© 2003 by European Society of Cardiology
Copyright © 2003, European Society of Cardiology
Effect of pacing and mexiletine on dispersion of repolarisation and arrhythmias in 
KPQ SCN5A (long QT3) mice
aDepartment of Cardiology and Angiology, University Hospital Münster, Münster, Germany
bInstitute for Arteriosclerosis Research at the University of Münster, Münster, Germany
cDepartments of Pharmacology and Cardiology, Georgetown University and VA Medical Center, Washington, DC, USA
dCenter for Transgene Technology, Leuven, Belgium
fabritzl{at}uni-muenster.de
* Corresponding author. Medizinische Klinik und Poliklinik C–Kardiologie und Angiologie, Universitätsklinikum Münster, D-48129 Münster, Germany. Tel.: +49-251-83-47638; fax: +49-251-83-47864.
Objective: It has been suggested that both pacing and treatment with mexiletine may reduce torsade de pointes (TdP) arrhythmias in patients with long QT syndrome 3 (LQT3), but it is not fully understood how these interventions could prevent TdP. We therefore studied the effects of pacing and mexiletine in mice with a heterozygous knock-in 
KPQ SCN5A/+ deletion (SCN5A-Tg), a murine LQT3 model. Methods: Three right and left ventricular monophasic action potentials (MAPs) were simultaneously recorded in Langendorff-perfused hearts of SCN5A-Tg and wild type (WT) littermates. AV block was induced, and pacing was performed at baseline and during mexiletine infusion (4 µg/ml). MAP recordings were analysed for action potential duration (APD), APD dispersion, and early afterdepolarisations (EADs) and related to spontaneous arrhythmias. Results: After inducing AV block, SCN5A-Tg hearts were bradycardic [SCN5A-Tg 532±60 vs. WT 284±48 ms cycle length (CL, mean±S.E.M., P<0.05(*))]. EADs occurred in 16/18, and polymorphic ventricular tachycardia (pVT) in 11/18 SCN5A-Tg but not in 19 WT. SCN5A-Tg had longer APD than WT hearts*. At CL of 200 ms and longer, APD dispersion was higher in SCN5A-Tg [dispersion (APD70): 12±3 ms vs. 5±2 ms at CL=200 ms*], and increased to 35±4 ms* directly prior to pVT episodes. Sudden rate accelerations initially increased APD dispersion due to EADs and APD alternans in SCN5A-Tg, but pacing then reduced APD dispersion. Pacing suppressed (n=9/9) and prevented (n=49/50) pVT. Mexiletine shortened APD at long CL*, and suppressed pVT (n=4/5*), but did not prevent pVT during normal rhythm. Conclusions: Bradycardia, increased dispersion of APD and EADs provoke ventricular ectopy and pVT in SCN5A-Tg hearts. Ventricular pacing reduces APD dispersion, suppresses EADs and prevents pVT in SCN5A-Tg hearts. These effects provide a pathophysiological rationale for pacing in LQT3.
KEYWORDS APD, action potential duration; AV, atrioventricular; CL, cycle length; EAD, early afterdepolarisations; ERP, effective refractory period; LQT3, Long QT syndrome 3; LV, left ventricle, left ventricular; MAP, monophasic action potential; pVT, polymorphic ventricular tachycardia; RV, right ventricle, right ventricular; SCN5A-Tg, mice with heterozygous knock-in KPQ SCN5/+ deletion; TdP, torsade de pointes
1 Larissa Fabritz and Paulus Kirchhof contributed equally to this paper.
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