High-Resolution Endocardial Mapping Using an Advanced 3D Mapping System

Arrhythmia localization in the EP laboratory has improved from simple recording of electrograms to cataloging electrograms on a 3D geometry, and now high temporal and spatial resolution within that geometry.¹ With the advent of electroanatomic mapping technologies, our ability to identify site of origin has improved considerably.^2,3 Although systems have improved, existing algorithms have suffered from lack of spatial resolution, inability to detect low-amplitude signals, and the need to adjudicate collected points, increasing procedural time. More recently, non-contact systems have offered the ability to follow wavefronts from a macroscopic multichamber view at the expense of spatial resolution.⁴ The Rhythmia^™ Mapping System (Boston Scientific) utilizes a combination of a high-density mapping catheter with a combination of prespecified mapping algorithms to increase sampling size, decrease mapping time, and improve sensitivity.^5,6 We present our early experience.

Mapping System, Algorithm, and Geometry

The system uses an 8 French basket catheter with bidirectional deflection. The basket electrode is designed as an array with eight splines (Figure 1). Each spline contains low-impedance electrodes measuring 0.4mm² with an inter-electrode spacing of 2.5 mm, comprising a total of 64 electrodes. The basket can be fully deployed to assume a spherical shape or retracted, with a range between 3-22 mm in diameter. The catheter is localized using a combination of a magnetic sensor in the distal catheter and impedance sensing on each basket electrode. The catheter is easily viewable on intracardiac echocardiography (Figure 2). 

The system utilizes four criteria to adjudicate recorded electrograms: (1) cycle length stability, (2) timing of reference electrograms, (3) electrode location stability, and (4) respiratory gating (Figure 3). These can be further modified for sensitivity and specificity. The system uses a combination of unipolar and bipolar electrograms for activation maps to reduce far field, as well as maximum negative dV/dt of the unipolar electrograms or maximum amplitude on bipolar electrograms. Multi-component electrograms are further sorted according to surrounding potentials. As the system detects sensitivities down to 0.01 mV, scar is defined as peak-to-peak bipolar and unipolar amplitude of <0.03 mV. 

The system generates 3D geometry on the basis of the outermost electrodes, restricted to collected electrograms within 1 to 5 mm from the surface geometry. The software gates to respiratory and cardiac cycles, also customizable. A probe allows the user to interrogate individual electrograms. 

Case #1

A 58-year-old female with hypertension and persistent atrial fibrillation (AF) refractory to flecainide presents for an elective Convergent hybrid ablation with concomitant posterior wall epicardial ablation and endocardial pulmonary vein (PV) isolation. The system by AtriCure, Inc. utilizes a unique epicardial ablation RF device that integrates suction, which draws the cardiac tissue into consistent contact, and perfusion, which uses circulating cooled saline to cool the external portion of the device and drive energy deep into tissue. The epicardial ablation device is placed through a cannula positioned through a subxiphoid incision. The energy is modulated using the ablation console, which adjusts wattage according to tissue impedance. During the epicardial component, we create overlapping lesions along the posterior left atrium, between the PVs. Following the epicardial portion, the midline incision is closed and the endocardial ablation commences using standard catheter ablation techniques to enter the left atrium. We employ Medtronic’s Arctic Front Advance^™ Cardiac CryoAblation Catheter to complete the PV lesion set, and have completed over 150 combined procedures to date. Prior to PV isolation, a voltage map was constructed of the left atrium. Figure 4 shows heterogeneity in the composition of the left atrial anterior wall (Figure 4A) as well as scar involving the endocardial posterior wall as a result of epicardial ablation (Figure 4B). The system allows for detection of voltage down to 0.01 mV to investigate for non-transmural ablation lesions. High sensitivity is necessary, especially as AF may produce low-voltage signals, which might be mistaken for scar. Completion of the ablation involved cryoablation, resulting in a contiguous lesion set extending from the PVs across the posterior atrium to the contralateral side. 

Case #2 

A 77-year-old female with symptomatic atrial tachycardia refractory to a Class 1C antiarrhythmic agent presents for ablation. The patient had relatively infrequent PACs after sedation, unresponsive to isoproterenol. The activation along the coronary sinus catheter was concentric, and this, in conjunction with the P wave morphology, suggested a right-sided origin. An IntellaMap Orion High Resolution Mapping Catheter (Boston Scientific) was deployed in the right atrium, and a map consisting of over 8000 points was constructed in a short period of time. An area along the tricuspid annulus at the 12 o’clock position revealed a site of earliest activation (Figure 5). A voltage map in this area showed heterogeneity compatible with a large area of scar, which explains the fairly diffuse area. The hemisphere showing gridlines is an approximation of the sampling size of the Orion catheter. A 4 mm radiofrequency ablation catheter was used to eradicate this site without further recurrence off sedation with isoproterenol. Shortly after, another group of foci was mapped adjacent to the sinus node (Figure 6). The sinus node was first mapped as was the phrenic nerve, and used as a reference for ablation of the perinodal sites. A cryocatheter was then selected to map the area before committing to full lesions, to avoid injury to the sinus node or phrenic nerve. 

Discussion

High-resolution mapping with automatic point adjudication provides the operator with improved site specificity while allowing for rapid characterization. Furthermore, electroanatomic systems decrease dependence on fluoroscopy, reducing radiation exposure to the physician and patient. Mapping systems have improved our ability to ablate arrhythmias; however, these systems remain an adjunct to classic electrophysiologic techniques based on surface ECG and electrograms. These devices still require oversight to help derive the greatest sensitivity and specificity from “automatic” mapping algorithms. Nevertheless, high-resolution mapping with automatic point adjudication allows for more sophisticated management of arrhythmia. Further research and device usage is needed to determine the complete system capability and optimal case selection.

Disclosures: The authors have no conflicts of interest to report regarding the content herein. Outside the submitted work, Dr. Makati reports reports grants and personal fees from AtriCure, Dr. Irwin reports personal fees from Medtronic, and Dr. Sherman reports personal fees from AtriCure. 

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