Heart Block (First-Degree) in Dogs

The heart is composed of four chambers. Upper two chambers are right and left atria (singular atrium) while the bottom two chambers are right and left ventricles. Contraction of heart depends upon electrical impulse generated by a group of specialized cells, known as sinus node or sinoatrial  (SA) node. This small mass of specialised cells is located in the right atrium and produces electrical impulses that keep the heart beating. For normal beating of the heart, the electrical impulse causes the atria to contract, pumping blood into the ventricles; the electrical impulse moves through the atrioventricular (AV) node and into the ventricles, causing the ventricles to contract and to pump blood to the lungs (right ventricle) and the body (left ventricle).

First degree atrioventricular block is a heart disease characterized by delayed or prolonged conduction of the electrical impulse from atria to ventricles. The electrical impulse takes more than normal time to reach ventricles through atrioventricular node. 

This condition is shown on an ECG recording as prolong PR interval. It shows time between main electrical impulse (P wave) and heartbeat (QRS complex).

Young dogs may suffer from first degree heart block due to a high vegal tone (impulses from the vagus nerve that produce an inhibition in the heart beat) while breed disposition is seen in older Dachshunds and Cocker Spaniels.

The dogs suffering from first degree heart block are typically asymptomatic. However, if overdose of digoxin (heart medication) is the cause of this disorder, the dogs may suffer from anorexia (poor appetite), vomitting, lethargy and diarrhea. 

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