Neural control of heart rate is an arrhythmia risk modifier in long QT syndrome
Schwartz PJ, Vanoli E, Crotti L, Spazzolini C, Ferrandi C, Goosen A, Hedley P, Heradien M, Bacchini S, Turco A et al. (2008) Neural control of heart rate is an arrhythmia risk modifier in long QT syndrome. J Am Coll Cardiol 51:9 920–9.
Abstract: OBJECTIVES: The purpose of this study was to test the hypothesis that differences in autonomic responses might modify clinical severity in long QT syndrome type 1 (LQT1) patients, those with KCNQ1 mutations and reduced I(Ks), in whom the main arrhythmia trigger is sympathetic activation. BACKGROUND: Some long QT syndrome (LQTS) patients experience life-threatening cardiac arrhythmias, whereas others remain asymptomatic throughout life. This clinical heterogeneity is currently unexplained. METHODS: In a South African LQT1 founder population segregating KCNQ1-A341V, we correlated major cardiac events to resting heart rate (HR) and to baroreflex sensitivity (BRS) on and off beta-adrenergic blockers (BB). RESULTS: In 56 mutation carriers (MCs), mean HR was lower among asymptomatic patients (p < 0.05). Among MCs with a QT interval corrected for heart rate <or=500 ms, those in the lower HR tertile were less likely to have suffered prior cardiac events (odds ratio [OR] 0.19, 95% confidence interval [CI] 0.04 to 0.79, p < 0.02). The BRS was lower among asymptomatic than symptomatic MCs (11.8 +/- 3.5 ms/mm Hg vs. 20.1 +/- 10.9 ms/mm Hg, p < 0.05). A BRS in the lower tertile was associated with a lower probability of being symptomatic (OR 0.13, 95% CI 0.02 to 0.96, p < 0.05). A similar trend was observed during BB. The MCs in the lower tertile for both HR and BRS were less frequently symptomatic than MCs with different patterns (20% vs. 76%, p < 0.05). Subjects with either ADRA2C-Del322-325 or homozygous for ADRB1-R389, 2 polymorphisms predicting enhanced adrenergic response, were more likely to have BRS values above the upper tertile (45% vs. 8%, p < 0.05). CONCLUSIONS: Lower resting HR and "relatively low" BRS are protective factors in KCNQ1-A341V carriers. A plausible underlying mechanism is that blunted autonomic responses prevent rapid HR changes, arrhythmogenic when I(Ks) is reduced. These findings help understanding phenotypic heterogeneity in LQTS and identify a physiological risk modifier, which is probably genetically determined.
- Study of a South African population with mutations in KCNQ1 that led to type 1 long QT syndrome. Mutation carriers who had not experienced symptoms were likely to have a lower resting heart rate and had lower baroreflex sensitivity. Mutation carriers in the lower tertile for both heart rate and baroreflex sensitivity had less frequent symptoms. Some polymorphisms were correlated with enhanced adrenergic sensitivity. Differences in adrenergic sensitivity among patients with epilepsy may predispose to SUDEP, though monogenic cases would likely account for only a small minority of the total number of cases.