Objective: To determine the effects of phenylephrine and dobutamine on repolarization lability in patients with genotyped long QT syndrome (LQTS). Patients and Methods: Between December 1998 and August 2000, 23 patients with genotyped LQTS (13 LQT1, 7 LQT2, and 3 LQT3) and 16 controls underwent electrocardiographic stress testing at the Mayo Clinic in Rochester, Minn. Aperiodic repolarization lability was quantified from digitized electrocardiograms recorded during catecholamine stress testing with phenylephrine and dobutamine. T-wave lability was quantified as a root-meansquare of the differences between corresponding signal values of subsequent beats. The magnitude of aperiodic T-wave lability was quantified by using a newly derived T-wave lability index (TWLI). Results: The TWLI was significantly greater in patients with LQTS than in controls (0.0945±0.0517 vs 0.0445±0.0123; P<.003). Marked T-wave lability (TWLI ≥0.095) was detected in all 3 LQTS genotypes (10/23) but in no controls (P<.003). There was no correlation between the TWLI and the baseline corrected QT interval. All high-risk patients having either a history of out-of-hospital cardiac arrest or syncope had a TWLI of 0.095 or greater. Conclusions: Beat-to-beat nonalternating T-wave lability occurs in LQT1, LQT2, and LQT3 patients during catecholamine provocation and is associated with a history of prior cardiac events. The quantification of this novel phenomenon may assist in identifying LQTS patients with increased risk of sudden cardiac death.
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