Heart Block (Mobitz Type II) in Dogs

The heart has four chambers known as atria (upper two chambers) and ventricles (lower two chambers). In order to contract in synchronization to pump blood through different chambers of the heart and out into the lungs and the body, the heart has an specialized conduction system. There are two nodes (masses of tissue) present in the heart that play an important role in this conduction system. One of these nodes is sinoatrial (SA) node which is a mass of similar cells located in the atrium and its function is to generate electrical impulses. The other node is known as atrioventricular (AV) node which is also a mass of similar cells, located in the right atrium near the ventricles. This node receives impulse from SA node and after a small delay, which allows ventricles to fill with blood from atria, passes the impulse to the ventricles.

Second degree heart block Mobitz type II occurs when an electrical impulse suddenly fails to reach the ventricles. In second degree type I AV block, a progressive delay in impulse conduction causes a skip beat where as in type II AV block, a beat is suddenly missed. This is evident in ECG recording as constant P-R interval before and after the dropped beat. 

Mobitz Type II AV block is less common than Type I but generally more serious. Because electrical impulses can’t reach the ventricles, an abnormally slow heartbeat may result. This condition is more serious since frequency and severity of the block is unpredictable. Type II AV block often leads to third degree AV block.

This condition rarely affects healthy dogs. Older dogs are more commonly affected due to pericardial fibrosis. A breed predisposition is seen in American cocker Spaniels, pugs, and dachshund breeds.

While some dogs may remain asymptomatic, some common signs of this disorder are weakness, lethargy, sudden collapse and syncope (loss of conciousness). However, if this disorder is secondary to digoxin overdose, prevalent symptoms may include vomiting, poor appetite, diarrhea and symptoms related to underlying cause. 

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