Beat-to-Beat Action Potential Duration Variability, Not Duration, Reflects β-Adrenergic Stimulation-Induced Repolarization Instability

Instability of the cardiac electrical system is a major cause of life-threatening arrhythmias. Current safety guidelines have traditionally emphasized QT interval prolongation on the electrocardiogram (ECG) as a primary biomarker of proarrhythmic risk. However, this approach does not capture all forms of electrical instability. In this study, we demonstrate that beat-to-beat variability in action potential duration (APD), a cellular surrogate of the QT interval, better stratifies proarrhythmic risk than mean APD prolongation alone. Using a validated computational model of a human ventricular myocyte, we simulated the effects of the β-adrenergic agonist isoproterenol (ISO). Increasing the ISO concentration shortened mean APD but paradoxically increased beat-to-beat APD variability, measured as the standard deviation of APDs. In our simulations, the emergence of early afterdepolarizations (EADs), abnormal oscillations of the membrane potential during the plateau phase of the action potential, at higher ISO concentrations was consistently preceded by a marked rise in APD variability, even while mean APD was reduced. Mechanistic analysis revealed that ISO-induced enhancement of L-type Ca 2+ current (I CaL ) and slow delayed rectifier K + current (I Ks ) steepens the APD restitution slope, which reflects action potential instability, even when the net effect is a reduction in APD. These results support APD variability as a more sensitive and mechanistically plausible marker of proarrhythmic risk than APD prolongation alone. Our findings suggest that QT interval variability may improve risk stratification and provide a more robust mechanistic foundation for drug safety evaluation. Significance Statement QT interval prolongation remains an important component of cardiac safety assessment, but changes in mean QT duration may not fully capture the dynamic stability of ventricular repolarization. Here, we show that beat-to-beat APD variability increases during β-adrenergic stimulation even when mean APD shortens, and increases further as the system approaches conditions associated with EAD formation. These findings identify APD variability as a complementary measure of repolarization stability and provide a cellular mechanistic basis for further evaluating QT interval variability as an additional marker of proarrhythmic susceptibility.