Cardiac I_Na and measurement protocols. (A) A diagram of the ventricular action potential, the transient I_Na during an AP (with an inset showing the first few ms during which the current activates and inactivates), and the late I_Na during an AP. (B) A schematic illustration of an activation protocol, the resulting currents with peaks indicated by circles, and the peak currents plotted against the test step voltage. (C) A schematic of an inactivation protocol, the resulting currents with peaks indicated by triangles, and the activation (green circles) and inactivation (purple triangles) curves derived from the peak currents. Numbers and shapes are chosen similar to András et al. (2021) for panel A and Nagatomo et al. (1998) for panels B and C. AP, action potential.