April 8th, 2014
Assessing the MRI Safety of New Pacemakers and ICDs
An estimated 50% to 75% of patients with a pacemaker or implantable cardioverter-defibrillator (ICD) have a clinical indication for MRI during the device’s lifetime. Device safety during MRI scanning is therefore an important issue to explore.
The Evidence Thus Far
Early concerns about device dislodgement and overheating during MRI have abated, thanks to evidence that the maximal force and torque acting on modern pacemakers/ICDs at 1.5T are unlikely to dislodge leads anchored in the myocardium and that small measured rises in temperature pose minimal risk for thermal injury. Several reported adverse electrical events — including diminished battery voltage, increased capture threshold, decreased pacing-capture threshold on repetitive MRI, decreased sensing amplitude and pace impedances, power-on-reset, and oversensing of electromagnetic interference — do represent real potential risks. However, many of the electrical changes detected on device interrogation immediately after MRI scanning have proven to be transient and clinically inconsequential. In general, ICDs’ tachycardia-detection capabilities and therapy can be temporarily suspended by placing a magnet over the device to change the built-in reed-switch function; therefore, unpredictable reed-switch behavior in an MRI environment could cause inappropriate shocks. However, a large clinical trial has failed to reveal any adverse events from the inappropriate shock during 1.5T MRI.
Ongoing Efforts to Ensure Safety
Although no device-related major adverse clinical events have been documented during MRI scanning, device makers have been working on features to further improve safety. We now have MRI conditional devices, which are those shown to pose no known hazards in a specified MRI environment under specified conditions of use. In 2011, Medtronic released an MRI conditional pacing system, Revo MRI SureScan, the first FDA-approved pacemaker system available in the U.S.; approval of the second-generation Advisa MRI SureScan followed in 2013.
Clinical trials have tested the safety and efficacy of the Revo and Advisa MRI pacemaker generators with CapSureFix MRI conditional leads, in a dual-chamber pacemaker design. The evidence shows no reports of MRI-related complications; no disturbances of pacemaker function during or after MRI; no ventricular arrhythmia inductions; and minimal differences in pre- and post-MRI pacing threshold, impedance, and sensing (similar to results in a control [no-MRI] group). CapSureFix MRI conditional leads have an encouraging safety and efficacy profile, with no lead displacement and no cases of high pacing thresholds or inadequate sensing. Compared with conventional leads, the CapSureFix MRI leads show no clinically relevant difference in pacing threshold, sensing, or impedance.
MRI conditional pacemaker design addresses the theoretical concerns of MRI and device interaction. The limited amount of ferromagnetic material minimizes the magnetic-field interaction and, therefore, reduces the risk for device dislocation. The reed switch is replaced by a Hall sensor and, consequently, mitigates program switching from unpredictable reed-switch behavior in an MRI environment. Internal circuits have also been improved to prevent internal power-supply interruption and to reduce the potential for cardiac stimulation, thereby diminishing the risk for electrical reset and tachyarrhythmia. To address concerns about thermal injury, leads are being modified to reduce RF lead tip heating from transmitted RF power.
Making MRI Conditional Systems Practical
Implanting an MRI conditional pacing system has been made more practical by giving the pulse generator a size of 12.7 cm3 and a weight of 21.5 g (Revo MRI) or 22 g (Advisa MRI), similar to the average size and weight of a conventional pulse generator. CapSureFix MRI conditional leads have greater diameter and stiffness, but there is no difference in procedural and fluoroscopy time, and the implant success rate for an MRI conditional pacing system is 100%. Ideally, MRI conditional devices should be implanted in patients with an anticipated future indication for MRI. However, a patient-selection strategy needs to be developed, in part because an MRI conditional device is $1000 to $3000 more expensive than a standard device.
What About Image Quality?
Susceptibility artifacts from MRI conditional devices continue to affect image quality on cardiac magnetic resonance (CMR). Most image distortions and artifacts are within 10 cm to 15 cm around the device generator — and are pronounced in CMR, especially on steady-state and inversion-recovery sequences. The artifacts are substantially greater for ICD than for pacemaker generators and for left-sided than for right-sided systems. Using gradient-echo and wide-band sequences helps to improve quality on cine and viability images, respectively. With these imaging-sequence modifications, sufficient image quality can be obtained in most cases.
Translating What We Know Into Practice
MRI conditional devices are considered safe in specified MRI environments and conditions of use when standard safety protocols are followed. However, experience with conditional devices is still limited in CMR and in 3T MRI, and we have no postmarketing data on long-term adverse events. Given the current evidence, what would you do if a patient with a conventional device is scheduled for CMR? Should patients with conventional devices switch to an MRI conditional device? Would it be cost-effective to implant an MRI conditional device in all patients with appropriate clinical indications?
Please share your answers to these questions here.