Cardiovascular Implantable Electronic Devices and MRI: A Challenge Met

The aging of the population guarantees that we will see more patients with implantable cardiovascular devices such as pacemakers and ICDs. However, it is estimated that more than half of those with cardiovascular implantable electronic devices (CIEDs) will have an indication at some point for MRI testing.¹ The Heart Rhythm Society, along with the American Heart Association, American College of Cardiology, and many U.S. and international organizations spanning the disciplines of cardiology, radiology, and oncology, recently released guidelines regarding CIED patient testing with MR and CT, as well as radiation treatments for oncology disorders.² The following is a review of the latest recommendations for patients with CIEDs who need to undergo MRI; also included is a discussion of the difference between MR conditional and MR nonconditional systems.

These are pacemaker or ICD systems that have been modified to allow the patient who has them implanted to undergo an MRI. The MRI environment involves a static magnetic field strength, a spatial gradient, a time-varying magnetic field, radiofrequency fields, and a specific absorption rate. The leads and generators of the past may not withstand these forces without being damaged. An MR conditional system has been manufactured in a different manner, and the generator and leads have been FDA approved.  

 

The MR conditional designation means that the device has undergone testing to assess the behavior of the system during an MRI. The types of testing include measurement of magnetically induced force and torque, current induction, RF heating, and modeling of potential electromagnetic interference from the MRI environment.

Any system that has not received the FDA designation of conditional is considered MR nonconditional. The entire system must be considered. Examples of nonconditional would include MR conditional generators with nonconditional leads or patient systems with abandoned leads.

Nuclear magnetic resonance spectroscopy is based on properties of specific atomic nuclei absorbing and emitting RF energy when placed in an external magnetic field.² Hydrogen nuclei are used most often to create anatomical images in clinical MRI. Since hydrogen is found in the body’s water and fat, MRI can map the location of water and fat in the body.

 

The MRI scanner generates pulsed sequences that cause the nuclei to spin. The testing involves different pulsed sequences that create contrasts between tissues. Testing also creates static magnetic fields, gradient magnetic fields, and RF — all of which can interact with metal objects and damage them.

 

There is a risk of injury in the MRI environment. As a result, defined physical zones within the MRI suite were created. Zone 4 is the scanner room itself, where there is the highest risk of flying objects to patients and staff; therefore, no metal objects should be brought in without proper screening. Zone 3 is located just outside the scanner room, and contains the area for patient holding and the control room. In these spaces, there is the potential for injury from MR scanner static and time-varying magnetic fields, so only properly trained staff are allowed. Zone 2 is used for patient reception and the interview/screening areas. Zone 1 is the general public area, where there are no restrictions (Table 1).

MRI testing can cause many potential device problems. For example, the gradient magnetic field can cause current in the conductive wires, which may lead to capture and induction of arrhythmias. RF fields may cause thermal damage to the device and surrounding tissues, possibly altering sensing and capture thresholds of the device. Magnetic fields may affect the reed switch activity of the device, and lead to asynchronous pacing as well as inhibition of tachycardia therapies. Battery status could be affected. There could also be reversion to a backup demand mode, inhibiting pacing and tachyarrhythmia therapies. The device may be changed from bipolar to unipolar, altering pacing therapies, and causing oversensing and pacing inhibition or inappropriate shocks. 

 

Additionally, generators can interfere with MRI images by producing artifacts. Special techniques can be employed during testing to reduce such artifacts.

All of the major manufacturers have produced pacemakers, ICDs, and leads that have been approved by the FDA as MR conditional (Tables 2 and 3). New MRI scanning techniques that are tailored to protect CIEDs have also been developed. MR conditional devices have been tested with specific leads, and only by using the leads and generator together does a system qualify as MR conditional.

 

Modifications in lead design have included structural changes to minimize heating at the tip. In addition, the antenna effect has been reduced so the resonant frequency does not cause rhythm induction.

 

Generators can be impacted by both magnetic fields and RF energy. Approaches to modification have included reduction of the ferromagnetic content, replacement of the reed switch with sensors that are more predictable with MRI, shielding with special filters to reduce risk of circuitry and internal power supply damage, and development of specific MRI programming.

 

Changes that have occurred in MRI protocols also reduce complication risks. These include the use of a lower static magnetic field system, alteration of slew rates, and limiting RF power.

The new guidelines for MRI and devices give specific guidance for screening potential MRI patients and conducting the MRI scan. It is recommended that institutional protocols for scanning be developed, and that there be collaboration between cardiologists with CIED expertise and MR imaging specialists. 

 

A patient assessment checklist is considered helpful to make sure that important details are not overlooked. This should include manufacturer and model numbers for generators and leads, including any abandoned leads. If the system is deemed conditional, the status of pacing and tachycardia mode activation are assessed. 

 

During testing, the patient is to be monitored with an MR-safe heart rate and rhythm monitor as well as pulse oximetry throughout the test, and ACLS-trained staff is to be in attendance in Zone 3 throughout imaging and recovery. The external defibrillator and programmer are kept in Zone 3 at all times. The manufacturer-recommended MRI programming is to be used prior to and after the test. If the system had been implanted less than 6 weeks prior to the scan, risk to benefit analysis should be assessed. For a nonconditional system, evaluation of system plus urgency/best test for condition evaluations should be done. 

 

The activity flow for MR conditional devices is: follow the recommended programming protocol; assess sensing, capture threshold, impedance, and battery voltage pre- and post-MRI. Follow-up recommended post-procedure management is to: monitor until stable; restore original programming; and schedule follow-up appointment. 

 

If resuscitation is required, the patient should be moved outside of Zone 4 to conduct. Personnel with the skill to program the device should be in attendance at all times.

 

In the case of the MR nonconditional system, it is considered reasonable to have MR imaging if there are no fractured, epicardial, or abandoned leads, and it is the best test for the condition. There should be an institutional protocol in place plus designated MR and CIED physicians in attendance. In addition to the pre- and post-system evaluations, it is recommended for these studies that a physician with the ability to direct CIED programming be available on the imaging facility premises. There are specific programming recommendations for the MR nonconditional device that differ from the recommendations for the MR conditional device.

The intent to implant an MR conditional system should be determined prior to the procedure. It is important to match an MR conditional generator and lead from the same manufacturer. Sometimes, a nonconditional lead that is already in place will have subsequently passed testing to become MR conditional. In this case, an MR conditional generator replacement could be considered.

 

Cost is also a factor in making the implant decisions. Conditional devices have been engineered differently to conform to MRI compatibility, and therefore, cost more. Device cost must be balanced against cost of future testing needs. In addition, nonconditional scanning is costlier due to a more extensive protocol before and after scanning, as well as the required presence of a physician during the test.

The future of CIED development will likely see improvements in programming. Automatic modulation of the device for the test environment is on the horizon. Also on the wish list is compatibility across manufacturers. The ability to program while in magnet mode would allow more therapy flexibility. The issue of abandoned hardware must be solved. Finally, as in all new medical endeavors, the creation of more registries to pool information and make it available to practitioners would expand the knowledge for all.