MR Conditional ICD Technology: Update

Magnetic resonance imaging (MRI) is a diagnostic technique used to obtain high-quality images of the human body. In the last few years, MRI has become the imaging modality of choice across a broad range of indications. Compared with other techniques such as computer tomography (CT) or x-rays, MRI is characterized by having much better contrast between different soft tissues of the body. Furthermore, MRI has no associated health risks compared to x-ray or other ionizing radiation. For these reasons, MRI has now become the gold standard for imaging the brain, spinal cord, musculoskeletal system, head and neck, complex congenital heart malformations, and other tissue structures. More recently, MR imaging has also been applied successfully to evaluate myocardial structure, wall motion, and perfusion.

The safety of MRI in patients with implanted pacemakers (PPM) or defibrillators (ICD) has been in question for a long time. The reason that manufacturers of PPM/ICDs and of MRI equipment consider the presence of a device an absolute contraindication for MRI is that some fatalities have been reported or published.^1,2

The most recent data have shown that MRI procedures have increased dramatically from 7.7 million to nearly 22 million in the last decade. Worldwide, approximately 60 million scans are performed each year. Similarly, more than 650,000 new pacing device implantations are performed annually. Moreover, the likelihood of MRI indication doubles after the age of 65 years, and this is the same demographic group that is most likely to need a PPM or ICD.^3,4 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 experience a medical situation for which MRI may be useful or even critical for optimal diagnosis and treatment.

As the average life of population and indications for ICDs expand, the number of patients worldwide who need an ICD also grows at a rate of about 10–15% annually. The soaring rates of patients needing ICDs as well as the increasing need for MR scans result in an estimated 50–75% probability that an ICD patient would be indicated for an MR scan over the lifetime of his or her implanted device. This means that worldwide, every five minutes, a patient is denied an MRI because he or she has an implanted cardiac device.^3-5 

Furthermore, in the last decade new indications for non-pharmacological therapy of heart failure (HF) have been arising as well, and HF patients increasingly receive devices such as  a cardiac resynchronization therapy (CRT) device or ICD. The total number of CRT implants (CRT-P and CRT-D) markedly increased from 46 million in 2004 to 99 million in 2008 (115%).

Pacemakers and ICDs contain metal with variable ferromagnetic qualities, complex electrical systems, and at least one lead implanted in the myocardium; therefore, permanent cardiac devices have historically represented a contraindication to MR imaging. Although clear guidelines are still not available, MRI in patients wearing devices is generally discouraged, and in every single case the physician should assess the risk-benefit ratio and make the final decision.¹ Although some studies suggest that with appropriate precautions, MRI scans can be conducted safely in patients with pacemakers or ICDs, it is apparent that MRI scans have the potential to cause serious deleterious effects in this patient population.

A powerful magnet generates a magnetic field roughly 50,000 times stronger than the natural background magnetism from the earth. Various types of clinical MRI systems currently use the superconductive magnet which utilizes 0.5–3.0 T.

Potentially deleterious effects have been identified, including inhibition of pacing, asynchronous pacing, lead tip heating, loss of capture, and alteration of programming with potential damage to the pacemaker circuitry. In fact, MRI systems put the patient through a static magnetic field which could drive to a malfunctioning of the device. These potentially harmful effects have been mainly identified in older pacemaker and lead technology.^6-8

During the last decade, a number of limited sample-size studies have asserted that MRI scans (at 0.5 T and 1.5 T) can be safely performed in patients with implanted pacemakers in carefully selected clinical circumstances when appropriate strategies are used.^9-16

Nevertheless, considerable controversy remains over safety issues. MRI is generally contraindicated in patients with pacemakers, and the American Heart Association’s guidelines recommend consideration of MRI only in exceptional circumstances, excluding the vast majority of pacemaker patients who might benefit from MRI examination.¹⁷

Recently, new pacing systems have been specifically designed for safe use in the MRI environment. In order to make the system “MR Conditional,” components have been redesigned and new features, especially with regard to lead design and geometry, characterize the system. Some recently published studies provided strong evidence that this system can be safely implanted without any significantly increased risk of complications and results in satisfactory short-term performance comparable to traditional pacing systems.^18,19 Moreover, other studies have been performed to assess whether these new pacemakers could safely be used in the MRI environment. In these series, there were no complications reported among subjects who underwent scans, and there were no MRI-attributed sustained ventricular arrhythmias, asystole, or pacemaker malfunctions.²⁰ This new pacing technology, in the current market available exclusively for PPM, has therefore proved to be safe and may allow patients with PPM to undergo MRI.

In general, exposure to an MRI system or to an MRI procedure has similar effects on an ICD as those previously described for a cardiac pacemaker, since some of the basic components are comparable. However, there are several unique aspects of ICDs that have impacted the possible safe performance of MRI procedures in patients with these devices.

One of the main reasons that it may be more complicated and potentially dangerous to perform an MRI in a patient with an ICD is related to the features of the ICD leads. All modern ICD systems can also function as pacemakers, being able to conduct an electrical pacing impulse from the generator to the tip of the leads and the heart, but the ICD lead is more complex than the pacemaker one, and it is as critical to the function of the ICD system as the ICD itself. These insulated wires have both pacing and defibrillation functions; high-voltage leads are required to withstand multiple chronic stresses: mechanical, thermal, electrical, chemical, and oxidative. As a result, they tend to be thicker than the average pacemaker lead, with an externally visualized “shocking coil” along the body. These shocking coils are metal spirals that allow for the internal defibrillation of the heart with a high-energy current delivery from the generator when a life-threatening tachycardia is detected. The ICD leads may be single-coil leads or dual-coil leads, depending on the presence of a second coil at the superior vena cava level. The length of the distal (right ventricular) coil is limited by the size of the RV. The proximal coil (superior vena cava), if present, is generally longer. Defibrillation efficiency is in part related to the total surface area of the coils. However, these coils, so critical for the effectiveness of the high-voltage ICD leads, are also the main concern regarding the safety of scanning patients wearing ICDs.

Another important issue that may explain the risk of scanning an ICD patient concerns the typology of patients wearing ICDs, who are usually sicker than those wearing a simple PPM, often due to a worse cardiac performance and severe co-morbidities. Such a difference makes a potential complication related to the MRI scan much more dangerous in an ICD patient compared to a PPM patient.

Several recent studies have described ICD patients examined by MR imaging without serious problems; however, potential problems remain even for modern cardiac pacemakers and ICDs, so extreme caution must be exercised when scanning patients with these devices.

MR Conditional Technology at San Filippo Neri Hospital

After receiving CE approval on November 18th, BIOTRONIK SE & Co. KG announced first implantations of its new Lumax 740 implantable cardiac defibrillators at a number of hospitals in Italy, Germany, and Austria (it is not approved in the US). San Filippo Neri Hospital was among the first sites in the world to use this MR conditional ICD technology. With the Lumax 740 ICD, it will be possible for patients with an ICD to undergo MRI scans and safely receive the diagnostic procedures they need.

The BIOTRONIK ProMRI^® tachycardia series marks the first time an ICD portfolio has been designed, tested, and approved to make MR scans possible. As a result, physicians will have the unique opportunity to select an optimal and modern device, offering their patients an advanced therapy along with access to MRI scanning. The new MR conditional ICDs are combined with BIOTRONIK Home Monitoring^®.

Summary

The ability to use MR scans is extremely important and will become even more important in the future. Until recently, patients with an ICD had been excluded from undergoing MR scans despite the fact that these scans are critical for the diagnosis and therapy of patients with serious medical conditions. BIOTRONIK’s ProMRI^® innovation in the Lumax 740 series represents an important breakthrough for patients needing ICDs.

References

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