In sinus exit block, what occurs?

• A. SA node fails to generate impulse
• B. SA node generates an impulse but exit is blocked
• C. AV node blocks conduction
• D. Ventricles fail to depolarize

Answer: (B)

Sinus exit block means the SA node is firing, but the impulse cannot exit into the atrial myocardium. The SA node continues to generate impulses at its normal rate, yet a block at the exit prevents those impulses from depolarizing the atria for that beat. On the ECG you’ll see a pause longer than the usual cycle with no atrial depolarization for that beat, and then normal P waves and onward conduction resume once the block clears. This is different from a failure to generate an impulse (sinus arrest), from conduction block at the AV node, or from ventricles failing to depolarize.