Author Affiliations: Tel-Aviv Sourasky Medical Center and Sackler-School of Medicine, Tel Aviv University, Israel.
The long QT syndrome (LQTS)1 and short QT syndrome (SQTS)2 are inherited diseases caused by mutations in genes coding for proteins that build up specific ion channels (located in the myocyte membrane) that are responsible for the electrical activity of the cardiac muscle.3 Different mutations will either prolong or shorten the action potential (depending on whether they diminish or increase repolarizing ion-current flow) and will consequently prolong or shorten the QT interval.3 Ironically, either excessive prolongation or shortening of the action potential (LQTS and SQTS, respectively) leads to similar clinical consequences: syncope or cardiac arrest caused by malignant polymorphic ventricular arrhythmias.3 This is because the density of ion channels along the myocyte membrane differs at distinct myocardial layers, creating action potentials that are always longer in the deep myocardium, shorter in the endocardium, and shortest in the epicardium. These differences amplify when genetic mutations exist, and it is this “dispersion of repolarization,” rather than its absolute lengthening or shortening, that ultimately leads to cardiac arrhythmias.4
Viskin S, Rosso R, Rozovski U. QT Interval and MortalityComment on “QT Interval Duration and Mortality Rate”. Arch Intern Med. 2011;171(19):1734-1735. doi:10.1001/archinternmed.2011.504