Baseline on an ECG is described as what?

• A. Line that separates the various cardiac waves; representative of the space between heartbeats while the heart is isoelectric
• B. Line representing ventricular depolarization
• C. Line indicating end of atrial repolarization
• D. Line marking start of P wave

Answer: (A)

Baseline on an ECG is the flat, isoelectric line that appears when the heart’s electrical activity is at rest. It serves as the reference level and marks the space between heartbeats, separating the different waves (P wave, QRS complex, and T wave). This line reflects moments when there is no net electrical activity, so the tracing sits at zero deflection. The line that represents ventricular depolarization is the QRS complex, not the baseline. End of atrial repolarization isn’t shown as a distinct line and is usually masked by the QRS complex, and the start of the P wave is part of atrial depolarization, not the flat baseline.