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 microg/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) (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.