Effects of Epinephrine and Phenylephrine on QT Interval Dispersion in Congenital Long QT Syndrome

Objectives. Measurement of QT interval dispersion during pharmacologic adrenergic stimulation was used to assess the effect of alpha- and beta-adrenergic stimulation on arrhythmic vulnerability in familial long QT syndrome (LQTS). Background. Nonhomogeneity in the ventricular action potential duration causes electrical instability leading to life-threatening ventricular arrhythmias and is markedly increased in LQTS. QT interval dispersion measured from the electrocardiogram (ECG) can be used as an index of nonhomogeneous ventricular repolarization. Methods. Sixteen symptomatic patients with LQTS and nine healthy control subjects were examined at baseline and during epinephrine (mainly beta-adrenergic agonist, 0.05 μg/kg body weight per min) and phenylephrine infusions (alpha-adrenergic agonist, mean 1.4 μg/kg per min). QT interval dispersion was determined from 12-lead ECG as interlead range and coefficient of variation measured to the end (QTend) and apex (QTapex) of the T wave. Results. At baseline QTend dispersion was greater in patients with LQTS compared with control subjects (mean [±SD] 68 ± 34 vs. 36 ± 7 ms, p = 0.001). QTend dispersion was markedly increased in patients with LQTS by use of epinephrine (from 68 ± 34 to 90 ± 36 ms, p = 0.002), but remained unchanged in control subjects. Phenylephrine did not affect QT dispersion in either group (all p = NS). Atrial pacing to achieve comparable heart rates during baseline and epinephrine and phenylephrine infusions did not influence the magnitude of QT dispersion in either group. QTapex dispersion analysis gave congruent results. Conclusions. Epinephrine but not phenylephrine increased QT dispersion, suggesting that beta-adrenergic stimulation provokes arrhythmias in patients with LQTS by aggravating nonhomogeneity of ventricular repolarization, whereas alpha-adrenergic stimulation is less important for arrhythmic vulnerability. The results also suggest that rapid pacing may not reduce vulnerability to arrhythmias in congenital LQTS.