New MRI Protocol for Patients With Implantable Devices May Eliminate Defibrillation-Threshold Testing

Reed Miller

February 24, 2010

February 24, 2010 (Southfield, Michigan) — A new MRI protocol for patients with implantable devices allows for the early detection and response to potential complications and might make routine post-MRI defibrillation-threshold testing (DFTT) and defibrillator safety-margin testing (DSMT) of patients with implantable cardioverter defibrillators (ICDs) unnecessary, according to the researchers [1].

Results of a study of the protocol in 38 patients, led by Dr Peter Thomas Burke (Providence Heart Institute, Southfield, MI), are published online January 29, 2010 in the Journal of Interventional Cardiac Electrophysiology.

Despite the risks of MRI disrupting the device's function, imaging of patients with these devices can be done safely, according to Burke and colleagues. "With careful and strict supervision under an electrophysiologist-guided team, we have developed a protocol that allows us to provide the lowest possible risk to [cardiovascular implantable electronic device] patients in need of an MRI," the authors explain.

Burke et al say the study is the first to incorporate the 2007 American Heart Association (AHA) recommendation that patients with an ICD undergo DFTT and DSMT after an MRI to ensure that the powerful magnetic field did not damage the device or interfere with its programming. Defibrillation threshold is defined as the lowest amount of energy capable of terminating an episode of induced ventricular fibrillation. But because the success of defibrillation is probabilistic, the exact defibrillation threshold cannot be established with certainty, so the defibrillator safety margin is tested. The defibrillator safety margin is the energy level capable of terminating two episodes of induced ventricular fibrillation and low enough to be at least 10 J less than the device's maximum output.

The authors argue that their protocol might obviate the need for DFTT and DSMT, which would be desirable because DFTT/DSMT requires anesthesia and is risky in some patients, such as those with intracavitary thrombus, persistent or permanent AF with unknown or inconsistent anticoagulation, suboptimal anticoagulation, or AF or left-ventricular thrombi or recent stroke, study coauthor Dr Christian Machado (Providence Heart Institute) told heartwire .

The authors also point out that the European Society of Cardiology guidelines do not recommend DFTT/DSMT post-MRI because their committee determined that the risks outweigh the potential benefits.

Protocol Calls for Careful Monitoring

The protocol recommends that an MRI of patients with an implantable device be overseen by an electrophysiologist, a cardiology fellow, a device manufacturer representative, and an electrophysiology nurse, all of whom are present before, during, and after the MRI scan. The staff should be prepared for worst-case scenarios, with on-site availability of transcutaneous and transvenous pacing and an external defibrillator, and should be prepared to transfer the patient to the electrophysiology laboratory to remove the device if it fails.

During MRI scans, the protocol team "vigilantly monitors" hemodynamics and cardiac parameters, including pulse oximetry and blood pressure, while communicating with the patient through an intercom. Immediately after the scan, the implantable device is reprogrammed to its original setting and interrogated for lead impedance, battery-life voltage, capture, and sensing thresholds. The interrogation is repeated three months later.

MRI Does No Damage

Burke et al's study enrolled 38 consecutive patients with cardiac rhythm management devices undergoing a total of 92 1.5-T MRI scans. A total of 10 ICD patients, four cardiac resynchronization therapy–defibrillator (CRT-D) patients, 13 pacemaker-dependent patients, and 11 non–pacemaker-dependent patients underwent scans.

The protocol specifies continuous monitoring during imaging plus device interrogation before and after the scan for lead impedance, battery life, pacing, and sensing thresholds. Pacemakers are to be reprogrammed to an asynchronous mode in pacemaker-dependent patients and to a nontracking/sensing mode in non–pacemaker-dependent patients. Plus, all tachyarrhythmia therapies are disabled during the scans.

All ICD patients underwent DFTT and DSMT post-MRI, in accordance with AHA guidelines. The follow-up testing showed that the MRIs caused no device circuitry damage, programming alterations, inappropriate shocks, or failures to pace. The MRI also did not cause any changes in sensing, pacing, or defibrillation thresholds in the implantable devices.

"To show that the defibrillator safety margin did not change at all in our patients is reassuring and, although it may not detect a minor change in defibrillation threshold, it reassures us that conversion should still occur in a device programmed with a 10-J margin," Machado said.

What's the Basis for DFTT/DSMT Post-MRI?

Given the risks associated with DFTT/DSMT, the study "raises the question of whether we should routinely perform DFTT/DSMT in ICD patients after a 1.5-T MRI," according to the authors. However, Burke et al write that until a larger randomized study of the routine use of DFTT/DSMT on post-MRI patients is conducted, there may be certain patients who warrant DFTT/DSMT post-MRI, such as those with previously inducible ventricular tachyarrhythmia (VT) at baseline, a history of sustained VT, and device-induced ventricular arrhythmias during scanning. But, "in the absence of these conditions, we feel that clinicians should refrain from routine DFTT/DSMT due to the inherent risks associated with such testing."

"With the proper protocol, we will continue to do MRI in our patients so as to avoid rejecting a life-saving diagnostic test on the basis of fear with no strong scientific evidence to back it up," Machado said, noting that patients undergoing MRI must be informed of the hypothetical risks.

Commenting on the study, Dr Bruce Wilkoff (Cleveland Clinic, OH) told heartwire that there is very little basis for the AHA recommendation for post-MRI DFTT. The AHA guidelines are based on an 18-patient study in which two devices failed, but in both cases, the problem could have been identified without the DFTT. "The risk with DFT measurement is low; however, there is both a logistical and monetary cost."

In an accompanying editorial [2], Dr J Rod Gimbel (Cardiology Associates of East Tennessee, Knoxville) commends Burke et al for demonstrating "that careful planning, monitoring, reprogramming, and supervision can facilitate a satisfactory outcome for patients with devices who need MRI."

The study also "highlights the overreaching of the AHA's scientific statement position that all ICD patients should undergo post-MRI DFT testing," Gimbel argues.

"In a time-pressured, cost-conscious, budget-limited healthcare environment, we should be mindful of the service burden our care pathways and expert recommendations commit us to," he says, noting that Medicare does not reimburse radiologists for MRIs for patients with implantable devices. "I find it difficult to believe that a radiology department and its physicians will cheerfully read dozens of scans per year and take the liability that comes with scanning device patients for free. This too, must be changed if we are to provide the much-needed service of providing MR scanning for device patients."

Gimbel argues that there is little evidentiary basis for the AHA guidelines recommending that all ICD patients undergo post-MRI DFTT. "One might ask whether or not manufacturers should issue a disclaimer in the physician’s product manual or a 'tech note' that reads, 'Should the ICD be exposed to MRI, we recommend post-MRI DFT testing, as we cannot guarantee appropriate function of the ICD otherwise.' More than two years has passed since the AHA has promulgated this controversial recommendation, and no manufacturer has endorsed it with such a caveat," he observes.

"No plausible explanation has been put forth that might explain mechanistically how the MRI-device interaction might lead to a rise in the patient's DFTs. In reality, what we’re concerned with is whether or not the ICD's functionality might somehow have been altered adversely by exposure to the powerful electromagnetic fields during MRI," he argues. "Given the risk and costs of DFT testing, I would recommend no DFT testing post-MRI unless an anomaly was detected at the mandatory thorough post-MRI interrogation."

He adds that the post-MRI interrogation of the ICD ought to include a test of the capacitor circuitry with a manual full charge, and DFT testing should be performed if there is an abnormality of the battery or charge circuitry.

Cardiologists Look Forward to MRI-Compatible Devices

Experts interviewed by heartwire pointed out that despite the good safety results in the study by Burke et al, patients and physicians shouldn't forget the real safety issues with MRI in patients with implantable devices.

Commenting on the Burke study, Dr Ariel Roguin (Technion-Israel Institute of Technology, Haifa) told heartwire , "We must exercise caution, however, given the wide range of available MRI systems, MRI scanning conditions, patient positions, pacemaker and ICD systems, and leads when extending these results to recommendations for routine use of MRI in these patients.

"The fact that several hundred patients with devices underwent uneventful MRI does not allow us to conclude that MRI in this population is indeed safe," he said. "All published studies were performed at centers with expertise in MRI and electrophysiology and were limited to patients with a true clinical need for MRI." He added that the diagnostic need for an MRI should be evaluated on a case-by-case basis and used only with patients with an implantable device in the absence of an alternative imaging modality and when the diagnostic benefit from MRI outweighs the presumed risks.

Wilkoff does not expect the FDA to lift the general contraindication of MRI for patients with implantable devices until one of the device manufacturers develops a MRI-compatible device that has been proven to be safe and effective.

He told heartwire , "Just because the physicians got away with scanning does not make it safe. Not only is there a potential for significant heating and capture threshold problems as well as arrhythmias, but without a strict care pathway, including hardware, software, and personnel, it will not be safe all the time."

The strict protocol in the Burke study required a team of physicians and nurses to be very vigilant and prepared for problems with the MRI. Wilkoff said that at most centers, "cardiologists and electrophysiologists are very willing to place risks and benefits in balance, [but] radiologists largely are not equipped to do procedures when there is the potential for risk. The support structure is not in place to handle this situation.

"[We are a long way from] the FDA approving this scenario for general use [and] also a long way from protecting physicians, hospitals, and MRI suites from the potential problems that will occur without a clear protocol and equipment," he said. "Without an FDA-approved device and care pathway, it is unlikely that MRI scanning will be a real option for most of these patients."

As reported by heartwire , Wilkoff led a randomized, multicenter clinical trial of Medtronic's MRI-compatible dual-chamber system (EnRhythm MRI SureScan and CapSureFix MRI leads) in 445 patients. In the trial, 211 underwent head and lumbar scans with 1.5-T MRI machines. There were no complications associated with the scans and no reported episodes of MRI-attributed sustained ventricular arrhythmias, asystole, or pacemaker malfunctions. The EnRhythm MRI is already approved for sale in Europe.

To make an MRI-compatible dual-chamber pacemaker system, Medtronic redesigned existing pacemaker components with a new geometry that requires the leads to be thicker and stiffer than comparable pacing systems. In a paper published online February 15, 2010 in Heart Rhythm, Dr Giovanni Forleo (University Hospital of Tor Vergata, Rome, Italy) and colleagues explain that thicker leads can cause more complications because they are harder to implant [3]. However, results from a 50-patient single-center study by Forleo's group show that the EnRhythm system performed about as well as a comparable non-MRI-specific device. The implantation success rate was 100%, and stimulation thresholds, sensing, and impedance were acceptable postimplant and at one year. There were no cases of high pacing thresholds or inadequate sensing and no complications or subsequent lead displacements.

Dr Claas Philip Nähle (University of Bonn, Germany), who has published research on implantable-device compatibility with MRI, explained that even DFTT/DSMT might overlook small myocardial scarring or edema of the tissue surrounding the lead tips caused by heating of the lead tip by the magnetic field, which could possibly result in the implantable device failing to detect ventricular fibrillation. "One cannot be 100% sure that subtle alterations of the tissue surrounding the lead tip can be excluded by all means, especially given the overall rather small study populations, even when adding all studies performed so far," he told heartwire . Therefore, he suggests that future studies of MRI-compatible implantable devices measure troponin I levels, which are a very sensitive marker of myocardial damage. "If one could demonstrate that, even in patients who had a increased troponin I level post-MRI--which then must be regarded as RF-induced due to heating of the lead tip--the ICD system will still detect VF correctly, that would be convincing."