Early Experience With the MR-Conditional ICD

Background

Magnetic resonance imaging (MRI) is considered the gold standard for many conditions such as stroke, neck pain, and back pain.¹ However, patients with an implantable cardioverter defibrillator (ICD) were previously contraindicated by the FDA from receiving MRI scans because of potential interactions between the MRI and device function, which might result in risk to patients.² There have been several concerns regarding the risks of MRI among ICD patients, including the induction of ventricular arrhythmias during the MRI procedure, adverse changes in pacing or sensing as a consequence of MRI, and abnormal defibrillation function. These MRI restrictions have resulted in a critical unmet need for ICD patients. It has been shown that 44% of non-ICD patients undergo an MRI within 3 days of stroke or transient ischemic attack vs 1% of patients with a traditional ICD. Twenty-two percent of non-ICD patients undergo an MRI within 30 days of back pain diagnosis vs 0.7% of patients with a traditional ICD. Seventeen percent of non-ICD patients undergo an MRI within 30 days of joint pain diagnosis vs 0.1% of patients with a traditional ICD.²

In September 2015, the FDA approved the first ICD system for use with MRI scans in the U.S.³ The FDA approval of the Evera MRI^™ SureScan^® ICD System (Medtronic plc) was based on safety and efficacy data from the Evera MRI Clinical Trial, a multicenter, prospective, controlled clinical study.⁴ The Evera MRI ICD system includes hardware and software design enhancements from previous generation devices that allow it to safely undergo full-body MRIs and still maintain the same longevity, shock reduction, and physiological size and shape of the original Evera ICD. In this article, we describe the first MRI ICD implant performed at the Plaza Medical Center in September 2015, as well as provide a brief review of the technology.

Case description

The patient is a 52-year-old male with severe left ventricular dysfunction with an ejection fraction (EF) of 25%-30%. The patient had non-ischemic dilated cardiomyopathy that was diagnosed 11 months ago when he was admitted with transient ischemic attack (TIA). An MRI brain scan revealed that the patient had small areas of lacunar infarcts. He was started on optimal medical therapy, and reevaluation showed his EF remained at less than 30%. We discussed the option of ICD implantation, specifically an MR-conditional ICD, given his TIA history.

The ICD implantation of the Evera XT was performed using the standard technique. A 9 French (Fr) sheath was used to place the DF4 ICD lead in the apical septum of the right ventricle. A 7 Fr sheath was used to place the right atrial lead (model 5076-45). Defibrillation threshold testing was undertaken at implant using the T-shock method of induction with appropriate sensing of ventricular fibrillation (VF) (at 1.2 mV) and successful conversion to sinus rhythm (SR) with an internal shock. The postoperative course for the patient was uneventful, and he did well upon discharge. 

About the Clinical Trial

The Evera MRI Clinical Trial was the first randomized study of an MR-conditional ICD system to allow for full-body 1.5 Tesla (the field strength of the magnet) MRI scans. This study included MRI scans of the chest region as well as full-body scans.

The study enrolled 275 patients (including pacing-dependent patients) at 42 centers around the world. Patients were randomized 2:1 to either undergo a series of MRI sequences (MRI group) or a 1-hour waiting period without MRI (control group). An additional subset of patients in the MRI group had VF induced following their MRI in order to characterize arrhythmia sensing, detection, and therapy delivery.⁶ 

The primary safety objective was to demonstrate >90% freedom from any one of the following MRI-related events: 1) sustained ventricular tachyarrhythmia (VT/ VF) during SureScan mode; 2) complication within 30 days and related to the MRI; or 3) loss of capture within 30 days of MRI.4 Primary efficacy endpoints included ventricular pacing capture threshold (VPCT) and ventricular sensing amplitude changes from the pre-MRI/waiting period visit to 1-month post-MRI waiting period. VPCT, measured at 0.4 ms in both the MRI and control groups, was defined as a failure if it increased by >0.5 V. Sensing amplitude was considered a failure if a >50% decrease in R-wave amplitude (or >25% decrease if<3 mV) was noted.⁴ 

The study met the safety endpoint, demonstrating 100 percent freedom from MRI-related complications (P<0.0001) in the MRI group, as well as both primary efficacy endpoints.⁶ No patients who underwent an MRI experienced a significant increase in the pacing capture threshold; these results were comparable to those in the control group who did not undergo an MRI.⁶ Additionally, 99.3 percent of MRI patients and 98.8 percent of control patients experienced either an increase or a ≤50 percent decrease in R-wave amplitude. The MRI scan also did not have any impact on the Evera MRI device’s sensing, detection, or therapy delivery in patients that experienced VT/VF post-MRI.⁶

SureScan Mode

Use of the SureScan mode during MRI facilitates automatic programming of the device.⁴ Asynchronous pacing for pacing-dependent patients is also available. SureScan disables tachyarrhythmia sensing and defibrillation therapies with a 6-hour timeout to prevent long-term inactivation of ICD therapy. The SureScan settings are stored in nonvolatile device memory to maintain these settings during an electrical reset.4 Additional changes were added to the device to make the ICD safer for MRI: ferromagnetic material was reduced, a hall sensor replaced the mechanical reed switch, filters to prevent gradient and radiofrequency energy coupling were added, and battery circuitry protection was added.4 We have a protocol for activating the SureScan mode when patients with a pacemaker or ICD require an MRI that is based on published protocols.⁵ 

Disclosure: The author has no conflicts of interest to report regarding the content herein. 

References 

1. ISO/TS 10974:2012: Assessment of the safety of magnetic resonance imaging for patients with an active implantable medical device. International Organization for Standardization. Published 2012-05-01. Available online at https://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=46462. Accessed October 19, 2015.
2. Nazarian S, Reynolds M, Ryan M, et al. Estimating the likelihood of MRI in patients after ICD implantation: a 10-year prediction model. J Am Coll Cardiol. 2015;65(10_S):doi:10.1016/S0735-1097(15)61090-7.  
3. Medtronic First to Receive FDA Approval for MR-Conditional Implantable Cardioverter Defibrillator System. Medtronic. Published September 14, 2015. Available online at https://newsroom.medtronic.com/phoenix.zhtml?c=251324&p=irol-newsArticle&ID=2087595. Accessed October 19, 2015.
4. Gold MR, Torsten S, Schwitter J, et al. Full-body MRI in patients with an implantable cardioverter-defibrillator: primary results of a randomized Evera MRI study. J Am Coll Cardiol. 2015;65(24):2581-2588.
5. MRI Scanning Checklist. Medtronic. Available online at https://www.medtronic.com/mrisurescan-us/pdf/MRI-Scanning-Checklist.pdf. Accessed October 19, 2015.
6. Medtronic Evera MRI ICD Clinical Study Meets Safety and Efficacy Endpoints. Medtronic. Published May 14, 2015. Available online at https://newsroom.medtronic.com/phoenix.zhtml?c=251324&p=irol-newsArticle&ID=2048255. Accessed October 19, 2015.