What is an isoelectric line?

The isoelectric line of an electrocardiogram designates the flat part of the diagram in between P waves and T waves, according to lecturer Chris Schwirian of Ohio University. The isoelectric line is completely flat, while positive waves are above the line and negative waves are below it.

Between cycles, the ECG recorder returns the base isoelectric line. Sonya Christensen of Andrews University explains that the entire cardiac cycle includes a P wave, a QRS complex and a T wave with interspersed isoelectric lines. The isoelectric line, also known as the base line, is in the middle of the ECG reading. This part of the ECG is most prominent after the last T wave until the next P wave.