John Day, MD, FHRS, FACC
T. Jared Bunch, MD
Atrial Fibrillation Section Editor
It is with great pleasure that we mark the first anniversary of Innovations in Cardiac Rhythm Management in this issue. This past year has been filled with tremendous changes, with constant innovation in our field that has enabled the growth of our Journal and web platform. We are excited to continue to feature these advancements as a complimentary offering to you each month, and to share in our overall efforts to advance patient care.
Within this month's issue we would like to further discuss the manuscript you will find in the Innovative Techniques section by Drs. Zimmerman and Nazarian, focusing on the current role of magnetic resonance imaging prior to electrophysiology procedures. This is an extremely well written and timely article, as there is a growing interest in applying MRI technology into Electrophysiology programs. With this increasing excitement and discussion concerning the role of MRI in EP, we would like to take a moment and share our experience and perspectives on this subject- particularly related to the role of MRI in atrial fibrillation ablation procedures.
MRI provides excellent tissue imaging in multiple user-defined planes without exposing patients or operators to ionizing radiation. Gadolinium, an MRI contrast agent, distinguishes healthy versus fibrotic, scarred, or abnormal myocardial tissue. These features have made contrast MRI a very attractive modality in electrophysiology to define mechanisms of arrhythmia and/or risk as a means of patient selection for invasive procedures. In particular, there is tremendous enthusiasm for MRI technology in the management of atrial fibrillation, the most common clinical arrhythmia. Recent studies have shown that MRI has practical application in atrial fibrillation management. For example, defining extent of left atrial fibrosis (5–20%, >20–35%, >35%) separates patients that do well with radiofequency ablation1 and also those that are at higher risk of stroke (independent of CHADS2 score).2
The utility of MRI imaging in the management of atrial fibrillation is likely to increase with technological advancements. For example, 3T scanners offer enhanced patient-to-patient consistency with higher spatial resolution compared to older 1.5T scanners. 7 T scanners will likely provide exceptional spatial resolution ≤0.5 mm3 that will further allow accurate imaging of the thin left atrium and scar-mediated channels for micro and macro-reentry.
In addition to the baseline information provided by the MRI scan, there is much interest in the dynamic use of this imaging technique. Technological advances in MRI systems have shortened acquisition times, which may be coupled with rapid rendering of 3D volumes to allow for the possibility of real-time MRI to guide catheter- and substrate-based ablation procedures. Recently, real-time MRI was used to successfully ablate gaps in ablative lines in a porcine model.4
Based on the emerging data surrounding MRI, should we ask the same question as William Stanly Merwin, “Are we asleep with compasses in our hands?” In other words, with the prognostic information derived by baseline MRI scans, why is this test not standard of care in the preablation management of our atrial fibrillation patients? Furthermore, is real-time MRI ready for prime time in our clinical practices? The answers to these two questions are perhaps and probably not.
One of the greatest challenges with interpreting the use of MRI as a guide to patient selection for atrial fibrillation ablation- and to guide ablation strategy- is that the majority of outcomes data available are from one center.5 This limitation should not minimize the significance of their findings, but prompt additional research and collaboration. As additional centers validate their findings the general acceptance and use will be justified and thereby increase. As with all research, the probability of concluded findings ultimately being true depends on the study power, bias, the number of studies asking the same questions, and if there is a financial or other interest present.
Regarding real-time MRI there are several significant problems. One of the most significant issues is the lack of MRI compatible ablation catheters that have features similar to those proven to be the most efficient with traditional approaches.6 However, recently an MRI compatible ablation catheter was used to successfully create linear ablation lines along the cavotricuspid isthmus.7 Unlike manually manipulated catheters, these MRI compatible catheters are not irrigated-tip technologies that are broadly utilized with manual or magnetic navigation approaches to enhance tissue energy delivery and minimize endocardial tissue disruption.
In addition, there are various logistical challenges of catheter ablation using MRI scanners. First is the acquisition and placement of a cardiac MRI scanner in a catheterization laboratory. The acquisition of such a tool will also require extensive staff and operator training. Current electrophysiology laboratory staff members are not credentialed or trained in MRI sequence acquisition and processing and few electrophysiologists have advanced MRI training. Lastly, the spatial constraints of a MRI machine will impact anesthesiology and rapid operator access to a patient in the case of emergencies. None of the proposed challenges are insurmountable if this technology continues to advance to the point that efficacy and safety are favorably altered in ablation procedures.
In conclusion of our commentary, MRI technology offers a safe and accurate assessment of the heart. Recent advances have enhanced the potential utility of MRI in the practice of electrophysiology. Large and ongoing strides are currently being made to define its use in patient and treatment selection. Although real-time MRI is not yet ready for prime time, animal studies are compelling and suggest feasibility.
We hope you enjoy this issue. The support received thus far has been outstanding; we have appreciated your audience throughout our inaugural year and look forward to becoming an even larger part of your practice into the next. Please continue to submit the manuscripts that you would like to see featured within the Journal, and of course your continued comments and commentaries that help to inspire the entire clinical community.