Differential effects of beta-blockade on dispersion of repolarization in the absence and presence of sympathetic stimulation between the LQT1 and LQT2 forms of congenital long QT syndrome

0BJECTIVES: This study compared the effects of beta-blockade on transmural and spatial dispersion of repolarization (TDR and SDR, respectively) between the LQT1 and LQT2 forms of congenital long QT syndrome (LQTS). BACKGROUND: The LQT1 form is more sensitive to sympathetic stimulation and more responsive to beta-blockers than either the LQT2 or LQT3 forms. METHODS: Eighty-seven-lead, body-surface electrocardiograms (ECGs) were recorded before and after epinephrine infusion (0.1 μg/kg body weight per rain) in the absence and presence of oral propranolol (0.5-2.0 mg/kg per day) in 11 LQT1 patients and 11 LQT2 patients. The Q-T_end interval, the Q-T_peak interval and the interval between T_peak and T_end (T_p-e), representing TDR, were measured and averaged from 87-lead ECGs and corrected by Bazett's method (corrected Q-T_end interval [cQT_e], corrected Q-T_peak interval [cQT_p ] and corrected interval between T_peak and T_end [cT_p-e]). The dispersion of cQT_e (cQT_e-D) was obtained among 87 leads and was defined as the interval between the maximum and minimum values of cQT_e. RESULTS: Propranolol in the absence of epinephrine significantly prolonged the mean cQT_p value but not the mean cQT_e value, thus decreasing the mean cT_p-e value in both LQT1 and LQT2 patients; the differences with propranolol were significantly larger in LQT1 than in LQT2 (p < 0.05). The maximum cQT_e, minimum cQT_e and cQT_e-D were not changed with propranolol. Propranolol completely suppressed the influence of epinephrine in prolonging the mean cQT_e, maximum cQT_e and minimum cQT_e values, as well as increasing the mean cT_p-e and cQT_e-D values in both groups. CONCLUSIONS: Beta-blockade under normal sympathetic tone produces a greater decrease in TDR in the LQT1 form than in the LQT2 form, explaining the superior effectiveness of beta-blockers in LQT1 versus LQT2. Beta-blockers also suppress the influence of sympathetic stimulation in increasing TDR and SDR equally in LQT1 and LQT2 syndrome.