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 LQF3 forms. METHODS Eighty-seven-lead, body-surface electrocardiograms (ECGs) were recorded before and after epinephrine infusion (0.1 mug/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-T-end interval, the Q-T-peak interval and the interval between T-peak and T-end (Tp-e), representing TDR, were measured and averaged from 87-lead ECGs and corrected Bazett's method (corrected Q-T-end interval [cQT(e)], corrected Q-T-peak interval [cQT(p)], an corrected interval between T-peak and T-end [cT(p-e)]). The dispersion of cQT(e) (cQT(c)-D) was was defined as the interval between the maximum and obtained among 87 leads minimum values of cQT(c). RESULTS Propranolol in the absence of epinephrine significantly prolonged the mean cQT(p) value but not the mean cQT(c) 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(c), 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.