Present and Future Perspectives in ElectrophysiologyImaging at HRS

This event was reported on by Jo Ann LeQuang. Article content was not reviewed by members of the faculty.
This event was reported on by Jo Ann LeQuang. Article content was not reviewed by members of the faculty.
Dr. Vivek Y. Reddy of Cardiac Arrhythmia Services at Massachusetts General Hospital talked about how they integrated new three-dimensional (3D) images from MR and CT scans into electroanatomical mapping for guiding procedures, in particular, for atrial fibrillation (AF) ablation. While 3D images were exciting, they could not yet be taken in isolation. He recognized that some approaches call for comprehensive left atrial (LA) mapping followed by a 3D reconstruction, but he found that approach extravagant. He reasoned that if you use a comprehensive LA map, why would one need a 3D image besides? Instead, he recommended using minimal electroanatomical mapping (primarily for real-time information) superimposed on a 3D reconstruction (primarily for detail). Adding electrical information onto the merged map/reconstruction creates the most useful image for guiding an EP study. Obtaining a 3D image from 2D datasets is easier than ever, but not without its nuances. MT and CT scans obtained in real-world clinical practice may sometimes be less than optimal for this purpose. For image-guided interventions, Reddy obtained data (such as from an MR angiogram or axial slices from a CT scan) and then segmented the areas of interest. The datasets from the 2D images were then aligned to an identical orientation and the 3D image was reconstructed during the registration process. However, there is wide variability among patients, and even variability in a single patient, with respect to such things as fluid volume. Biological factors, such as respiration, and catheter deformation can pose challenges in acquiring data. For example, Reddy recommended consistently obtaining data at the end of quiet respiration. In terms of how fast registration should be, Reddy advised using the aorta (or another landmark, such as the superior vena cava or pulmonary arteries) to identify proper initial registration. While his facility did not use 3D echo, he considered it a promising new addition to the image armamentarium, in particular because of its ability to approach real-time information. From the Allgemeines Krankenhaus St. Georg in Hamburg, Germany, Professor Karl-Heinz Kuck, MD, spoke about his clinic s experience using the Niobe ® remote navigation system from Stereotaxis. This system allows remotely controlled external magnets to generate a magnetic field that guides magnetic-tipped catheters through the vasculature. A physician using the Cardiodrive ® unit on a Navigant ® workstation can sit outside the EP lab and conduct the procedure. One advantage of the magnetically guided remote catheter system is that software options allow the operator to store precise positions in memory, aiding in gathering exact activation times or returning easily to a previous location. Reducing radiation exposure for electrophysiologists was another significant advantage of remotely guided procedures. Kuck s team found that both total procedure time as well as radiation exposure time decreased as operators gained familiarity with the system. In a study of 161 patients with AV nodal reentry tachycardia, total procedure time dropped from 142 minutes (for the first 20 patients) to 103 minutes (for the last 20 patients) with radiation exposure times decreasing from 8.8 minutes to 3.6 minutes, respectively. A prospective study sponsored by Siemens Medical Solutions evaluated radiation within the EP lab versus radiation experienced by those conducting the procedure from the control room (n = 43), and found remotely conducted procedures exposed clinicians to less radiation than in-lab procedures. Kuck recommended a randomized study to further validate these findings. Meanwhile, the Hamburg hospital was also looking at using the Niobe ® system in new ways. Kuck himself had earlier coined the term single-catheter approach for ablation procedures on simple substrates which could be diagnosed and ablated using only one catheter. The advantages of the single-catheter approach are primarily economic: it requires less hardware, saves time, and costs less. While clearly not all patients are candidates for the single-catheter approach to ablation, the hospital evaluated how well the remote system worked with this methodology. A total of 41 patients with documented or suspected paroxysmal supraventricular tachycardias indicated for catheter ablation were enrolled in the study. Of these patients, four could not be induced despite documented arrhythmias. Successful single-catheter approach ablations were performed in 34 of the 37 patients (92%). This demonstrates the potential viability of remote, magnetically guided, single-catheter AF ablation for a subset of AF ablation candidates. Dr. Warren Sonny Jackman spoke on the correlation of left atrial sites of high dominant frequency and location of left atrial ganglionated plexi. His work at the University of Oklahoma Health Sciences Center is exploring whether portions of the intrinsic nervous system of the heart (ganglionated plexi) might precipitate AF and the possible benefit of ablating such sites. So far, results have been promising if not yet fully understood. Ganglia in the heart are located in distinct regions under epicardial fat pads. These ganglionated plexi consist of efferent parasympathetic neurons (about 10%), efferent sympathetic neurons (about 10%), and connective neurons. The connectivity of these ganglia means that when stimulation occurs in one location, it may affect other ganglia. Using high-frequency stimulation (1-10 ms pulse width, 50 ms cycle length, one-half to 12 volts) from a bipolar, distal pair of electrodes on a mapping catheter and waiting for a vagal response (parasympathetic responses are more immediate than sympathetic ones), that is, a large increase (> 50%) in the R-R interval, it is possible to map areas of response. Jackman reports that high-frequency stimulation of the nerve centers at the epicardial fat pads consistently induces AF. The maps created by Jackman s team were tagged for areas with a vagal response to high-frequency stimulation as well areas which showed no vagal response. The tagged maps showed distinct areas of response that corresponded almost exactly to the epicardial fat pads and the ganglia. They were not very close to the pulmonary veins, Jackman observed. Next, his team contacted Dr. Wee Nademanee to work with some complex fractionated (continuous, low-amplitude) atrial electrograms (CFAEs) and found that they also contained positive areas which aligned with the epicardial fat pads and ganglionated plexi. As stimulation moved away from these sites, the response progressively diminished. In an instrumented dog, a short high-frequency atrial extra-stimulus (nicknamed a buzz) at a ganglionated plexus could produce atrial fibrillation, but when the stimulation occurred further away from the plexus, the response diminished. Jackman reported that in this canine model they timed a 48 ms effective refractory period (ERP) when the ganglionated plexus was stimulated, but a 112 ms ERP at other more distant locations. Stimulating the ganglionated plexus is fibrinogenic, Jackman concluded, saying that when stimulation in the dog occurred within 2 mm of a ganglionated plexus, a single extra-stimulus provoked AF 100% of the time. While many in the EP lab have observed atrial foci in patients, Jackman concluded that new visualization approaches have allowed clinicians to see that local triggered firing in the atrium might very well be able to induce and sustain AF. The role of ablation of such ganglionated plexi in the treatment of AF remains to be explored. Dr. T. Jared Bunch of the Mayo Clinic discussed the role of imaging in radiofrequency energy lesion delivery. His team uses ultrasound to guide catheter and lasso location during AF ablation. Ultrasound has worked well to help with anatomical assessment, in particular, the dimensions of the pulmonary veins. Advanced imaging techniques will help in locating the catheter more precisely during lesion delivery, resulting in more precise burn lines. Bunch cautions that the orientation of the catheter is very important. The tip of a large catheter is not necessarily predictive of tissue temperature, and temperature discrepancy can be very large if the catheter is positioned obliquely. One way to avoid overheating involves noninvasively monitoring the lesion formation by watching for microbubbles. Type I microbubbles are percolating events indicative of early tissue overheating, while Type II microbubbles are bursts of microbubbles that show tissue boiling. Even using cool-tip catheters, it is easy to see Type I and II microbubbles on intracardiac echocardiography (ICE) images. However, microbubbles lack specificity and should not be the sole arbiter of tissue temperature. Bunch reported that at the Mayo Clinic, the use of ICE imaging has decreased the morbidity associated with AF ablation by reducing the rate of pulmonary stenosis. Using high-intensity focused ultrasound balloon catheters, Bunch relied on Doppler for physiologic imaging to monitor leakage around the balloon. He has found leakage to be an independent predictor of unsuccessful outcomes for all types of balloon catheters. Doppler tissue analysis and advances in contrast echocardiography will help improve tissue delineation and advance our ability to visualize lesion dimensions, in particular, to see how they correlate to pathology. Using ultrasound and CARTO sensors, volumetric reconstruction can create 3D images, which can be further colored to show electrical activity (4D). These 3D (4D) images can help visualize the blood/tissue interface and to assess papillary muscles and other anatomical structures. Today, the role of ICE in AF ablation includes its accuracy in assessing left-atrial anatomy (particularly when compared to CT), its assistance in gaining transseptal access to the left atrium, and its utility in detecting microbubbles. Real-time 3D images could one day do all of that plus guide EP procedures. The program concluded with Dr. Nassir Marrouche presenting his thoughts on the role of 3D imaging and navigation systems, specifically in the ablation of AF. He discussed his experience at the Cleveland Clinic, which uses mainly intracardiac echocardiography for guiding AF ablation procedures. ICE has the advantage of real-time imaging, but only offers two dimensions. While AF ablation is an important and curative procedure, it is by no means an easy procedure. Depending on the lab, experience, operators and other factors, Marrouche reported that efficacy rates for AF ablation in the US are around 70-80%. Pulmonary vein (PV) stenosis and stroke are rare but significant complications. His interest in imaging techniques comes from his belief that AF ablation should be a safe and effective procedure available at hospitals all over the world. To accomplish this, not only are better operator skills required, but also better imaging techniques. For instance, he pointed out the role of 3D imaging in helping define PV stenoses. This is particularly important, since many patients can have severe occlusion and yet remain asymptomatic. Three-dimensional images can also help define pre-procedural scars in the LA, a difficult process that can impact outcomes. Finally, three-dimensional images show good sensitivity in identifying LA thrombus. At the Cleveland Clinic, many such patients have a pre-procedural CT scan to check for LA thrombus, but a 3D image may work even better. Siemens Medical Solutions developed special software upgrades for the Leonardo ® system at the Cleveland Clinic, which enabled some toolbars on the workstation monitor for novel segmentation protocols to improve imaging. This enhanced segmentation allowed CT and CARTO images to be generated faster and registered more precisely. While image integration has shown great promise, Marrouche pointed out some remaining challenges. The registration process in which datasets are aligned is still imperfect. Getting accurate datasets can be difficult, in particular because of cardiac movement and respiration. Monitoring lesion formation during ablation is of special interest because data indicate transmurality issues. MR images superimposed on fluoroscopy approach real-time accuracy, but are still not ideal. In particular, Marrouche looked forward to the widespread use of 3D imaging to better detect pre-procedural LA scars, define stenosis and check for LA thrombus. He could also foresee the use of real-time 3D images to facilitate ablation procedures so that they could be more widely available to the patients who need them. Despite our ever-improving abilities to visualize, map and navigate the heart in a closed chest, the true mechanisms of AF remain a mystery. Perhaps these new imaging techniques, in particular real-time 3D, will help us better understand what causes and what can cure atrial fibrillation.