MRI Safety for Implantable Cardiac Devices

MRI Overview

How many of your patients are excluded from the benefits of MRI because of their implantable cardiac device?

Most likely several since cardiac devices have not been considered safe in the MRI environment.

The Dilemma

As MRI use increases, so too does the number of patients benefiting from pacemakers and implantable cardioverter defibrillators (ICDs). This convergence results in an escalating number of patients who may be denied MR diagnostic imaging.

Convergence of Trends:
Many Patients Potentially Denied MR Scans

Detail - Pacemaker and ICD Implants - Prevalence of Implantable Pulse Generators (IPGs) and Implantable Cardioverter Defibrillators (ICDs) implanted in the U.S. 1993-2004.*

Detail - MRI Procedures - Total US MRI procedure volume, hospital and non-hospital sites, 1993-2003.†

The Impact

• MRI scans are potentially unsafe and off-limits for more than 2 million Americans because they have an implanted cardiac device¹
• In 2004 alone, about 200,000 implantable cardiac device patients were denied the benefits of an MR scan ²
• 40% of Europeans and 50-75% of Americans with implantable devices will be indicated for MR scanning over the lifetime of their device ²
• Every 3 minutes in the US, and every 6 minutes in Europe, a patient is denied an MRI because of the presence of a pacemaker or defibrillator ^2-5

Recently, the FDA approved the Revo MRI SureScan Pacing System designed for use in the MRI environment under specified conditions.

* Copyrighted information provided with permission from IMV Medical Information Division. 2004 MRI Benchmark Report.
† Cumulative implants minus mortality. Medtronic data on file.

Medtronic Viewpoint

Medtronic does not support the off-label use of its products and believes that the safe use of implantable cardiac devices in the MR environment cannot be guaranteed unless devices are designed to be safe from the ground up. And such devices must be backed by rigorous scientific testing as well as review from appropriate regulatory agencies.

Medtronic recognizes that under certain circumstances, physicians will determine that the medical benefit of conducting an MRI on a patient with an implanted cardiac device outweighs the associated risks. But Medtronic cautions the scientific community about drawing conclusions from limited studies that have reported no adverse events. MRI experts Kanal and Gimbel clearly state: …failing to identify an adverse event is not equivalent to demonstrating safety¹… in small studies with products not intended for the MR environment.

This is especially true since the hazards associated with scanning devices have a low frequency of occurrence. Furthermore, research needs to account for numerous variables, including inconsistency among MRI manufacturers, lead type and length, anatomical position of implant, and scan parameters. All these variables can change with the introduction of new cardiac devices and leads, and the continuous evolution of MRI technology.

The Revo MRI^™ SureScan^® Pacing System is the first MR-conditional FDA approved pacing system.

Medtronic has an ongoing commitment to design future device systems to be safe in MRI scanners. Physicians and their patients should accept nothing less than this commitment to safety and quality.

For other industry and government viewpoints, refer to the April 2005 issue of PACE.

Sources and Hazards of MRI Electromagnetic Interference

MRI has different sources of electromagnetic interference:

• Static magnetic field
• Gradient magnetic fields
• Pulsed radio frequency (RF) field
• Combined field effects

Electromagnetic interference (EMI) produced during the MRI has the potential to induce several hazardous effects on implantable cardiac devices:

Effects of Static Magnetic Field

• Force and torque on the IPG and leads resulting in damage to the implanted system, damage to surrounding tissue, lead dislodgment, or patient discomfort during MRI exam
• Magnetization of IPG components resulting in corruption of normal device functionality

Effects of Gradient Magnetic Fields

The gradient (changing) magnetic fields may induce voltages/currents on the implanted lead(s) causing the following hazards:

• Continuous capture or VT/VF
• Reduced energy of pacing pulses resulting in loss of pacing capture
• Over-sensing or under-sensing resulting in accelerated pacing or pacing inhibition
• IPG reset resulting in mode change to demand pacing and potential pacing inhibition
• Damage to normal IPG functionality or mechanical integrity
• Erroneous diagnostic data

Effects of Radio Frequency Waves

The RF waves may induce energy on implanted leads causing the following hazards:

• Lead electrode heating resulting in tissue damage and loss of sensing and pacing capture
• Continuous capture or VT/VF
• Reduced energy of pacing pulses resulting in loss of pacing capture
• Over-sensing or under-sensing resulting in accelerated pacing or pacing inhibition
• IPG reset resulting in mode change to demand pacing and potential pacing inhibition
• Damage to normal IPG functionality or mechanical integrity
• Erroneous diagnostic data

Effects of Combined EM Fields (Static Magnetic, Gradient Magnetic, and/or RF)

• Vibration of the IPG resulting in damage to normal IPG functionality or mechanical integrity
• Vibration of the IPG and leads resulting in tissue damage, lead dislodgment, or patient discomfort during MRI exam
• IPG case heating resulting in tissue heating and damage and patient discomfort during MRI exam

Clinical Controversy

Despite theoretical and actual risks of MR scanning to patients with cardiac devices, the safety debate continues.

• Limited studies have documented safe procedures and "no adverse events."
• Patient safety cannot be guaranteed by extrapolating findings from limited studies that documented no adverse events. Safety is proven only by more rigorous scientific testing and regulatory review.
• Clinicians must carefully balance the potential benefits and risks to patients.
• No implantable cardiac device is currently labeled for use in the MR environment.

“...failing to identify an adverse event is not equivalent to demonstrating safety-especially when only a limited number of patients are studied...”¹

J. Rod Gimbel, MD, FACC
Emanuel Kanal, MD, FACR

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