The cardiac pacing system consists of leads and an electrical pulse generator, which contains connectors, electronic circuits and a battery may be made from ferromagnetic components. The static magnetic field of most MRI systems used in clinical practice is 1.5 or 3.0 T and affects the ferromagnetic components of cardiac implantable devices in three different ways: 1) by exerting a force of attraction on the pacing system components; 2) producing a torque effect or 3) activating the reed switch on older devices.5,6

The force of attraction and torque effect are maximum at the center of the MRI tunnel and could theoretically result in the vibration or displacement of internal components in the pacing system or ICD. However, studies published so far have not shown any significant risks arising from this effect in conventional devices.6 Older pacemakers and ICDs also have a mechanical network, known as a reed switch. It is activated (closed) in the presence of magnetic fields, even of low intensity, resulting in a change to a fixed stimulation mode (DOO or VOO), with the deactivation of sensing functions on pacemakers and the deactivation of tachycardia detection on ICDs. However, the behavior of the reed switch in the presence of the MRI magnetic field is unpredictable. It can remain disabled when the torque effect is predominant or be activated, leading to the risk of asynchronous stimulation.7

During MRI, gradient coils create additional magnetic fields used to locate signals, which are repeatedly turned on and off very quickly. This can induce pulses on leads, with the associated risk of oversensing, undersensing or developing potentially fatal arrhythmias.4 Voltages induced by the magnetic field gradient can also result in a reconfiguration of the programming parameters of older conventional devices, with the activation of the emergency mode (typically ventricular demand pacing (VVI)).5

The most complex and significant interactions occurs between the device and the RF pulses that MRI machines use to produce images. The body absorbs part of this energy resulting in resistive heating (release of heat due to the passage of electrical current through a conductive material). Pacemakers leads and ICDs can act as antennas, receiving the RF signal, amplifying it locally and conducting energy to the lead tip and to the myocardium. Depending on orientation, the RF field can induce significant temperature increases at the lead tips (temperature increases of up to 20 °C).8 The antenna effect can lead to thermal tissue damage with edema or myocardial fibrosis, and also damage to the generator or battery circuit. The main risks associated with this effect are: irreversible increase in the pacing capture threshold, alteration in the sensing function or impedances, resetting of the device and battery depletion.4 Abandoned or fractured leads are especially prone to increases in tip temperature.8

In MR-conditional CIEDs the ferromagnetic content has been reduced to a minimum, in such a way that the effective force has decreased to lower than the gravitational pull and susceptibility artifact production is minimized.10 The use of non-ferromagnetic materials that maintain proper conductivity, durability and biocompatibility results in a reduction in the amount of ferromagnetic materials used. Generators have started to be hermetically sealed with stainless steel or titanium material6 and leads are being manufactured with non-ferromagnetic materials.4

Unlike reed switches on non-MR-conditional devices, which are unpredictable in the presence of a strong magnetic field, the Hall sensor is predictable and can be blocked when performing an MRI.4

The main objectives of modifications made to the leads on MR-conditionals CIEDs were: 1) to minimize heating at the lead tip, which could cause myocardial damage and changes in the pacing and/or sensing functions and 2) to reduce the antenna effect and the risk of electric currents induced by electromagnetic energy being conducted by the leads and causing myocardial stimulation and potentially fatal arrhythmias.11 Modifications were made to the lead geometry, including changing the winding pattern of the filaments that make up the internal coil and limiting the RFs that could be transmitted through the leads. The tips of the leads were also coated with a polarization resistant substance and an electromagnetic filter was implanted at the distal end of the lead to prevent it from heating during MRI.4,11

For an CIED to be considered MR-conditional, in addition to the generator, the leads must also be MR-conditional and from the same manufacturer. It is important to note that some leads not designed for this purpose were later tested and considered MR-conditional (retrospective approval).

CIED MR-conditionals are equipped with special filters that limit the transfer of certain frequencies and dissipate energy, reducing the risk of damage to the internal power source and circuits. Generator shielding has also been improved to minimize the transfer of electromagnetic energy.4

A very important feature of MR-conditional CIEDs is that there is a specific “MR mode”. The checklist and specific programming algorithm for MR-conditional CIEDs varies according to model and manufacturer. Usually, the MR mode must be selected before the MRI is performed and deactivated right after the study.4,6 Even with MR-conditional devices, it is necessary to observe a series of parameters so that the MRI can be performed safely. There must be a minimum period of 6 weeks between the implantation of the CIED and the performance of MRI, for adequate healing and fixation, thus reducing the torque effect.6 However, the use of some CIEDs is not recommended in higher MRI fields of 3.0 T. Some models also do not allow MRI when the isocenter is located in the chest (MRI of thoracic structures), a limitation that has been disappearing from more recent devices.6

In the literature, there are several prospective randomized studies that assessed the safety of MRI in patients with MR-conditional CIEDs. No clinically relevant events or complications were reported in any of these studies.12,13 It should be noted, however, that there is still little long-term information on the new MR-conditional CIED technology.

The first step in performing an MRI in patients with conditional-MR CIEDs is to check whether the device is in fact MR-conditional. This means identifying all the installed hardware, since it is only when the entire system meets the criteria that the device is considered MR-conditional.11 Therefore, an MR-conditional generator combined with non-MR-conditional components is non-MR-conditional. Examples of non-MR-conditional components are: abandoned leads; fractured leads; epicardial leads or active non-cardiac devices. Given the combination of components from different manufacturers in the same CIED, the system is also considered non-MR-conditional.11 The location of the device is also important, since a device located in the pectoral area is the only one that is considered MR-compatible in the case of transvenous systems.11 It should also be noted that when an MR-conditional CIED is programmed in any mode other than MR, this results in the device becoming non-MR-conditional.

Being labeled MR-conditional takes into account the whole CIED system. To obtain information on the compatibility of a specific CIED with MRI, the lists provided in the appendixes may be consulted (Appendixes 2 and 3). It is preferable to however consult the specific up-to-date databases provided by the device manufacturers as they are a more reliable alternative, given the constant introduction of new systems to the market and validation of compatibility of older leads with MRI (retrospective approval).11

If the device is MR-conditional, the following must always be checked to ensure that MRI is performed safely, especially:5

• 1

  Check that the entire system is MR-conditional (generator and leads)

• 2

  Confirm that the device was implanted more than 6 weeks ago

• 3

  Check that the leads are electrically intact

• 4

  Confirm implantation of the device in the chest region

• 5

  Check that the pacing capture thresholds are within the normal range

• 6

  Confirm battery longevity

• 7

  Exclude the presence of abandoned leads or additional components, such as adapters and lead extensions

• 8

  Exclude the presence of other cardiac implants.

After confirmation of the clinical indication, exclusion of contraindications and obtaining informed consent (which can be obtained on the day of the study), the next step is to schedule the study. The need for a prior visit to the electrophysiology laboratory or a pacemaker consultation, to program the device must be flagged. The scheduling of the procedure requires seamless coordination between the cardiology and radiology departments at the institution. Most centers with experience in performing MRI in patients with ICEDs choose to schedule the study on specific dates where they can guarantee minimum staffing requirements for device programming and supervision of the study.

Users of MR-conditional devices must be assessed by the CIED specialized team before and after performing the MRI.5 Before the MRI, the MR-conditional CIED need to be programmed to MR mode, available in the device software options, and includes the following:

• a)

  Asynchronous mode or deactivation of the pacing function

• b)

  Bipolar configuration of the leads

• c)

  Increased stimulation output

• d)

  Deactivation of additional stimulation functions such as rate-dependent stimulation, anti-tachycardia pacing and shocks.

Table 1 provides recommendations for programming CIEDs for MRIs according to the patient’s type of pacemaker or ICD and their baseline rhythm.

In the case of MR-conditional devices, the presence of an electrophysiologist or cardiologist with experience in device programming is not considered mandatory. In centers that do not have permanent support from an electrophysiologist, it is necessary to ensure support for the reprogramming of MR-conditional devices by properly trained professionals. However, the decision on the pacing mode is clinical and the responsibility of the cardiologist in charge of the study. Likewise, physical and human resources do not need to be available for the implantation of a provisional pacemaker. As in patients with non-MR-conditional CIEDs, the study must be performed under the supervision of a physician trained in advanced life support (ALS) and with continuous monitoring of the electrocardiogram, pulse oximetry and blood pressure (optional).11

The image acquisition protocol should be adapted according to the manufacturer's recommendations for each specific device. The main recommendation is to shorten the acquisition time and limit the sequence protocol to the clinical question posed, regardless of the device type.

Other general guidelines to consider when obtaining the images are:

• 1

  Not to exceed a slew rate of 200 T/m/s (check manufacturer's recommendations for each specific device)

• 2

  Place the patient in the supine position

• 3

  Preferably use a body antenna

• 4

  Limit the specific absorption rate (SAR) to 2 W/Kg.

Recommendations on specificities of the most suitable MRI mode and ideal timing of the schedule are summarized in Table 1. Unlike non-MR-conditional CIED, reassessment is not recommended 3 months after MRI.

Before requesting an MRI from a patient with a non-MR-conditional CIED, the risk-benefit ratio of the procedure should be considered. In terms of benefit, it is important that the study has a clear and relevant indication, and that the clinical question cannot be satisfactorily clarified using alternative imaging methods. With regard to risk, it is essential to identify the patient's baseline rhythm to assess the risk of developing arrhythmias and to determine the most appropriate device program for performing an MRI.5

Once a request for an MRI study has been formalized in a patient with a MR-conditional device, a clinical questionnaire (ideally associated with the MRI request) that addresses several safety issues that may contraindicate or limit the performance of the study should be completed.15 This questionnaire should screen for circumstances that contraindicate exposure to magnetic fields (absolute contraindications) or that have implications for the study methodology, including the presence of CIEDs. In this case, the patient's medical records should be consulted to obtain, the device model and year of implantation, as well as the existence of earlier devices, even if already removed. In the presence of a non-MR-conditional device, there are some circumstances that usually contraindicate MRIs:

• 1

  Year of implantation of the device prior to 1998 (pacemakers) or 2000 (ICD);

• 2

  Device implantation under 6 weeks ago;

• 3

  Presence of epicardial leads and fractured or abandoned leads.11,16

Performing MRI in patients with non-MR-conditional devices is not without risks, which means that a discussion between doctor and patient about the potential risks and clinical benefits of the procedure is required. To perform an MRI in patients with non-MR-conditional CIEDs, specific informed consent must be obtained, using a template that complies with the recommendations of the Portuguese General Health Directorate. An informed consent template is provided for this purpose as an attachment to this article (Appendix 4).

The scheduling of the MRI study in patients with non-MR-conditional CIEDs should follow the same principles as in patients with MR-conditional devices, particularly the need for coordination with the electrophysiology and pacing laboratory for device interrogation and programming.

The optimal programming of the non-MR-conditional device depends on the type of device and the patient's intrinsic heart rhythm. The first step is to check whether the patient is pacing-dependent or not.11

A) In pacing-dependent patients and in non-dependent but with symptomatic permanent bradycardia (intrinsic heart rhythm <50 bpm), the pacemaker must be programmed in asynchronous mode (DOO or FLIGHT or AOO) with a stimulation frequency significantly above the intrinsic heart rate of the patient. This is to avoid competition with the native rhythm and the development of pacemaker-mediated arrhythmias (Table 2). Pre- and post-MR programming should be carried out as close as possible to the study. In pacing-dependent patients with atrial and ventricular leads, the indicated programming mode is DOO (Fig. 3). The reprogramming of the original mode and verification of the normal functioning of the device must take place immediately after performing the MRI.5

B) In patients not-dependent on pacing with high or unstable intrinsic heart rates, there are two programming possibilities5:

B.1) Deactivation of the stimulation function by programming in ODO, OVO or OAO or, when none of the options is possible, as in the case of older pacemakers, reprogramming the stimulation energy below the threshold. This procedure requires the device to be reprogrammed immediately before and after the MRI.

B.2) Reprogramming in VVI/DDI mode, with the advantage that pre-MR programming can be performed at a different time and place than an MRI setting (since these modes prevent spontaneous episodes of symptomatic bradycardia). A maximum 24-h interval pre-MR is acceptable for programming. It is recommended that immediately after the MRI the original mode is restored and it is checked that the pacemaker works normally. The disadvantage of the latter is that, despite programming in VVI mode, asynchronous stimulation can occur during the study (the activation of the reed switch by the static magnetic field), with the potential risk of potentially fatal ventricular arrhythmias (Table 2).

C) In patients with cardiomyopathies with cardiac resynchronization therapy systems, disabling the pacing function or temporary programming in a non-biventricular pacing mode can have significant hemodynamic consequences.11 For these patients, it is justifiable to program the CRT device in an asynchronous pacing (FLIGHT or DOO), with a pacing frequency that avoids competition with the intrinsic rhythm.

Regardless of the type of device, there are other aspects of pre-MR programming that should be considered:

• 1

  Pacemaker/ICD output must be increased to 4 times the value of the pacing capture threshold or to 5.0 V/1.0 ms before MRI, to compensate for a possible increase in the pacing capture threshold due to lead heating induced by RF energy

• 2

  If possible, the sensing and stimulation polarity of the leads should be programmed to bipolar.

• 3

  Disable additional stimulation functions5,11:

  • a)

    Frequency response mode

  • b)

    Pacemaker-mediated anti-tachycardia pacing (PMT algorithms)

  • c)

    Left ventricular pacing triggered by sensing response to intrinsic right ventricular activity (triggered pacing) - CRT systems only

  • d)

    Anti-tachycardia therapies (including ATPs and shocks) - in ICDs

  • e)

    PVC-triggered pacing response (in response to ventricular extrasystoles sensing)

  • f)

    PAC-triggered pacing response (in response to atrial extrasystole sensing)

  • g)

    Atrial fibrillation therapies (such as rate smoothing and overdrive pacing)

  • h)

    Hysteresis

  • i)

    Magnet response (when option exists).

Performing MRI studies in patients with CIED requires appropriate supervision and monitoring during the study. In the particular case of non-MR-conditional devices, prior evaluation by an electrophysiologist or cardiologist with experience in device programming is essential. All MRI studies in non-MR-conditional devices should therefore be performed in centers with electrophysiology back-up support and with means immediately available if there is an urgent need for a temporary pacemaker implantation.

The following safety requirements must be ensured when performing MRI for patients with CIED:

• 1

  Immediate availability of drugs and equipment necessary for ALS, including an emergency car with defibrillator and capacity for external pacing

• 2

  Supervision of the study by a physician with recognized training in ALS

• 3

  Clinical surveillance through verbal contact (intercom) and, if possible, visual (camera).

• 4

  Electrocardiographic, blood pressure (optional) and pulse oximetry monitoring.

On the day of the study, after admission to the radiology department, the patient should then be referred to the Electrophysiology Unit or a Pacemaker Consultation for identification, interrogation and programming of the device. These procedures should take place in the MRI Unit, dependent upon the logistics and organization of the department. Only after a prior assessment by electrophysiology can the patient access the Security Zone 2 of the MRI Unit to prepare and plan for the study.