Introduction
It is estimated that up to 75% of patients with implantable electronic cardiac devices (IEDs) will have a lifetime indication for magnetic resonance imaging (MRI). Due to the characteristics of the devices, they were historically excluded from the list of patients considered eligible for the test.
The DCEI consists of electrode cables and generator. Each electrode lead is a metallic multi-filament spiral connector that connects the generator to the cardiac muscle. The generator, in turn, is made up of a battery, circuits and a connector for the electrode cables. The function of the electrode leads is to conduct electrical impulses with minimal energy sufficient to initiate a cardiac electrical impulse (p wave or QRS). Another essential function of the electrode leads is to transmit electrical information acquired in the myocardium (intracavitary electrogram) to the generator, that is, to sense the patient's native electrical rhythm, avoiding unnecessary stimulation.
The generator is usually located in the right or left infraclavicular space (or less frequently in the lateral region of the chest or abdomen). It has the function of interpreting the stimuli coming from the lead-electrodes, generating electrical impulse through electric current between the myocardium and the generator (using the lead-electrodes as a conductor). This contains the battery needed for the impulse (generally lasting between 7 and 15 years) and programmable circuits that allow a minimum frequency, movement-dependent frequency elevation, integration between electrode cables located in different cardiac chambers, among others. In exceptional cases, such as in children, the generator can be positioned on the abdomen) and leads (generally transvenous, eventually epimyocardial). Systems without lead-electrodes (generator implanted directly inside the heart – leadless pacemaker) or leadless (subcutaneous ICD) are available for use in selected cases.
The magnetic field generated by MRI can be interpreted by the DCEI as an abnormal cardiac electrical signal (QRS or p wave) and create interference that causes one of the following behaviors:
- Trigger artificial cardiac stimuli with high rate
- Inhibition of cardiac stimuli
- Damage to electrodes, generator or heating system
- Modification of forced stimulation parameters (reset)
- Deflagration of inappropriate shocks (when dealing with an internal cardioverter defibrillator)
Despite the initial apprehension, moving the device and twisting the electrode cables was not evident due to the adherence of the tissues to the subcutaneous tissue.
System heating, leading to component damage and injury to the myocardium around the lead, has been proven for non-conditional leads. This heating can lead to an increase in the threshold necessary for myocardial stimulation and patient discomfort.
All DCEI undergoing MRI must be reprogrammed before and after the scan. Some more recent devices have the ability to detect the MRI magnetic field when activated for a predetermined period, which makes programming adequate when the patient is inside the device's room (zone 4). Most, however, should be reprogrammed to asynchronous mode or adequate mode as late as possible and return to the original programming after the end of the exam in the shortest period of time deemed appropriate by the physician responsible for monitoring the patient with CIED. This guideline is based on the potential risk of asynchronous stimulation, competing with the patient's own rhythm and being potentially arrhythmogenic in pacemakers. Cardiodefibrillator carriers should not leave a supervised environment without adequate protection from antitachycardia therapies.
Device Settings
pacemakers: device that has stimulation and sensitivity function. The pacemaker ensures the patient's minimum stimulated rate. Programming is described by letters (see table annex 1)
Resynchronizers: Also called pacemakers or multisite stimulators, they are devices that allow simultaneous stimulation of the left ventricle through an electrode positioned in an epicardial vein tributary of the venous coronary sinus. It may or may not be associated with a defibrillator function, and if so, it is called a multisite defibrillator. Patients with cardiac resynchronizers have significant structural heart disease and compromised left ventricular ejection fraction (generally less than 35%). The LV lead is positioned in a tributary epicardial vein of the venous coronary sinus. Occasionally, the LV lead can be implanted through an epimyocardial access.
Cardiodefibrillators (ICD): DCEI with stimulation function identical to the pacemaker. It also has a capacitor that allows the device to deliver high-energy shocks. Their function is to control ventricular tachycardia or ventricular fibrillation and are generally implanted in patients with different degrees of ventricular dysfunction or at greater risk of cardiorespiratory arrest.
event monitors: Devices between 3 and 6 cm, placed in the anterior thorax, subcutaneously. Its function is prolonged monitoring of cardiac arrhythmias. All currently available event monitors are MR compatible. It is recommended, at the physician's discretion, to evaluate the data before the examination, due to the risk of losing the information collected so far or even suppressing them due to the acquisition of magnetic field artifacts.
conditional devices: These are DCEIs for which exposure to the magnetic field poses no risk to the patient. These are DCEI that contain only electrodes described by the manufacturer as conditional, connected to generators described by the manufacturer as conditional, and that do not meet the exclusion criteria. Lead wires and generators must be from the same manufacturer. The use of electrodes and generators from different manufacturers may not be guaranteed in case of damage to the system.
non-conditional devices: Devices that have not been extensively tested and/or are not warranted by the manufacturer against potential damage caused by the MR environment.
Minimum security parameters
The minimum safety parameters for ICD patients in the MRI room are:
- Heart rate monitoring (preferably ECG) and saturation in real time throughout the exam;
- Presence of a doctor and team capable of treating cardiorespiratory arrest (CPA) in the radiology section (immediately outside Zone 4, also known as the MRI room);
- Availability of material for CPA care immediately outside zone 4 in accordance with current ACLS guidelines;
- It is suggested to carry out these procedures in a hospital or clinic that has parameters 1, 2 and 3 and the ability to safely remove the patient to the intensive care unit in case of need;
- Have institutional standard operating protocol easily accessible to all laboratory members.
Specific guidelines for different devices:
conditional stimulators
Conditional implantable devices arrived in Brazil in 2012. Manufacturers developed electrode leads and generators that allow MRI scans to be performed initially with an exclusion zone (avoiding the thorax, cervical region and upper abdomen), and later authorized for the entire body. Other models have developed with similar technologies.
The pacemaker wearer's wallet contains the model of the patient's leads and generator with MRI compatibility information. The complete list of electrode cables and conditional generators is available on the ABEC website (https://abecdeca.org.br/medico)
The DCEI need, however, to be reprogrammed prior to exposure to the magnetic field. The purpose of this reprogramming is to make the DCEI indifferent to the magnetic field (asynchronous mode) and perform other modifications, such as increasing the stimulation energy. The reprogramming should also evaluate command thresholds and remaining battery power, to assess the safety of patient exposure to the magnetic field. Ideally, the battery should not have less than 30% of charge and the command thresholds should not be raised prior to the test, although they are not absolute contraindications to the procedure.
The parameters of 1.5T, gradient slew rate < or = at 200T/m/s and maximum SAR < or = at 2W/kg allow safety in all conditional devices regardless of the region of interest. Some devices already allow 3T, and this can be checked if it is in the interest of the patient and the physician responsible for acquiring the images.
conditional cardioverter defibrillators
Conditional cardioverter defibrillators also demonstrate proven safety for patient exposure to the magnetic environment. And they also need reprogramming prior to the exam and return to the original parameters after the end of the exam. In addition to the indifference to the magnetic field (asynchronous mode), the reprogramming aims to inhibit its inappropriate detection and its interpretation as ventricular tachycardia or ventricular arrhythmia.
This removes the device's inherent tachycardia protection during resonance-specific programming. Inhibiting tachycardia detection prevents the patient from being shocked during the scan.
Thus, in this context, if the patient spontaneously presents with sustained ventricular tachycardia, the treatment should be identical to that of patients who do not have an ICD, according to current ACLS guidelines.
non-conditional devices
There is extensive literature on case series of non-conditional CIED patients who underwent MRI without adverse events. Several studies are underway to validate the routine use of MRI in these devices.
Whenever MRI is an essential diagnostic method that cannot be replaced or is required on an emergency basis, the examination should not be avoided due to the presence of CIED. Ideally, it should be carried out in an environment that meets the minimum conditions suggested in this document. Most devices subjected to 1.5 T NMR tolerated the procedure, as well as scans lasting less than 40 minutes. Despite the lack of randomized studies, we suggest maintenance of safety parameters with short-term exams and field equal to or less than 1.5T.
The reprogramming of the non-conditional DCEI functionality to the “resonance compatible” parameters can be performed on any DCEI, even though the manufacturer does not guarantee the safety of the components in case of damage, such as: reduction or suppression of cardiac stimulation, change sudden change, heating of circuits and electrode cables, as well as failure to capture during the procedure or late.
In case of need to perform MRI in non-conditional ICD, we suggest the presence of a qualified physician to evaluate the functioning and reprogramming of the device in the radiology environment, with reassessment of the functionality of the ICD at the end of the procedure, by the same and before discharge from the supervised environment . The need for temporary cardiac pacing should also be anticipated in case of device dysfunction.
Exclusion criteria:
Exclusion criteria should be considered and discussed with the attending physician. In the case of patients with an absolute need for the test in the presence of exclusion criteria, the risk of potentially fatal events should be discussed with the attending physician and patient. DCEI should be considered non-conditional and treated as such (see description above under non-conditional device category):
- Presence of abandoned or non-conditional electrode leads
- Presence of epicardial electrode leads
- Implant less than 6 weeks ago
- Non-thoracic implants;
- Children;
Attachments:
- Table of programming modes
- Checklist for performing NMR
Annex 1
| STIMULATED CHAMBER | FELT CHAMBER | RESPONSE TO THE FELT EVENT | ADAPTIVE FREQUENCY |
| O = NONE | O = NONE | O = NONE | R= FREQUENCY RESPONSE ON |
| A = ATTRIUM | A = ATTRIUM | I= DISABLED | |
| V = VENTRICLE | V = VENTRICLE | T = DEFLAGRATED | |
| D = ATRIUM AND VENTRICLE | D = ATRIUM AND VENTRICLE | D = BOTH |
Annex 2
Check list:
Before the exam:
- Authorization from the stimulator physician informing – necessary check by the team responsible for the MRI:
- If the device is conditional
- If you have a function auto detect or similar scheduled
- If the programming will be done by a member of your team by a member of the radiology service immediately before the MRI and after the end of the exam.
- If the patient is able to perform the procedure without the need for reprogramming
- (If you meet institutional protocols)
- Check if there is a professional available on site for rescheduling, when necessary (Unconditional or unscheduled)
- Check if there is a team capable of assisting PCR
- Check if there is material available for PCR care
During the exam:
- Check for effective rhythm and saturation monitoring
After the exam:
- Check patient's vital signs
- Check if the device has been reprogrammed with the approval of the doctor responsible for the procedure
- Confirm with the responsible physician the safety of the patient's discharge.
Suggested additional literature:
- Indik JH, Gimbel JR, Abe H, 2017 et al HRS expert consensus statement on magnetic resonance imaging and radiation exposure in patients with cardiovascular implantable electronic devices Heart Rhythm, Vol 14, No 7, July 2017
- https://www.mayoclinic.org/medical-professionals/clinical-updates/cardiovascular/new-protocols-allow-mri-selected-pacemaker-patients Last visited 8/19/2018
- Mattei E, Gentili G, Censi F, et al. Impact of capped and uncapped abandoned leads on the heating of an MR-conditional pacemaker implant. Magn Reson Med 2015;73(1):390–400. 78.
- Boilson BA, Wokhlu A, Acker NG, et al. Safety of magnetic resonance imaging in patients with permanent pacemakers: a collaborative clinical approach. J Interv Card Electrophysiol 2012;33(1):59–67
- Burke PT, Ghanbari H, Alexander PB, et al. A protocol for patients with cardiovascular implantable devices and magnetic resonance imaging (MRI): should defibrillation threshold testing be performed post-(MRI). J Interv Card Electrophysiol 2010;28(1):59–66
Guideline:
Coordinators: Bruno Valdigem, Hilton Leão Filho, André D´Avila
Editorial Committee:
ABEC
Antonio Vitor Moraes junior
Bruno Pereira Valdigem
Cecilia Monteiro Boya Barcelos
Celso Salgado de Melo
Wilson Lopes Pereira
CBR
Hilton Muniz Leão Filho
Marco Antonio Rocha Mello
Cyro Antônio Fonseca Junior
Fernando Eduardo Nunes Mariz
Marcelo Rodrigues de Abreu
Patricia Prando Cardia
Paulo Roberto Vieira de Andrade
Simone Kodlulovich Renha
SOBRAC
Alexander Dal Forno
Andre Luiz Buchele D'Avila
Ricardo Alkmin Teixeira
Veridiana Silva de Andrade