Early afterdepolarizations (EADs) typically occur during which phases of the cardiac action potential?

• A. Phase 0
• B. Phase 1
• C. Phase 2 or Phase 3
• D. Phase 4

Answer: (C)

Early afterdepolarizations arise when the cardiac action potential is abnormally prolonged, so the depolarizing event occurs during the later part of repolarization. Specifically, they appear during the plateau (phase 2) and can carry into the early repolarization (phase 3). When the membrane potential remains depolarized long enough, inward currents reappear—most notably L-type calcium channels can recover from inactivation and reopen, creating a small inward calcium current that creates a new upward deflection during the repolarization phase. This premature depolarization interrupts the normal repolarization and can trigger arrhythmias, especially when the action potential duration is extended by drugs, electrolyte disturbances, or bradycardia.