Joseph Hejlik RN, Carla Bell None, Win K. Shen MD,
Michael J. Ackerman MD, PhD, Mayo Clinic, Rochester, MN.
Background: Identification of patients (pts) with long QT syndrome (LQTS) at-risk
for potentially fatal arrhythmias is difficult. Microvoltage T wave alternans (μTWA) is
associated with arrhythmia susceptibility in other cardiac diseases, but less
is known with LQTS. Since many LQTS-related events stem from activation of the
sympathetic nervous system, we sought to compare inducibility of μTWA during exercise testing and epinephrine infusion. Methods: 14 pts
(age = 25 ± 13 years, 6 F) with LQTS
(6-LQT1, 7-LQT2, 1-LQT3) underwent an exercise treadmill stress test (modified
Bruce protocol) and an epinephrine provocation (0.025 mcg/kg/min infusion
serially incremented to a maximum of 0.2
mcg/kg/min). μTWA was determined by the spectral method using a CH2000 system
(Cambridge Heart, Bedford, MA). A test was considered positive for μTWA if there was alternans voltage >
1.9 μV and alternans ratio > 3 present in
either 1 orthogonal or 2 adjacent precordial leads that was
sustained for > 1 minute with an onset heart rate (HR) < 120 bpm. The
onset HR for μTWA and the maximum negative HR were recorded for each pt during both
testings. The majority of pts (10/14) were not on beta-blockers at the time of
either study. Results: In this cohort, the baseline QTc was 458 ± 31 ms and 10/14 pts were asymptomatic prior to stress
testing. During exercise testing, all pts achieved a diagnostic heart rate >
120 bpm (148 ±
26). μTWA was evident in 4/14 pts (28.6%): 0-LQT1, 3-LQT2, and 1-LQT3. Exercise-induced
μTWA was not associated with a history of symptoms, use of beta-blockers,
or the resting QTc. In contrast to exercise-induced μTWA, none of the pts had epinephrine-provoked μTWA. However, the maximum negative HR achieved during epinephrine infusion was only
79 ± 12 bpm. Interestingly, during
epinephrine infusion, 7 of the 14 pts (50%) displayed nonsustained (< 1 min) episodes of μTWA. Conclusions: Exercise-induced μTWA was seen in over 25% of pts with LQTS. However, μTWA was not observed in LQT1 pts most susceptible to exercise triggered
cardiac events. Moreover, the role of μTWA in risk
stratification was not apparent. The significance of nonsustained μTWA during epinephrine infusion is unknown.