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).