Do new tools help us identify substrate to target for ablation in ventricular tachycardia?

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Catheter ablation of ventricular tachycardia is a well-established option for the management of patients with recurrent ventricular arrhythmias. In the context of structural heart disease, catheter ablation has been demonstrated to be an effective alternative to antiarrhythmic medications for reducing recurrence of ventricular tachycardia and ICD shocks. The most common mechanism of ventricular tachycardia in these patients is scar-related reentry and the traditional approach to identify the target of ablation involves activation and entrainment mapping of the ventricular tachycardia circuit. A primary limitation, however, is that the clinical tachycardia may be unable to be induced or, more frequently, unable to be tolerated hemodynamically. Consequently, substrate-based catheter ablation of VT has developed as a preferred treatment strategy because of the ability to pursue ablation without requiring prolonged maintenance of sustained VT. Multiple studies have identified distinct characteristics within ventricular scar and thereby proposed different targets and approaches for substrate-based ablation. In the present study, Nührich et al.1 describe the application of a new “ultra-high resolution” mapping system (Rhythmia HDx TM by Boston Scientific, Marlborough, MA, USA) to the characterization of scar for catheter ablation in the context of this evolution of the definition of the electroanatomic substrate for VT.
Fundamental to a substrate-based approach to VT ablation is the ability to identify accurately and reproducibly the diseased tissue with slowed or abnormal conduction supporting the ventricular arrhythmia. One must also be able to visualize and consume this information in a convenient manner to facilitate ablation. As described by the authors, the Rhythmia mapping system employs a novel basket catheter with physical characteristics well-suited to mapping scar-related substrate and software with automated electrogram annotation that also offers benefits in the characterization of scar and presentation of the data.
A principal approach for electroanatomic mapping of scar has been the identification of areas of low voltage. The contemporary bipolar voltage thresholds for defining abnormal scar tissue, <0.5 mV for dense scar and >1.5 mV for normal tissue, were identified in the context of a case control study and mapping was performed with a 4-mm tip electrode ablation catheter with 1- or 2-mm interelectrode spacing.2 Similarly, the standard unipolar voltage of <8.27 mV in the presence of normal endocardial bipolar voltage for patients with nonischemic cardiomyopathy was established when mapping with a 3.5- or 4-mm distal-tip ablation catheter.3 More recently the application of additional multi-electrode mapping catheters with smaller electrodes and different interelectrode spacing has resulted in a reassessment of a set voltage threshold for scar. An ex vivo study in a porcine infarct model has demonstrated a linear relationship between an appropriate voltage threshold for MRI-defined scar and the interelectrode spacing.4 The OrionTM basket catheter employed with the Rhythmia mapping system has 2.5-mm interelectrode spacing and 0.4 mm2 electrode surface area. The authors employ bipolar and unipolar voltage standards based upon previously published experimental and human VT ablation studies.5,6 Using a bipolar threshold of <0.1 mV for dense scar and >1.0 mV for normal tissue in both the endocardium and epicardium as well as an endocardial unipolar voltage <4.5 mV to indicate the presence of scar resulted in regions of scar that were internally consistent and of a similar extent to prior studies. The authors also note that all of the sites of slow conduction identified in sinus rhythm or CS pacing resided within the border zone established by these voltage thresholds. These findings help to define the appropriate voltage thresholds for this catheter for clinic VT ablation studies.
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