Sinoatrial Block or Sinoatrial Exit Block

Sinoatrial blocks (also called SA exit block or sinoatrial exit block) result when there is an abnormality in the conduction from the SA node to the surrounding atrium. As with AV block, SA block is characterized as first-, second-, and third-degree, with second-degree blocks subclassified as type I and type II.

First-degree SA block represents an increased time for the SA node’s impulse to reach and depolarize the rest of the atrium to form a P wave. It cannot be seen on the surface ECG because the impulse generation in the SA node do not produce a deflection.

Second-degree SA block can be seen on the surface ECG. It could either be type I or type II.

SA block type I

ECG Recognition:

There is progressive increase in the interval for each SA nodal impulse to depolarize the atrium and create a P wave until an SA nodal impulse does not depolarize the atrium at all.  

On the surface ECG, there is gradual shortening of the P-P interval with an eventual  "dropped" P-QRST complex

There is "group-beating" or may manifest as irregular sinus rhythm with pauses.

Second degree SA block type I or sinoatrial exit block type I. There is group-beating (3 QRS in a group). There is shortening of the PP interval until the missing P QRs complex. 

Sinoatrial exit block type I. There is group beating and shortening of the PP interval until there is a dropped PQRST. There is no evidence of a PAC on all the leads that can explain the sudden bradycardia.

Second degree SA block type II

There is a "dropped" P-QRST complex with the P-P interval surrounding the pause that is two to four time the length of the baseline P-P interval. This is because there is a consistent interval between the SA node impulse and the depolarization of the atrium until the SA nodal impulse is not conducted.

Second degree SA block type II. The surrounding PP interval around the pause is equal to 2x the baseline PP interval. In short, the expected PQRST complex did not show up. 

Sinoatrial exit block type II. There is the complete absence of the expected PQRST. The surrounding pause is equal to 2x the baseline PP interval.

Third degree SA block

Third degree SA block occurs when none of the sinus impulse failed to depolarize the atrium. This will appear as junctional rhythm with no P waves. 

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