Electronic Magnetic Interference and Magnetic Resonance Compatibility

Introduction

The implantation of permanent pacemakers (PPMs) and implantable cardioverter-defibrillators (ICDs) for the treatment of bradyarrhythmia and tachyarrhythmia has increased significantly over the past 30 years. In a review of the National Inpatient Sample, researchers found that 2.9 million PPMs were implanted in patients in the United States between 1993 and 2009. The National ICD Registry report states that there had been over 800,000 ICDs implanted by the end of 2011. Along with the increase in the use of cardiac implantable electronic devices (CIEDs), there has been an increase in the development and use of technology that emits electromagnetic interference (EMI), defined as any electromagnetic radiation from an external source that disturbs the behavior of a CIED with potential harmful consequences in CIED function. Because one of the essential components of CIED function is accurate sensing of intrinsic cardiac potentials, any interference by nonphysiologic sources may cause the system to interpret such interference as cardiac in nature. Noise sensing can have significant adverse clinical consequences, including the failure to pace when appropriate or the triggering of ICD therapies due to inappropriate identification of noise as ventricular tachycardia (VT)/ventricular fibrillation (VF). Thus electromagnetic compatibility (EMC) is of interest to physicians, clinical scientists, and device manufacturers to help identify sources of EMI and provide solutions to render CIEDs less susceptible to EMI. In order to understand the potential deleterious effects of EMI on CIEDs, it is important to understand some basic engineering principles.

Electromagnetic Field

The term electromagnetic field (EMF) refers to the combined properties of an electric and magnetic field. The concept of an electric field, first introduced by Michael Faraday, is the region that encompasses an electrical force generated by a source electric charge. The strength of the electric field is measured in volts per meter or Newtons per coulomb. Additionally, when electrically charged particles begin moving in a conductor, a magnetic field perpendicular to the direction of current flow is created. The properties of the magnetic field depend on the instantaneous velocity and the rate of change in velocity (acceleration) of the charged particles. The intensity of the resulting magnetic field is measured in amperes per meter. The magnetic flux density is measured in teslas (T), where 1 T = 10,000 gauss (G). As electric and magnetic fields are interdependent, the term EMF is used for the combined properties of these fields.

The EMF is characterized by three properties: wavelength, frequency, and field strength. The wavelength and frequency at which the electromagnetic force is radiating produce the electromagnetic spectrum. Frequencies from 0 Hz to 12 GHz used for communication can interfere with CIEDs. The strength of the EMF decreases in an inverse squared relationship from the source of the EMF. The EMF can interact with a CIED and result in a number of deleterious events, including:

1.
Device lead dislodgement secondary to translational and rotational forces.
2.
Inductive heating of tissue in contact with a lead that acts as a conductor of radiofrequency (RF) signals.
3.
Oversensing of noise created by EMI, leading to pacing inhibition and/or inappropriate tachycardia therapy if the noise has a rapid frequency that is mistaken for VF ( Fig. 12-1 ).
4.
Current induction produced by time-varying gradient magnetic fields through conductive wires, which may result in high-frequency myocardial stimulation and capture.
5.
Direct circuitry damage secondary to interaction with the metal oxide semiconductor by ionizing radiation, leading to a buildup of charge in the silicon dioxide insulation and ultimate leakage of current.
6.
Induction of a voltage gradient within the vessel perpendicular to the flow of electrically charged blood through the magnetic field, referred to as the magnetohydrodynamic (MHD) effect. Inappropriate sensing of peaked T waves caused by the MHD effect has been reported to cause pacemaker inhibition.

The degree to which EMI effects CIED function depends on a number of important factors, including the following:

1.
Intensity of the EMF and the distance of the EMI source to the CIED; the field intensity decreases inversely with the square of the distance from the source.
2.
Lead electrode configuration. Unipolar sensing has increased risk of oversensing compared with bipolar sensing, particularly as the strength of the electrical and magnetic fields increases. Further, the susceptibility of the CIED device to EMI increases as the programmed sensitivity increases. The atrial lead is more likely to be affected in pacemakers and dual-chamber ICDs due to the higher programmed sensitivity.
3.
The location of the generator may also influence susceptibility to EMI. Unipolar left-sided CIEDs are more susceptible to the negative effects of EMI, due to the increased semicircular field area produced between the lead and the generator, creating a larger magnetic field.

Ultimately, not all episodes of EMI are clinically important. Pacemaker dependency is an important factor, and patients who do not have a reliable escape rhythm can have prolonged periods during which pacing is inhibited. A classification scheme has been created to help categorize the clinical significance of interference with pacemaker function ( Table 12-1 ):

Class I: definitely clinically significant
Class II: clinical responses that were probably clinically significant
Class III: clinical responses that were probably not clinically significant

TABLE 12-1

Definition of Clinically Significant Interference With Pacemaker Function

From Hayes DL, Wang PJ, Reynolds DW, et al: Interference with cardiac pacemakers by cellular telephones. N Engl J Med 336(21):1473-1479, 1997.

Class I	Interference associated with symptoms of presyncope, syncope, dizziness, or shortness of breath
	Transient ventricular inhibition for 3 seconds or more
	Transient atrial inhibition for 3 seconds or more when the patient is in AAI or AAIR mode
	Persistent ventricular inhibition
	Persistent atrial inhibition in a patient in AAI or AAIR mode
	Any change in programmed settings
	Secondary events of supraventricular or ventricular arrhythmias
Class II	Transient ventricular inhibition (>2 sec <3 sec)
	Transient atrial inhibition (>2 sec <3 sec) in a patient programmed to AAI or AAIR
	Any type of interference and the presence of palpitations
	Persistent undersensing, tracking, safety pacing, rate-adaptive sensor-driven pacing, atrial or ventricular noise reversion mode, atrial inhibition, asynchronous pacing, mode switching
	Pacemaker-mediated tachycardia, rate-drop response, rate-adaptive pacing
Class III	Any other type of interference
Class III	Any other secondary events

Differential Diagnosis of Electromagnetic Interference

Other than EMI, there are several important potential sources of interference with CIED function that should remain in the differential diagnosis. Myopotentials, which are electrical signals arising from skeletal muscle, are a common source of inappropriate sensing in CIED recipients. In pacemaker-dependent patients, this can lead to long periods of atrial asystole caused by inhibition in the atrial channel, ventricular asystole caused by inhibition of pacing function in the ventricle, inappropriate triggering of ventricular response when in an atrial tracking mode, or even inappropriate ICD antitachycardial therapies when high-frequency myopotentials are mistaken for VT/VF. Diaphragmatic myopotentials have a typical pattern of respiratory phasic amplitude change. Myopotentials are often reproducible at the bedside through isometric maneuvers and reaching exercises, which can help differentiate these signals from EMI.

Make-break signals are often caused by a loose set screw, interaction between an active lead and an abandoned lead, or internal insulation breaks. High-amplitude make-break signals can lead to oversensing. The diagnosis can be made through reproduction of the signals with manipulation of the device in the generator pocket or through interrogation of the device, which often shows significant impedance changes compared with baseline. In contrast to EMI, which typically causes continuous or pulsatile high-frequency signals, make-break signals are random and occupy a small portion of the cardiac cycle.

Pacemaker and ICD Response to EMI

Inappropriate Sensing and Inhibition

The response of the CIED to EMI and the potential negative effects to the patient depend on the programmed mode of the CIED and whether the patient is dependent on the CIED for pacing. Potential adverse clinical complications of failure to pace secondary to inappropriate oversensing include lightheadedness, dizziness, syncope, or even death with prolonged asystole. With inhibited pacing modes (AAI, VVI, or DDI), noncardiac signals secondary to EMI that are detected by the pacemaker can lead to pacing inhibition, resulting in bradycardia or asystole if there is not a reliable native escape rhythm. Given the necessity for ICD systems to sense low-amplitude ventricular electrograms during VF, pacemaker-dependent ICD recipients are particularly vulnerable to prolonged inhibition by EMI. Programming within the ICD allows for automatic adjustment of the sensing threshold or gain, based on the intrinsic amplitude of the R wave, to appropriately identify low-amplitude VF without oversensing T waves. When the native conduction system fails to depolarize, the ICD appropriately begins pacing while also increasing the device's sensitivity to identifying potential VF. If EMI activity occurs during this vulnerable period, noncardiac signals can be oversensed, which can lead to inappropriate firing of the ICD. In clinical practice, it appears the risk is low. In one retrospective series, the risk of receiving an inappropriate shock secondary to EMI was <1% per year.

In devices programmed to atrial tracking modes (DDD or DDDR), EMI can lead to inappropriate rapid ventricular pacing and/or triggering of the CIED to “mode-switch” to an inhibited pacing mode (AAI, VVI, or DDI). EMI-generated signals are oversensed in the atrium (where the sense amplifier is usually more sensitive to electrograms of lower amplitude than in the ventricle), which may trigger pacing in the ventricle up to the upper tracking limit. Through the same oversensing mechanism, the device can potentially mode-switch to an inhibited mode caused by oversensing of EMI in the atrium that is misinterpreted as an atrial arrhythmia. In tachyarrhythmia systems, if the ventricular lead is oversensing EMI above the programmed detection rate, then inappropriate antitachycardial pacing or defibrillation can occur. Another mechanism by which EMI can trigger rapid pacing includes activation of an artificial rate-responsive minute ventilation sensor. The frequencies of some acoustomagnetic electronic article surveillance (EAS) systems are the same as those of the pulses used by some minute ventilation sensors in pacemakers that measure transthoracic impedance. Finally, other equipment that uses bioelectric impedance measurements can interfere with minute ventilation pacemakers. Such equipment includes cardiac monitors, apnea monitors, and respiration monitors.

In the presence of a very strong EMF, current induction and direct capture of the myocardium can be induced through the leads. The leads are composed of an outer and inner coil inside an outer and inner insulation layer. The inner coils are constructed in a 3D geometry to maximize conduction, flexibility, and durability. It has been reported that a 58-kHz acoustomagnetic EAS system is capable of generating 3.7 V in a pacemaker lead by inductance. Although very rare, rapid pacing has been reported during an MRI (which can produce 64-MHz RF power) at 100 beats/min.

Power-On Reset

When a strong EMI signal is encountered, the device may reset to factory settings, a phenomenon called power-on reset. Such an event is caused by interruption of communication between the microprocessor and the random access memory that contains the programmable settings. EMI that can activate power-on reset includes electrosurgery and external-internal defibrillation. Each device manufacturer has unique settings for when power-on reset is triggered, including switching from DDD to VVI mode. With some pacemakers, the power-on reset mode has a pacing mode and rate similar to those of the elective replacement indicator (ERI). Power-on reset caused by EMI can be distinguished from ERI because, when caused by EMI, impedance and assessment of battery voltage should be normal. With ICDs, power-on reset reverts to a “shock box” configuration where the pacing mode will revert to VVI and the antitachycardial therapies will provide the maximum number of ICD firings and shock energy. When power-on reset occurs, the CIED does not automatically revert to the previous programmed mode when the EMI signal is interrupted and must be reprogrammed to the desired settings.

Another consequence of EMI to an ICD is device deactivation and suspension of antitachycardial therapies (magnet reversion). Although most often the effects of EMI on CIEDs are temporary, rarely a “hard reset” or “cold reset” can occur if the EMI causes damage to the ICD microprocessor that is irreversible. If this occurs, pulse generator replacement is necessary.

CIED Components Designed for EMI

Zener Diode

Device manufacturers have created elements to protect the CIED from high-energy EMI by shunting energy away from the device, such as with a Zener diode. The Zener diode is a special kind of semiconductor that behaves as a short circuit by allowing current to flow in the opposite direction and away from the device if the detected voltage exceeds a certain value above the output voltage of the pulse generator.

Shielding

A Faraday cage, made of conductive material, protects the interior of the cage from static electric fields by distributing and thus canceling electrical charges around the exterior. The design of CIEDs has evolved to take advantage of the concept to include conductive or magnetic material to protect the pulse generator and lead circuitry from EMI. The CIED is often shielded by an airtight titanium or stainless steel case and must be capable of rejecting electrical fields of at least 2 MHz. A thin-film particle coating, called nanomagnetic insulation, is now being incorporated into CIED leads to decrease RF and magnetic field interference.

Bipolar Sensing

Bipolar sensing decreases the field of myocardium in which the device needs to properly sense and minimizes the sensing of external, noncardiac signals. In repeated studies of the effects of RF and magnetic field interference on pacemakers and ICDs, researchers found devices programmed with bipolar sensing (particularly dedicated tip-to-ring sensing) were less susceptible to EMI than when unipolar sensing was programmed. In one study of 23 patients with ICDs, electrocautery-induced EMI resulted in pacing inhibition in 22 of 23 cases and inappropriate VF detection in 17 of 23 cases with an integrated bipolar configuration of the coil to the tip. In contrast, dedicated bipolar configuration from the ring to the tip resulted in no pacing inhibition or inappropriate VF detection.

Bandpass Filter

Incorporated in CIED generators are bandpass filters that reject prespecified electromagnetic frequencies above or below a specified range. Although extremely effective at rejecting frequencies above and below the specified range, frequencies within the cardiac signal range (typically 0-60 Hz) cannot be reliably rejected. To address EMI frequencies within the allowable range, a noise reversion mode allows for temporary asynchronous pacing in the setting of spurious signals. Additionally, during the relative refractory period, there is a ventricular blanking period during which the device does not respond to electrical signals. During the ventricular refractory period, there is a noise-sampling window in which detected noise extends the blanking period and resets the noise-sampling window. Repeated resetting of the noise-sampling window triggers the pacemaker to pace asynchronously. Whereas asynchronous pacing is often benign when transient, prolonged episodes can lead to symptoms secondary to loss of atrioventricular synchrony and dyssynchronous right ventricular (RV) pacing. Given that ICDs are programmed to detect fast ventricular rates, implementing a noise reversion mode is more problematic, as failure to sense electromagnetic noise as VF could lead to a failure of lifesaving therapies.

Feedthrough Capacitor Filter

The feedthrough capacitor in a CIED is a conductor that carries signals through the casing. The feedthrough capacitor is able to hold a minimum amount of charge in order to contribute to unipolar sensing and pacing with the CIED generator while also acting as an antenna that carries EMI. The feedthrough capacitor filter shields and inhibits conduction of RF signals (most commonly from mobile telephones) from entering the CIED circuitry.

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