EXPLANATION


Patients with cardiac implantable electronic devices (CIEDs) must undergo safety screening for their CIED prior to magnetic resonance imaging (MRI). The screening process includes an evaluation of the device and the predicted performance in the MRI environment. The CIED is then classified as one of the following: a) “MR safe” - no hazard in any MR environments, b) “MR unsafe” - an object known to pose hazards in all MR environments, and c) “MR conditional”- an object in a specified MRI environment within specified conditions that does not pose a known hazard. CIEDs that do not meet the criteria to be MR conditional have a designation of a “MR nonconditional system”. This includes MR conditional generators that are combined with nonconditional leads.


Although there are currently no CIEDs with MR safe designation, several modern CIED systems are designated MR conditional by the Food and Drug administration (FDA). The development of MR Conditional CIED devices has involved modifications of the generator, leads, and/or MRI scan field conditions. Field conditions include the body sector for imaging, MRI operating criteria, lead and generator combination, and mode of programming, all of which can vary among manufacturers and specific devices.


Although the physics of MR imaging is extremely complex, it is helpful to understand how the scan is generated. To produce an MR image requires the application of a static magnetic field to align protons with or against the magnetic field, a source of radiofrequency (RF) pulsed waves to excite the nuclear spin of protons which causes an energy transition, and magnetic field gradients to localize the signal in space that is emitted after the RF signal is turned off. Pulse sequences are generated by applying a series of RF pulses to the anatomy of interest. The parameters of the pulse sequence(s) can be varied to generate contrast between tissues, based on the relaxation properties of hydrogen nuclei. These various electromagnetic fields, either alone or in combination, can interact with some ferromagnetic materials or damage sensitive electronic components of the device. Adverse effects of the MRI electromagnetic field on CIEDs include:


1. Torque and movement due to ferromagnetic materials. Generally, CIED generator movement is unlikely due to subcutaneous tissue encasing, especially six weeks and longer after initial CIED placement. Newer generators and leads are made with titanium and its alloys with limited ferromagnetic materials, making movement an unlikely event.


2. Electrical current induction by pulsed RF and gradient magnetic field. Low frequency electrial currents can be induced in leads within the field. In pacemakers (PMs), this can result in inhibition, leading to asynchronous pacing or overdrive pacing due to perceived cardiac electrical activity. In implanted cardiac defibrillators (ICDs), the consequences include inappropriate inhibition or induction of anti-tachycardia pacing or inappropriate shock delivery when a rapidly changing gradient electromagnetic field is interpreted as native cardiac activity. Newer CIEDs contain improved band pass filters, programming, and shielding to reduce background noise.


3. Tissue damage from heat generation. Radiofrequency fields can induce heating in the nonconditional components of the device, leading to tissue damage from heat generation and device malfunction. Lead sensing and capture thresholds may also become altered due to tissue damage near lead electrodes (termed functional ablation).


The decision to perform MRI on a patient with a CIED requires a detailed assessment of the risks versus benefits of the scan. The determination of MR conditional status requires knowledge of the device specifics, presence of abandoned or fractured leads, location of device components, and manufacturers of the device components. Although some current CIED systems have been FDA-approved for the 3 Tesla magnet, most of the MRI safety literature is based on scans performed in “normal operating mode using the 1.5 Tesla magnet.


Due to the variety of CIED systems and patient characteristics, an individualized approach is necessary to ensure safety and provide the optimal conditions to obtain the best diagnostic information from the scan. Recommendations by the Heart Rhythm Society (2017) for the management of patients with MR conditional CIED systems who undergo MRI include the following:


1) MR conditional devices should adhere to the product instructions, which includes programming the appropriate “MR mode” and scanning within the prerequisites specified for the device. The MR mode features include prescan system integrity checks, asynchronous pacing or nonstimulation modes (nonsensing modes), disabling of tachycardia detection, increased output during the scan, and restoration of prescan states and values.


2) A rigourous standardized institutional protocol specific for MR imaging in these patients should always be applied. Such protocols should include the benefits assessment of MRI versus alternative diagnostic methods, pre-MRI and post-MRI device evaluation, and appropriate MR conditional device programming specific to the CIED and patient characteristics. A protocol checklist should be used, preferably one that would be traceable in the electronic system.


3) Skilled personnel to perform advanced cardiac life support, arrhythmia recognition, defibrillation, and transcutaneous pacing are recommended to be in attendance with the patient during the time period when the device is reprogrammed until returned to baseline. The institutional protocol should specify whether this is nursing or medical staff.


4) MR-safe ECG and pulse oximetry monitoring is recommended during the time of MR-mode programming and continued until baseline or other clinically appropriate CIED settings are restored. Pulse oximetry is particularly useful, as it is less prone to electromagnetic interference when compared to the ECG.


5) Resuscitative efforts involving the use of a defibrillator/monitor, device programming system, or any other MR- unsafe equipment should be performed after moving the patient outside of Zone 4 and into a nearby designated magnetically safe area.


6) Personnel with the skills to program the CIED need to be available as defined by an institutional protocol. Such personnel are generally not required to be present during the scan.


7) Based on the risk and benefit to the patient, it is reasonable to perform an MRI scan earlier than the exempt period for conditionality of the system.


Based on these recommendations, the patient in the scenario should be managed as follows: (1) the ICD should be programmed to “MRI-mode” according to device instructions; (2) trained staff skilled to perform advanced cardiac life support, arrhythmia recognition, defibrillation, and transcutaneous pacing should be in attendance with the patient during the time period the device is reprogrammed, until returned to baseline; (3) vital signs monitoring in the form of ECG and pulse oximetry should be used until the patient’s device settings are restored back to baseline.


The Heart Rhythm Society’s expert consensus statement makes a special mention of CIEDs in adults and children with congenital heart disease. Often patients with congenital heart defects have CIEDs placed at a very young age. Infants and small children usually have epicardial leads and abdominal generators. These patients typically require multiple CIED revisions over the course of their lifetime and may have abandoned leads in situ. These combinations of current and abandoned leads and generators often render the system to be MR nonconditional. However, a MR nonconditional device does not preclude a MRI but does necessitate additional precautions. These include obtaining written consent, checking the device before and after the MRI, and the presence of personnel with the skills to recognize arrhythmias and abililty to provide cardiopulmonary resuscitation and transcutaneous pacing until device settings are restored.


The European Association of Cardiovascular Imaging and the Heart Rhythm Society emphasize that the presence of a CIED should not deny a patient the benefit of MRI. Instead, the decision to perform an MRI should be made thoughtfully and collaboratively, with robust institutional protocols in place to maximize the probability of safe outcomes.


REFERENCES


Indik JH, Gimbel JR, Abe H, et al. 2017 HRS expert consensus statement on magnetic resonance imaging and radiation exposure in patients with cardiovascular implantable electronic devices. Heart Rhythm. 2017;14(7):e97-e153. doi:10.1016/j.hrthm.2017.04.025


Stankovic I, Voigt JU, Burri H, et al. Imaging in patients with cardiovascular implantable electronic devices: part 2-imaging after device implantation. A clinical consensus statement of the European Association of Cardiovascular Imaging (EACVI) and the European Heart Rhythm Association (EHRA) of the ESC. Eur Heart J Cardiovasc Imaging. 2023;25(1):e33-e54. doi:10.1093/ehjci/jead273