The concept of substrate mapping and ablation evolved from t
The concept of substrate mapping and ablation evolved from these surgical experiences. Unlike surgeons who can visually identify the infarct region, electrophysiologists must locate the abnormal region using the guidance of local ventricular electrograms. Marchlinski et al. reported that voltage mapping in sinus rhythm using 3-dimensional electroanatomic mapping can characterize electroanatomic substrates . A peak-to-peak bipolar amplitude <0.5mV identifies areas with extremely abnormal signals (dense scars). The dense scar is typically surrounded by a border zone area with an electrogram amplitude of 0.5–1.5mV. This voltage criterion was recently validated by using fluorodeoxyglucose positron emission tomography . Delayed enhancement as depicted by contrast-enhanced magnetic resonance imaging can also exhibit a good correlation with the low-voltage region [30,31].
The surviving myocytes responsible for VT should form anatomically definable conduction channels within the dense background scar. Arenal et al. and Hsia et al. described that areas of relatively higher voltage corresponding to surviving bundles of myocardial Cilengitide within the scar tissue can function as conducting channels, which can be depicted on a color-coded electroanatomic voltage map [19,32]. They used different voltage cutoff values to define scars to identify conducting channels as corridors of continuous electrograms in the scar. By applying a stepwise reduction in the definition of abnormal voltage from 0.5 to 0.1mV, most conducting channels were found to have voltage scar definitions of ≤0.2mV. Recently, Mountantonakis et al. demonstrated the non-critical relationship between voltage-defined channels and VT critical isthmuses . Voltage channels can be identified in 87% of patients with mappable ischemic VT by adjusting the voltage limits of bipolar maps; however, the specificity of those channels in predicting the location of VT isthmus sites is only 30%. The presence of late potentials inside the voltage channel significantly increases the specificity for identifying the VT isthmus (Fig. 1). This study indicates the importance of identifying both abnormal voltage areas and abnormal local electrograms as optimal targets of VT substrate ablation.
Because precise assessment of the arrhythmia substrate pre- and post-ablation is mandatory, substrate-based approaches using 3-dimensional electroanatomic mapping systems are dependent on high-density mapping of the VT substrate during the baseline rhythm. Whereas point-by-point mapping can be time consuming and sometimes operator-dependent, multielectrode mapping may produce a high-density map in a timely manner. This potentially overcomes failure due to unrefined mapping. High-density electroanatomic mapping is performed with a multipolar mapping catheter (PentaRay, Biosense Webster, Diamond Bar, CA, USA). The splines of this catheter are extremely soft and provoke very few mechanical ectopics. The greatest value of the PentaRay is in epicardial mapping, as recently described . The elimination of epicardial arrhythmia substrates from endocardial ablation is feasible on using the PentaRay catheter. An epicardially placed PentaRay can be used for both high-density mapping and as a landmark of the target epicardial abnormal region, precisely guiding the operator to the facing endocardial site. It also enables real-time monitoring of the impact of endocardial ablation on epicardial arrhythmia substrates.
Substrate ablation strategies The presence of surviving bundles of myocytes that were electrically uncoupled from the surrounding myocardium by interstitial fibrosis is indicated by abnormal signal characteristics within low-voltage scars. These abnormal characteristics include fractionation, prolonged duration, and late potentials . There are several substrate-based ablation approaches to determine ablation targets and modify or eliminate the VT substrate.
Epicardial mapping and ablation