Response of beat-by-beat QT variability to sympathetic stimulation in the LQT1 form of congenital long QT syndrome.

OBJECTIVES: The purpose of this study was to test the hypothesis that the lability of beat-by-beat QT variability is prominent during sympathetic stimulation in LQT1 patients. We analyzed beat-by-beat QT variability using a newly developed program and applied cross-correlation methods in LQT1 patients before and after epinephrine infusion.
BACKGROUND: Studies suggest that cardiac events associated with sympathetic stimulation are more common in the LQT1 form than the LQT2 and LQT3 forms of congenital long QT syndrome (LQTS). Although beat-by-beat alternation of T-wave morphology is observed in LQTS, its objective estimation is difficult because of complicated T-wave morphology.
METHODS: Twelve-lead ECG was recorded under baseline conditions and during epinephrine infusion (0.1 mug/kg/min) in 14 LQT1 and five control patients. We measured beat-by-beat QT interval by a cross-correlation technique. Mean of successive changes in RR (DeltaRR), QT (DeltaQT), standard deviation of DeltaRR (SD-DeltaRR), DeltaQT (SD-DeltaQT), and QTI (QT/ RR) before and after epinephrine were compared between the two groups.
RESULTS: No significant differences in any parameters were observed between the two groups under baseline conditions. DeltaQT, SD-DeltaQT, and QTI were increased in LQT1 but not in control patients during epinephrine (LQT1: DeltaQT 2.3-4.2 ms, SD-DeltaQT 2.2-4.1, QTI 0.10-0.22, P < .005 vs baseline; CONTROL: DeltaQT 2.5-2.4 ms, SD-DeltaQT 1.9-2.1, QTI 0.08-0.09: P = NS vs baseline).
CONCLUSIONS: Beat-by-beat QT variability analyzed by the cross-correlation method was greater in LQT1 patients during epinephrine infusion, suggesting sympathetic stimulation accentuates beat-by-beat alternation of repolarization in LQT1 patients.