Neural control of heart rate is an arrhythmia risk modifier in long QT syndrome.

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
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.