In a reentrant circuit, what is the key usefulness of the excitable gap?

• A. It allows a premature stimulus to terminate the reentry
• B. It has no functional significance
• C. It sustains the tachycardia
• D. It eliminates the circuit

Answer: (A)

The excitable gap is the portion of the reentrant circuit that has recovered enough to be excited again, while the rest of the loop remains refractory. Its usefulness lies in providing a timing window for a premature impulse to enter the circuit and disrupt the circulating wave. When a premature beat occurs during this gap, it can propagate into the loop and collide with or preempt the ongoing wavefront, interrupting the reentrant cycle and terminating the tachycardia. If the timing misses the gap, the premature impulse may fail to affect the circuit or even fail to propagate.

Thus, the excitable gap enables termination of reentry by appropriately timed external stimulation.