Heart Block (Mobitz Type I) in Dogs

The term heart block refers to a delay or blockage of electrical impulse from SA node (impulse generating cells) to the ventricles. The impulses travel on a specified path known as electrical conduction system. Sinoatrial (SA) node is the name given to specialized cells located in the right atrium which generate the impulse. The electrical impulse stimulates the two atria, causing them to contract and force the blood into ventricles. The impulse passes through AV node (small mass of specialized impulse conducting tissues located on the border between the right atrium and the right ventricle) to the ventricles, contracting them so that blood is pumped out of ventricles into the lungs and the body. 

A second degree heart block is when some electricle impulses reach the ventricles while some do not. Second-degree AV block is more than just a delayed impulse at the AV node. The impulse is delayed and blocked resulting in both conducted and non-conducted P waves on the ECG. This translates into some P waves (electricle impulse) that are followed by a QRS complex (heartbeat) and some P waves that occur without a subsequent QRS complex. Second degree heart block is further divided in Mobiz type I and Mobiz type II based on character of the P-R interval.

Mobiz type I second degree heart block is also known as Wenckebach. In this type of heart block, electrical impulses are delayed more and more with each heartbeat until a beat is skipped. The P-R interval gradually lengthens until a QRS-T is dropped, thus the P-R interval is variable. This is the least serious type of second degree heart block; it may occasionally cause mild dizziness and does not usually require treatment.

The affected dogs are usually asymptomatic and it is commonly an incidental finding. Common signs include weakness, dizziness and syncope. However, if it is caused by overdose of certain drugs like digoxin, bethanechol, physostigmine, pilocarpine, the dog may show vomiting and lack of appetite as well.

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