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Figure 5.22. Abnormal epicardial electrograms (EGMs) characteristics and localization. A. Peak
ST-segment elevation (STE) magnitude map. Insets show ECG lead V2. B. EGM magnitude map
(EMM). C. EGM deflection map (EDM) showing number (#) of low-amplitude deflections. BrS
maps are in the three right columns; the left column shows corresponding maps from a healthy
subject for comparison. D. Unipolar EGMs from locations marked by the white dots in C: 1, Anterior
RVOT; 2, lateral RVOT; 3, RV free wall; 4, RV apex; 5, LV free wall (EGMs from other LV sites are also
normal). Red traces show the time derivatives of fractionated QRS. The derivatives approximate
bipolar EGMs and emphasize fractionation. Maps are shown in anterior view. PT: pulmonary trunk.
From Zhang et. al. [320] with permission of Wolters Kluwer Health, Inc.

The ECGI results demonstrate the presence of both abnormal repolarization and abnormal

conduction in the RVOT of BrS patients. Abnormal repolarization during phase 1 of the action
potential, due to reduced I on the background of large I in RVOT epicardium, can generate
Na to
potential gradients that give rise to the STE in the EGMs and the ECG. Slow discontinuous
conduction in the RVOT can cause the fractionation and low voltage of RVOT EGMs. Increasing

the heart rate should affect both. It diminishes the phase 1 notch in the action potential (due to
incomplete recovery of I ) and thereby reduces potential gradients and STE. It also compromise
to
conduction further by reducing I availability. Indeed, increased heart rate (by exercise or
Na
isoprenaline) in six of the BrS patients reduced STE and augmented EGM fractionation, providing

additional support for the co-existence of conduction and repolarization abnormalities in the
RVOT substrate of BrS patients. Note that the abnormal substrate in BrS is localized exclusively to
the RVOT. This stands in striking contrast to LQT syndrome, where location of the abnormal
substrate was highly variable.