There are many different kinds of arrhythmias in dogs. Some will primarily affect large breed dogs while others might affect specific breeds. Boxer dogs are most likely to develop arrhythmogenic right ventricular cardiomyopathy or ARVC, a kind of arrhythmia that can be fatal if left untreated. Boxers are also at risk of developing atrial fibrillation, as are Great Danes, Doberman Pinschers, Irish Wolfhounds, and Newfoundlands. ARVC and sometimes atrial fibrillation can be treated with medications that slow down the heart rate. Smaller breeds like Cocker Spaniels, Dachshunds, Miniature Schnauzers, and Westies are at a higher risk of developing sick sinus syndrome, which causes such a slow heart rate that surgical implantation of a pacemaker is necessary.
Structural changes to the heart can cause arrhythmias. For example, in dogs with dilated cardiomyopathy (DCM), the walls of the heart become very thin and literally dilate outwards. Because of this, the heart cannot pump blood efficiently, and the body becomes deprived of oxygen. As a result, the heart is asked to pump harder, and the heart rate becomes so fast that arrhythmias like supraventricular tachycardia (SVT) and atrial fibrillation can occur.
Infection or inflammation of the heart, akamyocarditis can increase the risk of ventricular arrhythmia or a heart block. Aheart block is when the electrical signal that moves through the heart to make it contract ends up being stopped. Most dogs can overcome this stop in signal thanks to a natural “secondary pacemaker” but this still results in a slow heart rate. Dogs with heart blocks, therefore, can appear very lethargic. Neurologic disease, respiratory disease, gastrointestinal diseases, and certain medications (especially those associated with general anesthesia) can increase the risk of heart blocks in dogs.