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asymptomatic patients (98±19 ms/cm), suggesting a possible role for ECGI in arrhythmic risk

stratification in LQT syndrome .
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Early Repolarization Syndrome 314.315

Early repolarization (ER) syndrome is defined in terms of its electrocardiographic signature,
that is, significant elevation of the QRS-ST-segment junction (J-point) in the inferior or lateral
precordial leads 316-318 . It has long been considered benign, but was recently shown to have
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greater prevalence in patients with history of idiopathic ventricular fibrillation (31.1% vs 5% in

control) 317,318 . The ECG lacks specificity and the J-point elevation (“J-wave”) could reflect delayed
activation and not necessarily early repolarization. We used ECGI to resolve this issue and test the
hypothesis that ER involves accelerated repolarization rather than delayed activation. An example
is provided in Figure 5.21.

Figure 5.21, top panel, shows the ECGI activation map during sinus rhythm, with activation
starting from the superior anterior septum (a normal variant), followed by activation of the RV and
LV in an apex to base fashion. This activation sequence is normal and excitation spread is uniform,

without regions of slow conduction, delayed activation, or conduction block. Figure 5.21, middle
panel, shows the ECGI repolarization ARI map for the same sinus beat. Strikingly, there are regions
of very short ARI (reflecting short APD) in midanterolateral RV (ARI = 140 ms, dark blue, marked
1 in right anterior oblique (RAO) view) and inferior basal RV (ARI = 160 ms, light blue, marked 3 in

left posterior oblique (LPO) view). Corresponding normal ARI value in these regions is 235 ms. Im-
portant for arrhythmogenesis, presence of these short ARI regions introduces steep repolarization
gradients: 107.4 ms/cm in region 1 and 102.2 ms/cm in region 3. For comparison, ARI gradients in
normal hearts range from 4.5 ms/cm to 11.3 ms/cm. ECGI-reconstructed EGMs from the regions

of short ARI show marked J-point elevation (inset 1), whereas those from neighboring regions do
not show any evidence of early repolarization (inset 2). The patient in Figure 5.21 had several pre-
mature ventricular complexes (PVC) of identical morphologies during the ECGI procedure. Figure
5.21, bottom panel, shows ECGI-generated epicardial potential maps for early activation (left) and

early repolarization (right) during a PVC. The local negative potential minimum of early activation
(left panel, asterisk, dark blue) locates the PVC origin. The location is verified by a potential
maximum that replaces the minimum at the same location (right panel, asterisk, red) during early
repolarization. The PVCs originate from an apical region. If the PVC generates an excitation wave

that interacts with the substrate of steep dispersion in a specific way (at a particular time window,
and at a particular velocity, direction and orientation relative to the substrate), unidirectional block
could occur and reentrant arrhythmia could be triggered. This illustrates the probabilistic nature
of cardiac arrhythmogenesis, which requires co-occurrence of multiple specific conditions. A later

study in 29 ER patients confirmed these results, showing marked abbreviation of ventricular
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repolarization in areas with J-wave EGMs, and regions of steep repolarization gradients. In two
patients, PVCs were mapped by ECGI with origins closely related to the abnormal EP substrate.