Sinoatrial block

Occurs when the SA node fails to excite the atria uniformly.

May be associated with abnormal intrinsic nodal function, a failure of the SA junction, or a failure of propagation in the surrounding tissue.

3 forms of SA block are first-, second-, and third-degree block.

First-degree SA block is characterized by a delay in the propagation of the action potential from the SA node to the atria.

Unlike first-degree atrioventricular (AV) block, this delay is not reflected in the ECG.

Second-degree SA block is characterized by an occasional dropped P wave), reflecting the inability of the SA node to consistently transmit an action potential to the surrounding myocardium.

In third-degree, or complete, SA block, the ECG is identical to that of sinus arrest, with absent P waves.

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