Sinus block is best described as

• A. SA node fires but impulse is blocked from exiting the node; P waves march out from previous beat
• B. SA node stops firing completely
• C. AV node blocks conduction
• D. There is no P wave

Answer: (A)

Sinus block occurs when the SA node generates an impulse, but that impulse is blocked at the level of the SA node and fails to exit to atrial tissue. On an ECG you’ll see a pause in atrial activity with no P waves corresponding to the blocked impulse, and then the next orderly P waves resume after the pause as the next impulse exits the node. This explains why the beat is skipped even though the SA node continues to fire at its intrinsic rate—the conduction pathway from the node to the atria is momentarily blocked.

This is different from a sinus arrest, where the SA node actually stops firing for a period, leading to missing P waves due to no impulse at all. It’s also distinct from an AV block, where the conduction problem is below the atria at the AV node or His-Purkinje system, resulting in dropped QRS complexes with intact P waves before each QRS. Understanding that the block is at the SA node itself clarifies why a beat is skipped but the rhythm otherwise remains regular.