Dynamic assessment of atria substrate through pacing
Wong et al. reported significant variation in left atrial voltage,
burden of LVAs and electrogram fractionation during atrial pacing,
depending on the pacing site and atrial rate (156). The prognostic
implication of these changes was no examined. Kim et al., previously
demonstrated steeper action potential restitution curves associated with
AF (157). Williams et al. evaluated electrophysiological responses to
incremental atrial pacing in a cohort of patients with PAF (158).
Patients displayed highly variable changes in electrogram voltage and
fractionation, however greater rate-dependent activation dispersion was
associated with increased AF vulnerability. Finally, Hunter and
colleagues assess conduction velocity dynamics in patients undergoing
ablation for PsAF (84). Most sites with early rate-dependent conduction
slowing were located in regions of low voltage, many of which harbored
rotational drivers for AF.
Some groups have implemented electrophysiology study-guided LA substrate
modification in PsAF with favorable results. In patients without LVAs on
the posterior wall of the LA, Yamaji et al. performed posterior wall
isolation if regions of the LA demonstrated pro-arrhythmic
repolarization properties or AF/AT was inducible during
electrophysiological testing (159). Patients who underwent posterior
wall isolation displayed greater freedom from AF/AT than those without
LVAs who had PVI alone, suggesting that atrial voltage assessment in
isolation may underestimate the risk of AF recurrence.
Finally the majority of studies in the current era of substrate
modification have focused on endocardial evaluation of the LA. Recent
high-resolution EAM has demonstrated significant burden of LVAs in the
right atrium as well as other pro-arrhythmic markers such as conduction
slowing and block (160). Comprehensive substrate modification in the
future will likely involve concomitant assessment of the right atrium. A
recent analysis of atrial LVAs have shown that these may be located
exclusively in the epicardial layer and correspond to normal bipolar
voltages when measured endocardially (161). Conventional bipolar mapping
and novel omnipolar technology have a limited field of view, and thus
are less sensitive to epicardial voltage when mapping endocardially, and
vice versa. Unipolar measurements do seem to improve the detection of
such epicardial LVAs to some degree, but a large proportion of
potentially arrhythmogenic areas may still be missed (162).
In summary, novel approaches to atrial substrate assessment are moving
towards a genuinely individualized ablation strategy, guided by
multimodal assessment of atrial electrophysiology. Progress on this
front will require a reassessment of historical definitions of low
voltage, appreciating the influence of clinical and technical factors.
Moreover, it is clear that low voltage in isolation is unlikely to
capture the arrhythmogenic potential of the tissue fully, and further
functional evaluation may be required to identify sites critical to AF
maintenance correctly. Finally, efficacious treatment of AF requires not
only accurate substrate characterization, but also means of delivering
durable ablation lesions safely.