Cardiac Electrical Failure in Dogs

The sinoatrial node (SA Node or SAN), also called the sinus node generates electrical impulses within the heart, triggering the heart to beat or contract. Sinus arrest is a disorder of heart beat impulse caused by the failure of the sinoatrial (SA) node to initiate an impulse. Sinoatrial block is a condition when an impulse formed within the sinus node fails to be conducted through the heart or it is delayed. Usually the rhythm of the sinus node is not disturbed in a Sinoatrial block.

Sinoatrial block is classified into first, second, and third-degree SA block. It is difficult to diagnose first and third-degree SA block from an electrocardiogram (ECG). Second-degree SA block is the most common type of SA block and can be recognized with an ECG. There are two types of second-degree SA blocks: Mobitz type I or Wenckebach periodicity and Mobitz type II.

First-degree sinoatrial block represents slowed conduction

Second-degree sinoatrial block is an intermittent Failure to conduct. Mobitz type I/Wenckebach periodicity is the condition when the speed of conduction slows gradually until failure of impulses to reach the atria occurs. Mobitz type II intermittent block is all, or none, until complete conduction failure takes place. While this can be identified on ECG, the two types cannot be differentiated on a surface ECG. 

Third-degree sinoatrial block is a complete failure to conduct

Depending on the degree the condition could present with some common symptoms or could be asymptomatic. Common symptoms may include weakness, fainting, pale gums, very slow heart rate and arrhythmias.

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