The isoelectric line after the P wave corresponds to AV node depolarization.

• A. SA node depolarization
• B. AV node depolarization
• C. Atrial repolarization
• D. Ventricular depolarization

Answer: (B)

The interval after the P wave that appears flat on the ECG is the PR segment, and it reflects conduction through the AV node. After atrial depolarization, the impulse slows as it passes through the AV node, providing a crucial delay that allows the ventricles to fill before they are activated. During this transit, there’s no net depolarization underway in the heart tissue detected by surface leads, so the tracing remains isoelectric. That’s why this flat line corresponds to AV nodal conduction (and the associated delay) rather than the initiation of atrial activity or ventricular activation. The SA node activity is manifested as the start of the P wave, atrial repolarization is masked by the QRS complex, and ventricular depolarization appears as the QRS complex itself.