During which phase are cardiac cells most vulnerable to injury or arrhythmias?

• A. Depolarization
• B. Systole
• C. Repolarization
• D. Diastole

Answer: (C)

Repolarization is when the heart muscle cells reset their resting electrical state after a beat. During this phase, the cells pass through the relative refractory period, meaning they’re not fully available to fire again unless a stronger-than-normal stimulus arrives. Because recovery is uneven across the tissue, some cells can be re-excited while others haven’t finished resetting, creating the setup for abnormal impulses to propagate or trigger reentrant circuits. The ECG’s T wave marks ventricular repolarization, so a premature beat that lands during the latter part of repolarization can provoke dangerous arrhythmias. That’s why this phase is the most vulnerable.