Sinoatrial arrest pathophysiology
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
In this disorder, the sinoatrial node is generating electrical complexes, but the electrical activity fails to conduct in the atrium. Sinoatrial automaticity is preserved, and the P waves are generated at a regular rate in a regular pattern which are a multiple of the basic sinus cycle. In patients with sinus exit block, the block is more frequently associated with either an atrial or an AV junctional escape rhythm.
Pathophysiology
First degree SA exit block
There is a conduction delay (a slowing of conduction not a block or completion cessation in electrical conduction) in the impulse traveling from the sinus node to the atrium and there are no discernible changes on the surface EKG.
Second Degree SA exit block
This condition refers to intermittent conduction block (not a delay or slowing of conduction but a completion cessation of conduction) between the sinus node and the atrium.
Type I (Wenckebach phenomenon) sinoatrial exit block
This is an example of group beating. The P-P cycle (time between two P waves) is progressively shortened until there is a pause. This pause is less than twice the shortest P-P interval. The cycle is then repeated. The pause is due to the dropped P wave and measures less than twice the P-P cycle. It is similar to the behavior of the RR intervals in type I second-degree AV block.
Type II second-degree sinoatrial exit block
There is an unexpected drop of the P wave. Following this drop, there is a pause in the sinus cycle which is a multiple of the basic sinus cycle. Blocked atrial premature beats sometimes mimic second-degree sinoatrial block.
Third-degree sinoatrial exit block
This SA block cannot be distinguished from sinus arrest when the sinus node ceases to fire. Under such circumstances, subsidiary pacemakers in the AV junction or ventricles may take over.