Action potential parameters measurement.

<p>Shown are APs (top), first and second time derivatives (middle and bottom, respectively). For each beat, the following parameters were automatically measured: MDP (maximum diastolic potential, mV), as the most negatively polarized value reached by membrane potential during each cycle; V_peak (mV), as the most positively polarized value reached by membrane potential during each cycle; APA (action potential amplitude, mV), as the difference between the two preceding parameters; TOP (take-off potential, mV), as the V_m taken at the time of the peak of the second derivative of V_m with respect to time; DD (diastolic depolarization phase, ms), as the time interval between MDP and TOP; DDR (diastolic depolarization rate, V/s), as the average value of the first derivative of V_m over DD (marked in red in the middle panel); APD (action potential duration, ms), as the time between the peak of the first derivative of V_m and the time when V_m reaches the value of −40 mV.