Early Experience with a High Resolution Mapping System

Vancouver General Hospital (VGH), a tertiary care hospital in Vancouver, British Columbia, is the second largest hospital in Canada. The cardiac electrophysiology program at VGH was founded in 2012. Together with our partner program at St. Paul’s Hospital, we perform over 1100 ablation procedures and 1500 device implant procedures annually. In addition, the VGH program provides support for the pediatric electrophysiology program based out of the Children’s Heart Center at B.C. Children’s Hospital.

In Spring 2019, VGH was the first program in Western Canada to acquire the Rhythmia HDx electroanatomic mapping system (Boston Scientific). This system was purchased to complement the existing CARTO 3 (Biosense Webster, Inc., a Johnson & Johnson company) and EnSite Velocity/Precision (Abbott) systems currently in use in our program.

Our interest in the Rhythmia HDx mapping system was based on its ability to rapidly acquire ultra-high resolution electroanatomic and activation maps with the 64-pole basket mapping catheter (IntellaMap Orion; Boston Scientific), the highly reliable automatic annotation algorithm, and the ability of this system to differentiate low amplitude signals from areas of dense scar. Specifically, the incorporation of small unidirectional electrodes (0.4 mm² with 2.5 mm spacing) into the IntellaMap Orion catheter has been shown to significantly lower background noise to a level below the perceptible cut-off of other contemporary mapping systems. As such, we were interested in the ability of the Rhythmia HDx mapping system to detect and visualize low-voltage critical isthmuses, with a specific focus on how this system may allow us to target previously unmappable tachycardia circuits.

Our first cases were chosen to familiarize ourselves with the Rhythmia HDx system and catheters. For these early cases, we chose ablation of typical right atrial flutter. While we have performed cavotricuspid isthmus (CTI) ablation with Velocity guidance in the past, it is our current practice to perform two catheter procedures, using a decapolar catheter in the coronary sinus and an irrigated ablation catheter, without 3D mapping. We felt that this arrhythmic substrate was the best means to gain a better understanding of the key features of this system as well as its mapping capabilities.

Case Series

Our first cases were de novo ablation for typical flutter. For these cases, we performed electroanatomic activation mapping of the atrial flutter circuit, with the goal of comparing the rapid mapping capabilities and automatic annotation algorithm between two catheters. For the first case, we used a two-catheter setup similar to our usual workflow, excepting that we performed activation mapping using the IntellaNav MIFI Open-Irrigated (Boston Scientific) ablation catheter. For the second case, we again performed activation mapping but instead employed the IntellaMap Orion. Owing to its multielectrode design, the Orion was able to obtain double the electrogram density despite processing 2/3 of the beats. Representative maps obtained with these two catheters are presented in Figure 1.

Following these initial cases, we evaluated the more advanced LUMIPOINT algorithms during cases of recurrent atrial flutter or atrial fibrillation. This allowed us an opportunity to familiarize ourselves with the SKYLINE, ACTIVATION SEARCH, and COMPLEX ACTIVATION features.

For these cases, electroanatomic voltage and activation maps were created using the IntellaMap Orion during proximal coronary sinus pacing (flutter) or distal coronary sinus pacing (fibrillation). In addition to voltage and activation time annotation, each electrogram was processed by the LUMIPOINT algorithms. These algorithms evaluate all activations above baseline noise within each electrogram and across the whole mapping window, irrespective of local activation time. In doing so, the LUMIPOINT algorithm is able to automatically highlight areas with electrograms having specific characteristics (e.g., “complex activation” — double or fractionated potentials) or timings (eg, deflections within a user-defined window of interest, irrespective of the index EGM annotation).

The SKYLINE feature computes the surface area associated with active electrograms for each moment in time as a fraction of the total surface area of the map, effectively converting the 3-dimensional spatial and temporal activation data into a 2-dimensional graphical representation of the full chamber activation. This global activation histogram (GAH) displays the relative surface area (y axis) with activation at a given point in time (x axis). A high value on the y-axis (ie, closer to 1) corresponds to larger areas of chamber activation, while a low value (ie, closer to 0) corresponds to a smaller activation region. These troughs can be used to highlight critical isthmuses (re-entry), or define earliest activation (focal arrhythmia).

A case of recurrent atrial flutter is shown in Figure 2. During proximal coronary sinus pacing, activation mapping demonstrates the presence of a breakthrough across the mid-portion of the cavotricuspid isthmus (middle panel), with timing that corresponds to the onset of an activation peak on the SKYLINE GAH (ie, the temporal interval prior to the activation of the largest right atrial mass – highlighted in bottom left). The COMPLEX ACTIVATION feature, which highlights regions of the map that activate within the time-of-interest period and exhibit user-adjustable activation components (eg, single, double, or fractionated potentials), demonstrates a region of fractionated double potentials overlying the site of breakthrough (right panel), with local signals demonstrating prolonged fractionated potentials (left panel).

A case of PV reconnection is shown in Figure 3. This figure demonstrates an activation map of a patient who had previously undergone PVI three years prior. The activation map demonstrates delayed activation within the right superior pulmonary vein. In this case, the ACTIVATION SEARCH feature was used to define the site of right superior PV reconnection. This feature presents an adjustable time-of-interest window, facilitating the identification of regions with electrogram activity during a user-defined period. These electrograms are highlighted by the algorithm as long as there is activity in the window of interest, regardless of the electrograms’ activation annotation. In this case, through sequential modification of the time-of-interest window we were able to trace the activation from atrial myocardium into the PV and easily identify the region of reconnection.

Figure 4 demonstrates a case of recurrent atrial fibrillation in a patient 4 years post index cryoballoon ablation. Representative bipolar voltage maps created with CARTO 3 and Rhythmia HDx are pictured in the left and right panels, respectively. Reconnection is identified in the right superior and left superior pulmonary veins. The COMPLEX ACTIVATION feature was applied, demonstrating a region of fractionation and double potentials overlying a site of breakthrough into the right superior pulmonary vein (bottom right).

Following these initial introductory cases, we undertook more complex arrhythmic substrates.

Figure 5 demonstrates a case of recurrent atrial fibrillation and atypical left atrial flutter 4 years after index cryoballoon ablation. Electroanatomic activation (left panel) and voltage mapping (middle panel) demonstrate a region of breakthrough into the right PVs originating from the posterior carina. COMPLEX ACTIVATION identified regions of double potentials in the anterior and posterior carina (top right panel), with complex fractionation at the site of breakthrough (* - bottom right panel). Ablation in this location resulted in arrhythmia termination and PV re-isolation.

Figure 6 demonstrates a case of atypical right atrial flutter in a patient with previous atriotomy (9 years ago), cavotricuspid isthmus ablation for typical flutter (4 years ago), and severe tricuspid regurgitation. Surface ECG demonstrated atypical atrial flutter with alternating atrial cycles (260 msec, and 320 msec). Electroanatomic activation mapping demonstrates a dual loop lateral right atrial flutter with a common critical isthmus in the free wall near the lateral aspect of the cavotricuspid isthmus. Ablation in the common isthmus terminated both flutters.

Figure 7 demonstrates a case of atypical left atrial flutter in a patient with previous PVI, mitral isthmus ablation, and CFAE ablation (3 years prior). Electroanatomic activation and voltage mapping were performed (center panel). Interrogation of the SKYLINE localized a microreentrant flutter to the base of the left atrial appendage, with critical isthmus activation corresponding to a trough in the global activation histogram (bottom left). In this region, COMPLEX ACTIVATION identified an anatomic region (inset, bottom right) with an electrogram displaying fractionated activation throughout the tachycardia cycle (left panel). Ablation in this region resulted in arrhythmia termination and non-inducibility.

Figure 8 demonstrates a case of atrial tachydysrhythmia in a young patient with intermittent palpitations on the background of dysautonomia and neurocardiogenic syncope. She had previously been diagnosed with inappropriate sinus tachycardia; however, a transition in her care questioned the diagnosis. An electrophysiology study was performed, which demonstrated absent VA conduction, single AV node physiology, and no inducible tachycardia with single and double extrastimuli. At baseline, an electroanatomic activation map was performed to define sinus node activation. With isoproterenol infusion, there was a shift in atrial activation to a more superior location (central panel, upper frame). With aggressive stimulation, a stable atrial tachycardia was induced. Electroanatomic activation mapping demonstrates a focal source at the inferior right atrium near the septal isthmus (*). Ablation in this location resulted in arrhythmia termination and non-inducibility.

Summary

As demonstrated in the above cases, the Rhythmia HDx mapping system has offered an incremental benefit to our program through its innovative approach to arrhythmia mapping, diagnosis, and treatment. For appropriately selected patients, the Rhythmia HDx mapping system offers rapid high resolution electroanatomic map creation, with improved visualization of low-voltage critical isthmuses. 

Disclosures: The authors have no conflicts of interest to report regarding the content herein. The purchase of the Rhythmia HDx system was by generous donations through VGH & UBC Hospital Foundation.