Reentry must occur beyond which physiological period?

• A. ERP
• B. APD
• C. DAD
• D. RRP

Answer: (A)

Reentry depends on the wavefront finding tissue that has recovered excitability after its refractory period. The end of that refractory state is the effective refractory period (ERP). If the premature impulse arrives while the tissue is still within the ERP, it cannot propagate and reentry cannot be sustained. Therefore, reentry must occur beyond the ERP, when the tissue is excitable again.

The other terms don’t define this timing: the action potential duration describes how long the cell stays depolarized, not when it can be re-excited; delayed afterdepolarizations can trigger beats but don’t specify the readiness of tissue to conduct again; and the relative refractory period is a window of partial recovery with unreliable conduction, not the point at which complete refractoriness ends.