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.