br Entrainment As the principles of entrainment
Entrainment As the principles of entrainment initially described by Waldo for atrial flutter could be fulfilled by scar-mediated VT, characterization of the reentrant circuit provided a central isthmus as a favorable target [33,34]. In the late 1980s, the collective descriptions of entrainment, or continuous resetting of VT with overdrive pacing, from regions of slow conduction helped to establish the gold standard definition for critical components of a reentrant circuit [35–39]. The current construct for reentry meandering between areas of postinfarct fibrosis was popularized by Stevenson et al. . In this model, the circuit contains an entrance, central isthmus, and exit, which results in the breakout site that results in formation of the QRS (Fig. 4). Regions of the circuit not constrained by scar are termed outer loop sites and regions that are proximal to the entrance and constrained by scar are termed inner loop sites. Bystander sites are ineffective sites for ablation and may be remote or attached to the circuit. The postpacing interval is equal or within 30ms of the tachycardia ampa length at any site that is in the reentrant circuit. Concealed fusion is seen in regions constrained within scar, such that antidromic capture and collision with the orthodromic wavefront occurs entirely within the circuit. The stimulus to QRS exceeds the electrogram to QRS interval at bystander sites attached to the circuit. The electrogram to QRS interval approximates proximity to the circuit exit, where <30% of the TCL is distal, 30–70% is central, and >70% is proximal within the isthmus. The ideal site for ablation within a central isthmus is a middiastolic potential during VT that exhibits concealed fusion during entrainment with a postpacing interval that is equal to the tachycardia cycle length (Fig. 5).
Electroanatomic mapping and substrate-guided ablation Since initial validation studies in 1999, electroanatomic mapping has become an essential tool to optimize mapping of VT and to guide ablation lesions. Using a threshold of <1.5 for low voltage and an arbitrarily defined dense scar threshold of 0.5, voltage on the surface mapped can be displayed as a three-dimensional reconstruction. Prior to the advent of electroanatomic mapping, fluoroscopy was used as a crude method for navigating the ablation catheter within scar. With scar extent and architecture displayed on a mapping system, electrogram sites and ablation lesions can be tagged with a high degree of navigation reproducibility . Two commercially available mapping systems are commonly used with magnetic field localization (CARTO, Biosense Webster, Diamond Bar, CA) and impedance-based localization (NAVX, St. Jude Medical, Minneapolis, MN), although these technologies have evolved toward incorporating both forms of technologies to optimize geometry and mapping accuracy. Both of these systems have been validated in animal models with excellent correlation with gross and histopathology [42–44] (Fig. 6). As ablation is often performed in sinus rhythm, accurate and detailed scar characterization for the delineation of border zones and identification of abnormal electrograms within scar. The placement of linear lesions guided by electroanatomic mapping was first described in the 2000 by Marchlinski et al., in patients with previously termed “unmappable” VT . Higher mapping density can be achieved with multielectrode catheters and has been shown to improve the identification of late potentials in regions of heterogeneity . Multielectrode mapping can expedite VT ablation using any commonly employed techniques including pacemapping, activation, and entrainment mapping and may be a more sensitive method to confirm abolition of late potentials  (Fig. 7). All current mapping systems have evolved to enable and incorporate multielectrode-capable contact mapping.
Epicardial ablation and substrates A significant advancement in the field of VT ablation is the ability to percutaneously access the pericardial for epicardial mapping and ablation. Initially described by Sosa et al. in 1996 to address arrhythmogenic epicardial scar in patients with Chagas, testes approach has facilitated major conceptual advances in our understanding of the transmural and epicardial predilection of scars across various disease states . Further, the presence of epicardial predominant scar may be an important mechanism for ablation failure, where ablation from the epicardium allows for a second dimension of attack in cases where endocardial ablation alone is inadequate (Fig. 8). Although the incidence of RV puncture may approach 20%, procedural related mortality and surgical conversion is exceedingly uncommon [48,49]. Anatomic barriers to ablation include coronary vessels, epicardial fat, and the left phrenic nerve [19,50,51]. In its current state, epicardial mapping and ablation should be performed at experienced centers with surgical backup. In patients with previous cardiac surgery, surgical access via a subxiphoid approach or limited anterior thoracotomy can be performed safely in the EP lab [52,53].