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Findings within the last year (2004) suggest that SQTS is a genetically heterogeneous disease similar to Long QT Syndrome. In two out of three families with SQTS studied by Brugada et al, two different missense mutations were discovered, revealing the same amino acid change (N588K) in the S5-P loop region of the cardiac I_Kr channel HERG (KCNH2). These mutations dramatically increased I_Kr, leading to heterogenous abbreviation of action potential duration and refractoriness, while reducing the affinity of the channel to I_Kr blockers.

In a sporadic case of SQTS, Bellocq et al identified a g919c substitution in the KCNQ1 gene encoding the K⁺ channel KvLQT1. This also leads to a gain in function of a delayed rectifier K current (I_Ks) with abbreviation of the action potential and refractoriness. Shortening of the effective refractory period combined with increased dispersion of repolarization are a likely substrate for reentry, which in the atria can lead to atrial fibrillation and in the ventricles to ventricular fibrillation and sudden cardiac death.

Mechanism Behind Arrhythmias in SQTS

 DOR: Dispersion of repolarization

ERP: Effective refractory period