Evaluating MRI-Compatible Pacemakers

Abstract and Introduction

Abstract

The worldwide use of both magnetic resonance imaging (MRI) and pacing devices has vastly increased in recent years and an important number of implanted patients likely will need an MRI over the course of the lifetime of their device. Although some studies have demonstrated that, given appropriate precautions, MRI can be safely performed in patients with selected implantable pacemakers, MRI in pacemaker patients is still contraindicated. Recently, new pacing systems have been specifically designed for safe use in the MRI environment and the first experience reported suggests that the technology is safe and may allow patients to undergo MRI. This review will describe the outstanding issues and controversies surrounding the safety of MRI imaging in pacemaker patients and the potential benefits of the new MRI-conditional technology. We will also discuss how to approach the decision whether or not an MRI-conditional system should be implanted and highlight key issues that warrant further studies.

Introduction

Magnetic resonance imaging (MRI) is the gold standard for soft tissue imaging and is widely used in a broad range of conditions such as cancer, musculoskeletal system, and neurologic disorders. MRI has several advantages over other diagnostic procedures: first MRI is a noninvasive imaging technique that does not involve exposure to ionizing radiation; it provides a noninvasive alternative to x-ray, angiography, and computed tomography for diagnosing problems of the heart and blood vessels; in addition, the contrast material used in MRI exams is less likely to produce an allergic reaction than the iodine-based contrast materials used for conventional x-rays and computed tomography scanning.

The safety of MRI in patients with implanted pacemakers (PPM) has been debated for years. The reason that manufacturers of both pacemakers and of MRI equipment consider the presence of a PPM an absolute contraindication for MRI is the fact that some fatalities have been reported.^[1,2] Additionally, other studies have demonstrated specific effects during MRI scanning of the heart, which could result in tissue damage.^[3]

A number of human and animal studies, including several devices but with limited sample sizes, have reported no adverse events as a result of MRI scans. Some of this preliminary information has been claimed (especially in the non-medical press) to represent MRI safety. This has only added to the confusion. As a result the American Heart Association (AHA) and the American College of Radiology (ACR) have issued guidelines stating that a careful risk/benefit must be established and that cardiac implantable devices remain a relative contraindication to MRI.^[1]

According to the AHA general recommendations, MRI examination of nonpacemaker-dependent patients is discouraged and should only be considered in cases in which there is a strong clinical indication and in which the benefits clearly outweigh the risks. MRI examination of pacemaker-dependent patients should not be performed unless there are highly compelling circumstances and when the benefits clearly outweigh the risks. Therefore, MRI in PPM patients are generally discouraged and in every single case the physician should assess the risk-benefit ratio and take the final decision.^[1]

The number of MRI scans performed worldwide has increased dramatically over the past few years, reaching the considerable number of more than 30 million scans per year. Similarly, more than 650,000 new pacing device implantations are performed annually worldwide. Moreover, the likelihood of MRI indication doubles after the age of 65 years^[4] and this is the same demographic group that is most likely to need a pacemaker.^[5] As the long-term survival of this population improves and utilization of cardiac implantable devices continues to grow, there is an increased likelihood that these patients will develop a medical condition for which an MRI may be useful or even critical, for optimal diagnosis and treatment. This likelihood is difficult to assess because the value of MRI relative to alternatives may vary.

It is still unknown what is the real number of PPM patients who will require an MRI during the lifetime of the device. Previous studies have shown that MRI is required in many PPM patients within 12 months of device placement and that, according to the guidelines,^[1] very probably the majority of them will likely be denied a MRI. Data derived from a questionnaire survey in Japanese patients suggest that 17% of PPM patients should require an MRI within 8 years of device placement.^[6] It is important to acknowledge that these patients enrolled more than 10 years ago in this survey do not represent the full spectrum of PPM patients who are going to be implanted in the current times in western countries. Other data reported in literature are derived from a Medtronic marketing survey and therefore should be interpreted cautiously.^[5] Given the inherent limitations of the aforementioned studies, it would be incorrect to extrapolate data regarding the real number of patients who will need MRI scans after pacemaker implantation. Prospective studies of patients implanted with MR-compatible systems are required to provide a definitive answer to this important topic. Indeed, no recent firm data exist, collected by an independent source, on how many people with implanted electronic devices are really being denied access to MRI examinations. Nevertheless, the longer expectation of a patient's life together with the wider and wider use of MRI should allow us to believe that a significant number of PPM patients with old devices and leads will need MRIs for multiple reasons during their lives.

The increased need for MRI in PPM patients has prompted the development of a specifically designed pacemaker and lead system tested for safe use in the MRI environment and in early 2011, the US Food and Drug Administration (FDA) approved the first cardiac pacemaker designed to be used safely but conditionally during MRI examinations.^[7]

Table I. Major MRI-Related PPM Malfunctions
Heating at the lead tip and at the lead tissue interface Force and torque on devices Image distortion Alteration of programming with potential damage to the pacemaker circuitry Rapid atrial pacing Pacing at multiples of the radiofrequency pulse and associated rapid ventricular pacing Reed switch malfunction Asynchronous pacing Inhibition of pacing output Induction of ventricular fibrillation Electrical reset Component damage Death

Table II. MRI-Conditional Devices Available
	MEDTRONIC*	ST. JUDE MEDICAL^†	BIOTRONIK^‡
Full body scan	yes	yes	no
MRI machine	Cylindrical bore magnet, clinical MRI systems with a static magnetic field of 1.5T
SAR limitation	≤ 2.0 W/kg	≤ 4.0 W/kg	≤ 2 W/kg
Max nr of scans	no	no	yes each scan ≤30 min maximum time for the system: 10 hours
Patient positioning	supine or dorsal	supine or dorsal	only dorsal
Available leads (fixation)	active^§	active	active and passive
Published clinical evidence on MRI environment	yes^¶	no	no
FDA approval	yes^#	no	no
CE mark	yes**	yes^††	yes^‡‡

MRI = magnetic resonance imaging; SAR = specific absorption rate; FDA = Food and Drug Administration.
*Medtronic SureScan technical manual.
^†St. Jude MRI procedure information.
^‡Biotronik ProMRI manual.
^§Passive leads available as of January 2012.
^¶Heart Rhythm 2010; 7:750–754[20]; Herzschrittmacherther Elektrophysiol 2011; 22:233–242²¹; Heart Rhythm 2011; 8:65–73.²²
^#Revo MRI and 5086 lead.
**Advisa MRI and 5086 lead.
^††Accent MRI and Tendril MRI lead.
^‡‡Evia Pro MRI and Safio and Solia leads.

Table III. Pacing System Integrity Checks
•Implant timing prior MRI:
Pacemaker and both leads implanted for more than 6 weeks (Biotronik, Medtronic)
Stable pacing capture threshold values (St. Jude)
•Pectoral implantation (all)
•No other active pacing, ICDs, or leads (all)
•No abandoned leads, lead extenders, or adaptors (all)
•Acceptable lead parameters before MRI:
Capture threshold of ≤ 2.0V at a pulse width of 0.4 ms (Medtronic, Biotronik)
Capture threshold ≤ 2.5 V at a pulse width of 0.5 ms (St. Jude)
Impedance of ≥ 200 and ≤ 1500 Ohms (all)
No diaphragmatic pacing at 5.0 V at a pulse width of 1.0 ms (Medtronic, St. Jude)
Battery-charging status at least 30% (Biotronik)
•MRI conditional programming modes (all)