During which phase are cardiac cells most vulnerable to arrhythmias?

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

Answer: (A)

The most vulnerable time for cardiac cells to develop arrhythmias is during repolarization. This window—encompassing the latter part of the action potential as the cell returns to its resting state—is when the membrane is transitioning from excited to resting and refractoriness is changing across the tissue. If a premature impulse hits during this period, it can trigger early afterdepolarizations, especially when the action potential is prolonged (long QT), leading to dangerous rhythms like torsades de pointes. Additionally, differences in how quickly different regions recover excitability during repolarization create dispersion of refractoriness, which sets up reentry circuits that sustain arrhythmias. Depolarization is the rapid upstroke of the action potential with high immediate excitability, while systole and diastole are mechanical phases; the electrical vulnerability that most often leads to arrhythmias arises during repolarization.