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Third Degree Atrioventricular Block

What Makes the Heart Beat?

The heart is made up of four chambers. The two upper chambers are the atria, the two lower chambers are the ventricles.  In addition to the upper and lower chambers, the heart also has right and left sides.  The atria fill with blood and pass it to the ventricles. The right ventricle pumps poorly oxygenated blood to the lungs. The left ventricle pumps well oxygenated blood to the body (see ‘Normal Cardiac Circulation’).


What Causes the Heart Muscle to Contract?

Heart muscle contracts after it is stimulated by an electrical impulse.  This starts within a small area of specialized tissue in the right atrium called the sinus node. The sinus node initiates the electrical impulse, which then spreads rapidly through the muscles of both atria, stimulating them to contract. The electrical impulse pauses briefly in a region of specialised tissue called the atrioventricular node (AV node) before spreading down to both ventricles to stimulate them to contract. The electrical activity of the heart can be recorded on an electrocardiogram (ECG).

What Are the Signs of 3rd Degree AV Block?

Dogs with 3rd degree AV block have a low heart rate (20-40 beats/minute) with no ability for this to increase. Hence, the first sign usually noticed is a reduced ability to exercise (as the heart rate is not able to increase to accommodate this). Sometimes collapsing episodes can also occur with activity. In a very sedentary dog, sometimes no signs are noticed by the owner.


How is 3rd Degree AV Block Diagnosed?

You vet will likely find a slow heart rate of 20-40 beats/minute on examination of your pet. To confirm 3rd Degree AV Block, an electrocardiogram (ECG) is required.  The ECG looks at electrical activity of the heart and will confirm the lack of electrical conduction from the atria to the ventricles. An ‘atropine response test’ is then recommended. This is an injection of atropine given under the skin of your pet. If the 3rd degree AV Block is due to primary disease of the AV node, there will be no response to atropine. If your pet’s heart rate does increase with atropine, they may be a secondary, treatable cause of the disease.  To look for potential secondary causes an echocardiogram (heart ultrasound), chest xrays and blood work are recommended.


Is There Treatment for 3rd Degree AV Block?

If your dog is diagnosed with 3rd Degree AV Block and does not respond to atropine, the only treatment available is surgical implantation of a permanent pacemaker. The pacemaker lead is passed from the right jugular vein into the right ventricle. It is then attached to a pacemaker battery placed behind the right shoulder blade. The pacemaker will stimulate your dog’s ventricles to beat at rates of 80-140 beats/minute (depending on level of activity).

At Veterinary Cardiac Referrals, we have done many pacemaker implantations. If successful, pacemaker implantation should restore a normal quality of life and lifespan for your dog.

A normal ECG

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