Atrioventricular block (AV block) occurs when electrical impulses do not travel normally from their origin in the upper chambers of the heart (the atria) to the lower chambers of the heart (the ventricles) to cause a heartbeat. In second degree AV block some impulses are conducted normally, while others are “blocked” at the AV node (located at the junction between the atria and ventricles). In high-grade second-degree AV block more impulses are blocked than are conducted, while in third degree/complete AV block all impulses are blocked. The result is a heart rate that is too slow (bradycardia).
The most common cause of advanced AV block in dogs is progressive fibrosis/degeneration of the AV node, but other causes include inflammation or infection of the heart muscle or the aortic valve (myocarditis and endocarditis, respectively), physical disruption of the AV node secondary to other cardiomyopathy (disease of the heart muscle), electrolyte abnormalities, and exposure to certain toxins or medications.
If the heart rate becomes excessively slow secondary to AV block, the heart has a “back-up” system (called an escape rhythm) that takes over beating for the heart. This escape rhythm usually beats at a slower than normal rate, and is unable to increase the heart rate, even when the body needs a higher heart rate (such as during exercise or stress). Additionally, the escape rhythm may not always be reliable, allowing the animal to go several seconds without a heartbeat. In some cases, the escape rhythm fails completely and sudden death results.
Dogs with high grade second degree AV block often show signs of lethargy, exercise intolerance, and collapse. To alleviate clinical signs and prevent sudden death, the recommended treatment for high grade second degree AV block and third-degree AV block is a permanent pacemaker. When functioning properly, a permanent pacemaker prevents the dog’s heart rate from dropping too low. Medical management of the slow heart rate may be attempted until a permanent pacemaker can be placed, but this is not effective in many cases.
Advanced AV block in cats has been reported in association with primary cardiac diseases, systemic/metabolic diseases (i.e. hyperthyroidism) and because of age-related fibrosis of the AV nodal tissue.
The most commonly reported clinical signs of advanced AV block in cats include labored breathing and collapse, which may appear like seizure activity. However, in one study up to 1/3 of cats were free of clinical signs and advanced AV block was an incidental finding. For cats with clinical signs attributable to advanced AV block, or for cats who have significant structural heart disease that would benefit from maintenance of a normal heart rate, the recommended treatment is placement of a permanent pacemaker. When functioning properly, a permanent pacemaker prevents the cat’s heart rate from dropping too low. In cats the permanent pacemaker is usually placed surgically with the pacemaker lead attached to the epicardium of the apex of the heart (epicardial system rather than transvenous system).
The risk of sudden death appears to be lower for cats than for dogs with advanced AV block, and cats may live for several years with this disease.
High grade second degree AV block