Figure 2. The cycle length restitution curve.

(A–B) Simulated spontaneous action potentials during a protocol to determine the cycle length restitution curve. A 10-ms stimulus is applied with varying amplitude at the maximum diastolic potential to accelerate or delay the subsequent spontaneous action potential. Time between 2^nd and 1^st APs following perturbation (CL₂) is then plotted as a function of time between 1^st perturbed and steady-state APs (CL₁). (C) CL restitution curves for control (black), low [Na⁺]_o (red) and I_Kr block (gray). Note that low [Na⁺]_o results in a curve with an abrupt transition from a relatively flat region to a very steep region (maximal slope >>−1 indicated by arrow; dashed line has slope of −1 for reference).