What is the isoelectric line on an ECG?

• A. A line indicating the heart is actively contracting
• B. Horizontal line that represents the absence of electrical activity in the heart (at rest)
• C. The peak of the QRS complex
• D. The end of the T wave

Answer: (B)

The isoelectric line is the flat baseline on an ECG, representing a moment when there is essentially no net electrical activity in the heart for that lead. It serves as a reference point for measuring how big the waves are and how long the intervals take. You see it as a horizontal segment between waves—commonly between the P wave and the start of the QRS complex (the PR segment) and during other quiet moments. It does not indicate active contraction; that is signaled by the QRS complex, the sharp deflection that shows ventricular depolarization and contraction. The end of the T wave marks the completion of ventricular repolarization, not the baseline. So the isoelectric line is the flat line that reflects electrical quiet, not a moment of contraction or a peak or end of a wave.