Heart Block (Complete) in Dogs

Third degree atrioventricular block, also known as complete heart block or third-degree heart block, is a disorder of cardiac conduction system in which SA nodes are blocked at AV node and there is no conduction through the atrioventricular node (AVN).

The heart is a pumping organ which consist of four chambers. The upper two chambers are called atria (sigular atrium) while the lower two chambers are known as ventricles. The cardiac impulse-generator or "pacemaker" is sinoatrial (SA) node. The SA node starts the impulse which causes atria (upper two chambers of heart) to contract,  the blood is pumped into ventricles. The impulse travels through atrioventricular (AV) node into the ventricles, causing them to contract and pump the blood into the lungs (right ventricle) and body (left ventricle).

In complete heart block, all impulses generated by SA node are blocked at AV node, resulting in atria and ventricles pumping without any coordination and a complete dissociation occurs. The ventricles are able to contract by virtue of a secondary “escape” pacemaker site (either near the AV node or in the ventricle) which stimulates the ventricles. 

Since there is no conduction between the atria and ventricle, ECG studies usually show a ventricular rate slower (< 40 beats per minute) than the atrial rate (> 60 beats per minute), equating to more P waves than QRS complexes and no relationship between the two. The P-P and R-R intervals are usually constant. The QRS complexes are usually wide and bizarre when the ventricle is supplying the pacemaker, and normal when the escape pacemaker is located in the lower AV junction (above the bifurcation of the bundle of His) in patients without bundle branch block.

A breed predisposition is seen in Cocker Spaniels, Pugs and Doberman. Older dogs are most frequently affected.

Common symptoms associated with third-degree heart block include weakness, difficulty breathing, slow heart rate (bradycardia), fainting (syncope) and exercise intolerance.

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