isoelectric zero line

What is going on when there is an isoelectric zero line?

The ECG records voltage differences. There are two situations in which there is no difference in voltage between the myocardial cells: at rest, when the ventricular myocardium is positively charged; and at the time of maximum excitation, when the entire ventricular myocardium is negatively charged. So at these stages of the cardiac cycle, no electrical difference is present between the myocardial cells. Consequently, there is no vector. When the heart is in this state, the ECG records an isoelectric zero line.