Background-Experimental studies suggest that the interval between peak and end of T wave (Tpe) in transmural ECGs reflects transmural dispersion of repolarization (TDR), which is amplified by ␤-adrenergic stimulation in the LQT1 model. In 82 patients with genetically identified long-QT syndrome (LQTS) and 33 control subjects, we examined T-wave morphology and various parameters for repolarization in 12-lead ECGs including corrected QT (QTc; QT/R-R 1/2 ) and corrected Tpe (Tpec; Tpe/R-R 1/2 ) before and during exercise stress tests. Methods and Results-Under baseline conditions, LQT1 (nϭ51) showed 3 cardinal T-wave patterns (broad-based, normal-appearing, late-onset) and LQT2 (nϭ31) 3 patterns (broad-based, bifid with a small or large notch). The QTc and Tpec were 510Ϯ68 ms and 143Ϯ53 ms in LQT1 and 520Ϯ61 ms and 195Ϯ69 ms in LQT2, respectively, which were both significantly larger than those in control subjects (402Ϯ36 ms and 99Ϯ36 ms). Both QTc and Tpec were significantly prolonged during exercise in LQT1 (599Ϯ54 ms and 215Ϯ46 ms) with morphological change into a broad-based T-wave pattern. In contrast, exercise produced a prominent notch on the descending limb of the T wave, with no significant changes in the QTc and Tpec (502Ϯ82 ms and 163Ϯ86 ms: nϭ19) in LQT2. Conclusions-Tpe interval increases during exercise in LQT1 but not in LQT2, which may partially account for the finding that fatal cardiac events in LQT1 are more often associated with exercise.

Our findings suggest that loss-of-function SCN5A mutations resulting in Brugada syndrome are distinguished by profound bradyarrhythmias.

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Multiple mutations in different LQTS-related genes may modify clinical characteristics. Expanded gene survey may be required in LQT1 patients who are resistant to beta-blocker therapy.