Which statement differentiates sinus node pause from sinus exit block?

Which statement differentiates sinus node pause from sinus exit block?

• A. Pause results from SA node not generating impulse; exit block from SA node impulse generated but blocked exiting
• B. Pause and exit block both due to SA node failure to generate impulse
• C. Both involve AV node conduction delay
• D. Only ventricles are involved

Answer: (A)

The key idea is distinguishing whether the SA node fails to generate an impulse or whether it generates an impulse but cannot exit into the atria. In a sinus pause, the SA node simply stops firing for a brief interval, so no atrial depolarization occurs and a gap (pause) appears until the node resumes pacing. In sinus exit block, the SA node does fire, but the impulse cannot exit to depolarize the atrial myocardium, so there is a pause in atrial activity despite SA node firing. This is exactly why the statement that a pause results from the SA node not generating an impulse is the best description, while an exit block would involve an impulse that is generated but blocked exiting. The other ideas implicate the wrong mechanism or structures (like the AV node or ventricles), which aren’t the primary issue here.

The key idea is distinguishing whether the SA node fails to generate an impulse or whether it generates an impulse but cannot exit into the atria. In a sinus pause, the SA node simply stops firing for a brief interval, so no atrial depolarization occurs and a gap (pause) appears until the node resumes pacing. In sinus exit block, the SA node does fire, but the impulse cannot exit to depolarize the atrial myocardium, so there is a pause in atrial activity despite SA node firing. This is exactly why the statement that a pause results from the SA node not generating an impulse is the best description, while an exit block would involve an impulse that is generated but blocked exiting. The other ideas implicate the wrong mechanism or structures (like the AV node or ventricles), which aren’t the primary issue here.