (R.B.) Search for more papers by this author Originally published24 Dec 2013https://doi.org/10.1161/CIRCULATIONAHA.113.005697Circulation. 2013;128:2799–2809According to a recent worldwide survey, current rates of de novo pacemaker and implantable cardioverter-defibrillator (ICD) implantation are higher than those of previous years at >700 000 and >200 000 per year, respectively.1 Parallel to the growth in cardiovascular implantable electronic device (CIED) use, technological advances have led to new sources of electromagnetic radiation. Modern CIEDs utilize shielding, filters, and bipolar leads to mitigate electromagnetic interference (EMI). Nevertheless, EMI sporadically results in harmful consequences. Physicians attending to patients with CIEDs must be aware of typical EMI sources and strategies to prevent malfunction of CIEDs.EMI: Engineering PrinciplesTerminology and DefinitionsElectromagnetic field is the term used to describe combined electric and magnetic fields. Electric fields exist whenever electric charges are present (ie, whenever electricity or electric equipment is in use). A magnetic field is produced when electric current flows in a conductor with magnetic field lines perpendicular to the current flow. EMI can occur as a result of conducted or radiated electromagnetic energy. Medical equipment such as transcutaneous electronic nerve stimulators or poorly grounded electric equipment may result in directly conducted electromagnetic energy in the form of galvanic currents. Electromagnetic radiation is the term used to describe electromagnetic energy radiating away from its source. Electromagnetic radiation can be described as ionizing versus nonionizing radiation. Ionizing radiation consists of very short wavelengths, such as x-rays, which have enough power to move electrons off their nuclear orbits. In contrast, nonionizing radiation consists of longer wavelengths with less power and is incapable of moving electrons off their orbit around the nucleus.Electromagnetic fields are characterized by wavelength, frequency, and field strength. Radiofrequency energy, defined by a frequency range between 0 Hz and 450 MHz, is emitted by sources including magnetic resonance imaging (MRI), electrosurgery, and radio/television broadcasting. Cellular telephones, microwave ovens, and radar transmitters typically emit microwave energy, defined by a frequency range between 450 MHz and 12 GHz. The electric field strength and magnetic field intensity components of electromagnetic fields are measured in volts per meter and amperes per meter, respectively. The magnitude of energy deposition in tissues is measured by the specific absorption rate measured as watts per kilogram. Electromagnetic field energy decreases as an inverse squared function of distance from the source. Thus, doubling the distance from the source results in a 4-fold reduction in exposure. For example, in the setting of MRI or static magnetic fields created by linear accelerators or betatrons, the intensity of static magnetic fields decreases as a function of distance from the source and creates a spatial gradient magnetic field. The intensity of the static magnetic field is typically reported in units of tesla, where 1 T is the equivalent of 10 000 G and 796 A/m. In general, it is accepted that local field strengths of >10 G are sufficient to induce EMI. In contrast to static and spatial gradient magnetic fields, gradient coils in MRI scanners produce time-varying gradient magnetic fields for spatial encoding of the MRI signal.2 Acoustic radiation involves pressure waves that are emitted from medical equipment such as lithotripsy equipment.Summary of Potential Transient and Permanent Effects of Electromagnetic FieldsDisturbances in CIED circuitry or behavior attributable to electromagnetic radiation emitted from an external source are known as EMI. Electromagnetic radiation properties and CIED distance from the EMI source, as well as CIED design, materials, shielding, programming, sensitivity, and filtering properties, modulate the extent of EMI.Spatial gradients in static magnetic fields result in translational and rotational forces on ferromagnetic objects.3 If the translational force exceeds counterforces from sutures, scarring, and tissue ingrowth, permanent and dangerous effects may occur from dislodgement and movement of CIEDs. A transient effect of spatial gradients in static magnetic fields is the magnetohydrodynamic effect, which occurs as a result of the conductive effect of blood that results in a voltage difference across the vessel in a direction perpendicular to the blood flow. This effect is dependent on the velocity of blood flow, magnetic field strength, vessel diameter, and angle of flow with respect to the magnetic field. Peaked T waves may be sensed as a result of the magnetohydrodynamic effect and may lead to transient oversensing by CIEDs.4 A case of transient pacemaker inhibition has been attributed to the magnetohydrodynamic effect.5 Exposure of older CIEDs with reed switches to static magnetic fields may transiently cause asynchronous pacing and inhibit tachycardia therapies. Modern CIEDs have largely replaced the reed switch with integrated solid state detection, Hall sensors, telemetry coils, or giant magnetoresistor circuits.Time-varying gradient magnetic fields may be associated with transient and permanent effects on CIEDs. The main concern in this setting is the potential for current induction within conductive wires in the field that may result in transient inhibition of pacemaker output and direct stimulation of the myocardium, in addition to permanent effects attributable to heating of the conductor and tissue burns.Radiofrequency fields can be associated with tissue heating and burns. Conductive implants and leads, particularly when placed in a loop configuration, can greatly increase the risk of burns as a result of inductive heating of the lead conductor from radiofrequency fields.4 In addition, transient EMI effects can be observed when radiofrequency energy is used for thermal ablation of the myocardium or other tissues. When radiofrequency ablation is performed, current flows from the conducting electrode toward a dispersive or ground electrode. Proper placement of the dispersive electrode may result in a current path that minimizes EMI with CIEDs; however, appropriate precautions are warranted if the path includes the thorax.Ionizing radiation can have cumulative and permanent effects on CIEDs. The complementary metal oxide semiconductor circuitry in CIEDs uses silicon and silicon dioxide as insulators between circuitry components.6 Exposure of the semiconductor to ionizing radiation results in buildup of charge in the silicon dioxide insulation and leakage current within the circuits, thus risking unpredictable function including programming changes, loss of diagnostics, power-on-reset, and circuitry failure.Acoustic radiation may transiently activate the rate response features of some CIEDs and permanently damage others if in close proximity.7Electromagnetic CompatibilityThe term electromagnetic compatibility (EMC) describes efforts to minimize the possibility of EMI. These efforts include shielding, which includes protecting the CIED generator or lead circuitry with electromagnetic field barriers composed of conductive or magnetic materials. CIED generators are typically shielded by hermetically sealed titanium or stainless steel cases. Nanomagnetic insulation has been utilized recently in lead design to improve shielding from radiofrequency and time-varying gradient magnetic fields. Additionally, body tissues add to the shielding of leads because leads placed in a conductive medium are poor antennas. The extent of CIED shielding varies depending on the manufacturer and can affect product weight, handling, and dimensions. In the case of the generator, the shielding must be adequate to reject electric fields >2 MHz. The use of bipolar sensing and low-pass filters reduces conducted and radiated interference. Additionally, feedthrough capacitor filters are utilized to prevent EMI from a wide range of frequencies including mobile phone frequencies.8 The term feedthrough describes a conductor used to carry a signal through the pacemaker casing. The conductor is capable of holding a guaranteed minimum value of charge and is therefore called a feedthrough capacitor. The conductor is meant to participate in unipolar (to generator) sensing/pacing functions but may also act as an antenna that carries EMI. Feedthrough capacitor filters decouple and shield against conduction of radiofrequency carrier signals from mobile phones into the pacemaker circuitry. In addition, incorporation of bandpass filters allows rejection of frequencies outside the range of interest. However, electromagnetic radiation with frequencies between 0 and 60 Hz overlaps the cardiac signal range. Therefore, rejection of this frequency range by band-pass filters would affect proper pacemaker function.7 To address EMI that bypasses filtering schemes, the noise reversion feature, typically programmed for asynchronous pacing, activates when signals are detected in the noise-sampling period of the atrial and ventricular refractory periods. Table 1 summarizes design features to enable EMC.Table 1. Electromagnetic Compatibility Features and Corresponding Electromagnetic Interference TypesIonizing RadiationElectric FieldsStatic and Spatial GradientMagnetic FieldTime-VaryingGradientMagnetic FieldRadiofrequencyMicrowaveShielding the generator and leads++++Bipolar sensing+++Feed-through capacitor filters++Low-pass filters++Band-pass filters++Noise reversion+++Use of Hall sensor (instead of reed switch)+Increasing lead conductor resistance++Reduction of ferromagnetic components++CIED Response to EMIIn addition to the EMI characteristics, programming of CIEDs is a significant determinant of device response to EMI. During inhibited pacing modes (AAI, VVI, or DDI), EMI results in pacing inhibition and bradycardia or asystole. In contrast, while in tracking mode (DDD), preferential sensing of EMI in the atrial channel (which often occurs because of higher programmed sensitivity) could result in (1) increased rate of ventricular pacing or (2) atrial arrhythmia detection and CIED “mode-switch” to inhibited pacing (AAI, VVI, or DDI). Detection of EMI may result in pacing at the sensor-triggered upper rate limit if rate response is activated. In ICD recipients, sensing of EMI in the tachyarrhythmia frequency range could result in inappropriate therapies. Activation of the magnet response by EMI may lead to asynchronous pacing in pacemakers and transient or permanent disabling of tachycardia therapies in certain ICD systems. Importantly, although rare, CIEDs may revert to a back-up programming mode known as “power-on-reset” when exposed to high-energy EMI. Power-on-reset events are generally resolved by reprogramming the CIEDs to patient specific settings. However, it is important to realize that during power-on-reset, pacing is typically set to an inhibited mode (VVI), and tachycardia therapies are enabled.Everyday Sources of EMIRoutinely used equipment seldom causes EMI with CIEDs. Table 2 summarizes potential sources of EMI, documented effects, and recommended precautions. Several common sources are also discussed below.Table 2. Sources of EMI and Their Potential EffectsType of FieldEnvironmentSourcePotential EffectsWarranted PrecautionsDamage to CIED CircuitryPacemaker InhibitionNoise TrackingAsynchronous PacingNoise ReversionCommunication FailureCurrent InductionArrhythmia Detection/ICD ShocksInhibition of Tachyarrhythmia DetectionSensing and Pacing Threshold ChangesPower-on-ResetRate Adaptive Sensor ActivationElectromagneticDaily lifeDigital music players9,10+Avoid close proximity (<30 cm) to generatorStereo speaker and headphones11(magnetic field strength ≥10 G)+++Avoid close proximity to generatorNeodymium-iron-boron magnets incorporated into toys, jewelry, and clothes12,13+Avoid close proximity to generatorMetal detectors14,15++Alert security personnel regarding implanted deviceElectronic article surveillance devices16–19+++++Patients should not linger near surveillance devices to minimize the duration of EMI that might be detected by CIEDsCellular phones20–26++++Avoid close proximity to generatorHand-held home phones27++Avoid close proximity to generatorTasers28–30++++Household leakage current from improper grounding31–34++Avoid contact with improperly grounded equipmentAutomotive engine11+++Avoid close proximity of ignition system, engine, and alternator to generatorWork and industryArc welding equipment, electric motors, and degaussing coils35++++Advise employer regarding implanted medical device and consider environmental testing by a trained EMI consultant (list of consultants can be provided by CIED manufacturer); maintain minimal distance of 2 ft (61 cm) from welding arc and cables or large motors; do not exceed tested currents with welding equipment, wear insulated gloves while operating electric equipment, and verify that electric equipment is properly groundedAircraft cockpits36++++RFID equipment37++++High-voltage power coils38,39++Assessment of threshold safe distance for EMI and follow-up device interrogations should be arrangedMedicalMRI40++++++MRI should be performed at experienced centers utilizing a strict protocol for patient monitoring and CIED programming (refer to Table 3)Electrosurgery41,42++++No intervention or magnet application is needed for subumbilical procedures; for procedures above the umbilicus, programming or magnet application should be performed to activate asynchronous pacing for pacemaker-dependent patients as well as deactivation of tachyarrhythmia therapies for ICD recipients; position the ground electrode to direct the current path away from the CIEDsLVAD43–45++Cardioversion46–48++CIEDs should be programmed to a bipolar sensing configuration and should be excluded from the current path by use of anteroposterior patches/paddles position, placed >6 inches away from the generator48; immediate access to a CIED programmer is recommendedDental equipment49–51+Radiofrequency catheter ablation52–55++++++Deactivate tachyarrhythmia therapies and activate asynchronous pacing for pacemaker-dependent patientsSpinal cord56,57 and deep brain stimulators58,59+Program bipolar sensing configuration; test for interactions intraoperatively and when modifying programmingGastroscopy, endoscopy, and colonoscopy60No anticipated effects in absence of electrosurgeryCapsule endoscopy61–64+++Tachyarrhythmia therapies should be turned off before procedureTranscutaneous electric nerve stimulation:65–67++++Magnet application or programming to an asynchronous mode (VOO/DOO) is recommended in pacemaker-dependent patients; ICD therapies should be disabled during treatmentTelemetry monitors68++Consider deactivation of minute ventilation sensorsElectroconvulsive therapy69,70+Availability of magnet or CIED programming equipment is advisedIonizing radiationRadiotherapy71–75+++++CIEDs should be interrogated within 24 h of each treatment when using a direct beam to the chest or high-energy photon radiation for pacemaker-dependent patients43; programming the CIEDs to a higher pacing rate than baseline allows rapid recognition of power-on-reset or device malfunction when onsite interrogation is unavailableAcoustic radiationLithotripsy76–79++++Patients should be monitored continuously; synchronization of the hydraulic shocks to the R wave should be performed; dual-chamber pacemakers should be reprogrammed to VVI mode; ICD tachyarrhythmia therapies should be deactivated during treatment; CIEDs should be interrogated after procedureCIED indicates cardiovascular implantable electronic device; EMI, electromagnetic interference; ICD, implantable cardioverter-defibrillator; LVAD, left ventricular assist device; MRI, magnetic resonance imaging; and RFID, radiofrequency identification.Digital Music Players, Headphones, and Neodymium MagnetsDigital music players may result in isolated telemetry EMI without direct impact on CIED function.9,10 Portable headphones contain strong magnets that may interfere with CIED function at close range. Similar neodymium magnets are used in the jewelry and clothing industry and can lead to magnetic field effects.12,13Security Systems and EquipmentWalk-through metal detector security systems use either continuous-wave (5–10 kHz) or pulsed-wave (200–400 Hz) modes. In contrast, handheld detectors operate in higher-frequency continuous-wave mode (80–130 KHz) with lower magnetic field strength and lower likelihood for EMI.14 The risk of EMI appears to be very low in the setting of walk-through15 and handheld metal detectors.80 Nevertheless, CIED recipients should alert security personnel regarding the implanted device as recommended by the Transportation Security Administration. Electronic article surveillance devices utilizing magnetic, acoustomagnetic, radiofrequency, or microwave transmissions are used to prevent theft in stores and libraries. Instances of EMI with tracking and pacing inhibition attributable to ventricular oversensing, asynchronous pacing, current induction with myocardial capture/extrasystoles, and inappropriate shocks have been reported with close exposure to magnetic, radiofrequency, and acoustomagnetic surveillance systems.16–19,81 CIED recipients should not linger near surveillance devices, thus avoiding adequate EMI duration for activation of tachyarrhythmia therapies by the CIED. Tasers are weapons that deliver short pulses of current with a peak voltage of 1000 to 1500 V. Instances of inappropriate sensing resulting in pacing inhibition, noise reversion, and tachycardia detection, in addition to ventricular capture secondary to direct delivery of the Taser pulse to the myocardium, have been reported.28–30Cellular PhonesInstances of EMI with CIEDs resulting in oversensing, tracking, pacing inhibition, and asynchronous pacing were readily observed with early cellular phones placed directly on the device.22 Studies of modern cellular phones and CIEDs with routine use of dedicated feedthrough filters have noted minimal or no EMI.23,26 In the setting of ICDs, only transient telemtery loss,24 without clinically significant EMI, has been reported.20,21,25 Current recommendations indicate holding cellular phones to the ear contralateral to the CIEDs, avoiding close contact between the CIEDs and the phone, and avoiding the use of cellular phones during programming of CIEDs.Other Routine or Workplace SourcesRare reports of malfunction of CIEDs attributable to EMI from other sources including slot machines,82 washing machines,33 power drills,83 and alternative medicine devices84 exist in the literature. In general, recipients should be advised to avoid close contact of electric equipment with a CIED generator.Potential industrial sources of EMI include welding equipment, electric motors, and degaussing coils.35 High electromagnetic radiation levels exist in aircraft cockpits, but, on the basis of in vitro studies, they appear unlikely to cause significant EMI with CIEDs.36 Similarly, high-voltage power lines are unlikely to result in EMI with routine exposure and bipolar sensing configuration.38,39 If a CIED recipient is in an area with industrial equipment, assessment of the threshold source distance for EMI and follow-up device interrogations should be arranged. Additionally, the employer should be notified regarding CIEDs and the importance of environmental testing. This type of assessment can be performed by a trained EMI testing consultant at the employer’s request. The CIED manufacturer can provide a list of consultants.Medical Sources of EMIThe medical environment has abundant sources of EMI with the potential to cause malfunction in CIEDs. Cardioversion procedures may cause transient or permanent changes in CIED function including lead sensing and capture parameters.48,85 This may be secondary to shunting the energy through the lead to the electrode tip, causing electric injury and impedance changes at the electrode-myocardial interface.86,87 Power-on-reset and pacing mode changes have also been observed.46,88 Radiofrequency ablation generates signal frequencies between 500 and 1000 kHz in a unipolar configuration. Asynchronous pacing, pacing inhibition, loss of capture, power-on-reset, and, rarely, runaway pacing have been reported in the setting of radiofrequency ablation and older CIEDs.52,53,55 Because of the abundance of EMI sources in the medical environment and for brevity, most have been summarized in Table 2. However, MRI, radiotherapy, and electrosurgery are discussed in detail below.Magnetic Resonance ImagingThere are 3 distinct sources of EMI in the setting of MRI: the static magnetic field, radiofrequency fields, and time-varying gradient magnetic fields. The static magnetic field can exert force and torque on CIEDs and may change the reed switch state in older devices, causing unpredictable pacing behaviors. The radiofrequency and gradient magnetic fields may induce electric currents in leads within the field if the current loop is completed through the body. Current induction may lead to myocardial capture and induction of fatal ventricular arrhythmias. Additionally, the generator and leads can act as an antenna that amplifies local radiofrequency energy deposition, resulting in heating at the device-tissue interface. Finally, EMI may result in inhibition of pacing, asynchronous pacing (Figure 1), power-on-reset events, inappropriate tracking, inappropriate ICD therapies, and runaway pacing with induction of potentially life-threatening ventricular arrhythmias.Download figureDownload PowerPointFigure 1. High-frequency noise and noise response ventricular pacing are noted on the bipolar ventricular channel from a single-chamber pacemaker during magnetic resonance imaging.Because of the benefits of MRI, our group and others have developed protocols to safely perform imaging in CIED recipients. At our institution, we have performed MRI at 1.5 T on >1000 patients with CIEDs using a protocol that incorporates device selection and programming and patient monitoring. We exclude patients with (1) recently implanted leads (<6 weeks); (2) abandoned leads; (3) nontransvenous epicardial leads; and (4) dependence on the pacing function of an ICD. Additionally, we require the presence of a provider experienced in advanced cardiac life support and programming of CIEDs. Before MRI, baseline interrogation is performed, and system parameters are recorded. If the patient has a hemodynamically stable underlying rhythm, an inhibited pacing mode is utilized (AAI, VVI, or DDI). If the patient is dependent on pacing, the pacemaker is set to an asynchronous mode. All other pacing functions and tachyarrhythmia therapies are disabled. On completion of the MRI examination, the device is reinterrogated. Battery status and lead parameters are rechecked, and programming is restored to baseline settings. The protocol is summarized in Table 3. We have published our experience of utilizing this protocol for performing 555 MRI examinations on pacemaker and ICD recipients. Immediately after MRI, clinically insignificant decreases in right ventricular sensing and lead impedances were noted. Long-term differences included increased right ventricular capture threshold, decreased right ventricular sensing and impedance, and decreased battery voltage. These changes did not warrant CIED system revision or reprogramming. In our study, subclinical changes in sensing and battery voltage were associated with thoracic imaging. Given the extent of image susceptibility artifacts that affect the anterior and apical left ventricle,89 the potentially elevated risks versus benefits of thoracic MRI must be weighed carefully in patients with left-sided ICD systems. Artifact reduction methodologies are currently under development and will improve the performance of MRI in patients with left-sided ICD systems. Additionally, we observed acute power-on-reset events in 3 patients.40 Power-on-reset is important to recognize because inhibition of pacing or delivery of tachycardia therapies may occur if EMI is sensed before the CIEDs are reprogrammed. The use of MRI at 3 T for neurological, musculoskeletal, abdominal, and cardiovascular applications is increasing owing to increased signal-to-noise ratio, spatial resolution, and imaging speed. However, there is only limited experience with MRI safety of CIEDs at 3 T.90,91Table 3. Safety Protocol for MRI in the Setting of CIEDsBefore procedure• Consider alternate imaging modalities• Discuss benefits and risks of MRI in the setting of CIEDs, including risks of EMI, device failure, and arrhythmia, and obtain informed consent• Exclude patient with the following characteristics• CIEDs implanted before 2000• Recently implanted leads (<6 wk)• Abandoned leads• Nontransvenous epicardial leads• Dependence on pacing function of ICD• Perform baseline interrogation and programming• For hemodynamically stable underlying rhythm: inhibited pacing mode (AAI/VVI/DDI)• For pacemaker-dependent patients: asynchronous mode (VOO/DOO)• All other pacing functions (magnet, rate, PVC, noise, ventricular sense, and conducted atrial fibrillation response) and tachyarrhythmia therapies should be disabledDuring procedure• A provider experienced in CIED programming and advanced cardiac life support should be present to monitor symptoms, blood pressure, ECG, and pulse oximetry• Resuscitation equipment should be immediately availableAfter procedure• Reinterrogate CIEDs with recording of battery status and lead parameters and reprogramming of the system to baseline settings• Schedule follow-up interrogation in 3–6 moCIED indicates cardiovascular implantable electronic device; EMI, electromagnetic interference; ICD, implantable cardioverter-defibrillator; MRI, magnetic resonance imaging; and PVC, premature ventricular contraction.It is important to note that MRI in the setting of standard CIEDs is performed off-label and generally considered to be a contraindication. The Center for Medicare and Medicaid Services will cover MRI for beneficiaries with implanted CIEDs (1) through Coverage with Evidence Development/Coverage with Study Participation in approved clinical studies of MRI or (2) in the setting of “MRI-conditional” pacemakers (discussed below) used according to the Food and Drug Administration–approved labeling for use in the MRI environment.92 The American Heart Association, American College of Radiology, and European Society of Cardiology recommendations emphasize the necessity (1) to perform MRI at experienced centers utilizing a strict protocol for patient monitoring and device programming and (2) to individualize the decision-making process on the basis of the risk-benefit ratio of MRI and the utility of modalities alternative to MRI.93–95In recent years, device manufacturers have made substantial efforts to develop MRI-conditional CIEDs. This terminology implies the lack of known hazards when MRI is performed with the use of predefined device programming, patient monitoring, and imaging protocols. MRI-conditional CIEDs contain fewer ferromagnetic materials and incorporate lead design changes to mitigate heating. A Hall sensor is used instead of the reed switch to reduce unpredictable behavior within the MRI scanner. Other modifications include the availability of an easily programmable MRI mode, which requires a mandatory system integrity check before enabling increased pacing output and selection of asynchronous or inhibited modes (AAI, VVI, or DDI). The US Food and Drug Administration approved the Revo MRI SureScan Pacing System (Medtronic, Inc, Minneapolis, MN) in early 2011. To assess implantation safety, 100 patients with pacing indications were randomized to receive the MRI-conditional versus a conventional system. No excess complications were observed; however, there was a trend toward lower successful cephalic vein access.41 Next, MRI safety was assessed with the use of a randomized prospective multicenter study. Up to 12 weeks after implantation, 211 patients with the MRI-conditional system underwent MRI (1.5 T, specific absorption rate ≤2 W/kg, and gradient slew rate <200 T/m per second). The isocenter position was limited to regions above the superior surface of C1 and below the inferior surface of T12 vertebra. No MRI-related events or overt complications such as failure to capture, lead dislodgement, or pericardial effusion were noted, and recorded measurements were similar between the study groups.42 Importantly, the system requires the use of manufacturer-specific leads, and the approved M