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

Objectives 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 min) 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-Tend interval, the Q-Tpeak interval and the interval between Tpeak and Tend (Tp-e), representing TDR, were measured and averaged from 87-lead ECGs and corrected by Bazett’s method (corrected Q-Tend interval [cQTe], corrected Q-Tpeak interval [cQTp] and corrected interval between Tpeak and Tend [cTp-e]). The dispersion of cQTe (cQTe-D) was obtained among 87 leads and was defined as the interval between the maximum and minimum values of cQTe. Results Propranolol in the absence of epinephrine significantly prolonged the mean cQTp value but not the mean cQTe value, thus decreasing the mean cTp-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 cQTe, minimum cQTe and cQTe-D were not changed with propranolol. Propranolol completely suppressed the influence of epinephrine in prolonging the mean cQTe, maximum cQTe and minimum cQTe values, as well as increasing the mean cTp-e and cQTe-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.