Insights from VT substrates
Techniques for delineating the atrial substrate requires refinement and perhaps useful insights can be gained from the evolution of ventricular tachycardia (VT) mapping strategies. While activation and entrainment mapping remain preferable in VT ablation, this is not feasible in the majority of cases due to poor hemodynamic tolerance. Thus substrate guided ablation remains the mainstay of VT ablation strategies, with voltage maps used to identify abnormal tissue. Marchlinski et al. posited 1.5 mV as a threshold to distinguish normal tissue from areas of scar based on the voltage at the 5th centile of all mapping point, with a value of 0.5 mV arbitrarily used to segregate dense scar from the scar border zone (146). Cassidy et al., described electrogram characteristics within such LVZs denoting arrhythmogenic sites and with more recent refinements to better discern these electrograms (147,148).
Traditionally, post-infarct VT has been considered as a re-entrant arrhythmia through anatomically defined and somewhat static circuit. However more recent insights highlight the importance of functional properties in arrhythmia initiation and maintenance. Both animal and human studies suggest functional modulation of electrophysiological properties play a significant role in the genesis of a re-entrant circuit, and some of the footprints of the VT circuit may therefore not be apparent when mapping in sinus rhythm. Canine studies in early post-infarct VT demonstrated the participation of lines of functional block in the supporting VT. More recent studies in a porcine model of post-infarct VT also suggested arrhythmias were primarily determined by functional block (149). Cain and colleagues demonstrated development of slowed conduction and functional lines of block in the mechanism of VT in 8 patients with a history of ischemic heart disease (150). Segal et al. more recently observed formation of unidirectional block and lines of functional block at the border of LVZs at the initiation of VT using non-contact mapping in patients with post-infarct VT (151).
These observations and the potential for components of the re-entrant circuit being masked during mapping in SR have spawned novel approaches to VT substrate mapping. Indeed significant variation in late potentials is seen when pacing from different sites (152). In another study, sites showing functional conduction slowing and/or electrogram fractionation during S1S2 pacing protocols more accurately identified critical substrates for VT than conventional late potential mapping (153). The Decremental evoked potential (DeEP) mapping protocol involves imposing a drive train and extra, assessing for development of local activation delay and electrogram fractionation after the latter. Early studies have suggested improved specificity in identifying the critical isthmus than conventional late potential mapping (154). In a subsequent multicenter study, limiting ablation to only DeEP identified sites demonstrated improved acute outcomes compared to late potential ablation (153). In a variation of this dynamic mapping approach, mapping during single sensed extras was better able to identify late potential and LAVAs compared with sinus rhythm (155).