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  • br Ablation strategies from targeted to homogenization The t

    2019-04-23


    Ablation strategies – from targeted to homogenization The techniques and principles originally developed in postinfarction scar have been translated and applied to patients with various etiologies of fibrosis and confluent scar including Chagasic cardiomyopathy, nonischemic cardiomyopathy, arrhythmogenic RV cardiomyopathy, sarcoidosis, hypertrophic cardiomyopathy, noncompaction, and surgically repaired congenital adenosine receptor disease. However, the electrical footprint for these conditions exhibit significant differences with regard to transmural propensity, anatomic location, and degree of fibrosis [56]. Various ablation strategies have been implemented to eliminate critical reentrant circuits within scar. There is no uniform agreement on the optimal ablation strategy and the number of targets within the substrate and hemodynamic tolerance of VT often dictates the procedural approach. Our approach to ablation is summarized in Fig. 11. High-density delineation of scar is first performed and regions of slow conduction, as evidenced by fractionated or late potentials are tagged and pace mapped to assess for a relation with the targeted VT. Pacemapping can be an effective method to focus ablation to specific regions within scar that exhibit a morphologic match to the targeted VT. Pace maps with stimulus-QRS delays (>40ms) are more specific as conduction slowing out of scar and sites that exhibit multiple QRS morphologies may suggest a common conducting channel [70,71]. The initiation of VT during pacemapping (pace mapped induction) can be seen when pacing from an isthmus and successful termination is seen in >90% of cases. When VT is not inducible or hemodynamically unstable, activation and entrainment mapping cannot be performed. Therefore, “substrate-based” ablation lesion sets within scar guided by electroanatomic mapping are commonly employed. (Fig. 12) Short linear lesions guided by electroanatomic mapping were first demonstrated to be effective at reducing recurrent VT [2,41,72]. Ablation at the interface between normal and low voltage areas has the potential to close off exits from channels originating deep within scar. Hsia et al. demonstrated by entrainment mapping within different regions of scar that 84% of isthmus sites were localized to dense scar, whereas Vera et al. correlated isthmus sites proven by entrainment mapping to border zone tissue in 68% [73,74]. “T-shaped” lesions were implemented in SMASH-VT at the border zone of a good pacemap site, with perpendicular linear ablation into scar to potentially ablate deeper in a conducting channel [75]. As areas of slow conduction within scar serve as the requisite substrate for reentry, the identification of locally uncoupled and delayed local electrograms, or late potentials is an important strategy during mapping in sinus rhythm. Delayed and isolated late potentials have been shown to have specificity for induced VTs and ablation aimed at eliminating these abnormal electrograms improves clinical freedom from VT [76,77]. Late potentials have been shown to be more specific for critical sites than macroscopic voltage channels visualized by lowering voltage thresholds [78,79]. Regions of localized delayed conduction may not always manifest with fixed conduction slowing after the QRS offset, as many critical sites are functionally influenced by anatomic location, wavefront, and rate, where an extrastimulus can reveal uncoupled local activation [77,80–82]. Extensive and diffuse ablation, or “scar homogenization” aimed to eliminate all abnormal electrograms within postinfarct scar is an important trend seen in VT ablation [83,84]. While the elimination of all electrograms has been shown to be more predictive of clinical success than inducibility, it may be difficult to achieve in large scar substrates [81]. Ablation targeting earlier late potentials may eliminate or modify downstream activation of a channel and expedite homogenization strategies [85,86]. Consistent with the trend to ablate scar more extensively, many institutions first target the scar and induce VT after homogenization is performed, whereas others maintain a targeted approach first to eliminate the clinical VT before diffuse ablation is undertaken.