Arrhythmias (Abnormal Rhythms) in Dogs
What dogs get arrhythmias?
All breeds and ages of dogs can get arrhythmias. Some specific arrhythmias are identified in specific breeds. The cause and the treatment vary widely depending on the diagnosis.
Boxers, bulldogs, German shepherds
A common disease identified in boxers is arrhythmogenic right ventricular cardiomyopathy or ARVC. This is commonly called "Boxer Cardiomyopathy". The arrhythmia seen in these dogs is primarily from the right ventricle, but they may also come from other locations in the heart. Bulldogs also get a variation of this disorder. These ventricular arrhythmias may occur in rapid succession and this is called ventricular tachycardia. When ventricular tachycardia occurs it may lead to a decrease in blood flow to the body. When the perfusion to the brain decreases enough dogs may collapse. This arrhythmias may degenerate into ventricular fibrillation which is a fatal abnormal rhythm. Therefore, some dogs with ventricular arrhythmias must be treated with antiarrhythmics (e.g. sotalol). Most often to determine if this is required electrocardiograms are recorded and 24-hour electrocardiograms are monitored (Holter monitoring). These same tests are used to monitor the response to treatment. In addition to treating the arrhythmias associated with ARVC, dogs need to have other diagnostics to understand the extent of the structural and functional problems in addition to the electrical disorder of the arrhythmia.
German shepherds have an inherited ventricular arrhythmia that affects young dogs between 3 and 24 months of age. Some dogs die suddenly of these arrhythmias most commonly between 5 and 9 months of age. A diagnosis usually requires a Holter monitoring period to catch the dangerous arrhythmia. After 24 months of age the arrhythmias disappear and the dogs are no longer at risk, although when used for breeding they have affected offspring when mated to a dog that has the genetic background of risk.
Great Danes, Doberman Pinschers, Newfoundlands, Irish Wolfhounds, Boxers and other large breeds
Atrial fibrillation may be associated with underlying cardiomyopathy that has resulted in poor heart function and enlargement of the heart or it may occur with when the heart is structurally and functionally within normal range. The diagnosis is made from an electrocardiogram. However, most importantly, for the best care, 24-hour ambulatory electrocardiographic monitoring (Holter monitoring) is required to fully understand the rhythm disturbance. Atrial fibrillation most commonly causes the heart to beat too fast. In dogs, cardioversion may be successful in returning the heart to a normal rhythm, but often the rhythm returns to atrial fibrillation in the dog. Consequently, treatment for atrial fibrillation involves the use of drugs to slow the rate. Such drugs include dilitiazem, atenolol and/or digoxin. It is vital in the management of this arrhythmia that follow-up Holter monitoring be used to optimize treatment with the correct dose. Follow-up echocardiography and radiography also are important to determine the need for other medications.
Sick Sinus Syndrome
West Highland White Terriers, Dachshunds, Miniature Schnauzers, Boxers, Cocker spaniels
Sick sinus syndrome is characterized by a heart rhythm whereby the sinus node (which normally initiates the beating heart) does not discharge an impulse to trigger the heart to contract. As a result the heart literally stops beating. If the heart stops for more than 8 seconds then the dog will collapse/faint. Sometimes the heart will have another part of the heart initiate a beat to rescue the heart from complete arrest. Most of the time the sinus node will eventually start up again to do its job but the dog has a rhythm with many long pauses. Some dogs with sick sinus syndrome have a more constant sinus bradycardia (too slow) because the sinus node has a low firing rate. Other dogs with sick sinus syndrome will have periods of excessive tachycardia (rapid rate) in addition to the pauses or bradycardia. When a dog has clinical signs of sick sinus syndrome it is almost always required that a pacemaker be implanted. The implantation of a pacemaker is today a common procedure in dogs. Veterinary cardiologists who are experienced in the implantation of pacemakers and the programming of these pacemakers can best insure the best treatment for afflicted dogs. The response to treatment is usually very good. In dogs that also have the tachycardia this is treated with medication(s) if it does not subside after pacing.
Any breed, may affect cats too
Normally the heart beat is initiated by the part of the heart called the sinus node. The impulse conducts through the atria and to the junctional point between the atrial and the ventricles. The junction is known as the atrioventricular node or AV node, in short. Here, the conduction is slowed allowing time for the atria to fill the ventricles with blood before the ventricles are triggered to contract. In dogs and cats with heart block the conduction to the ventricles is not just slowed, but eliminated. This means that the ventricles are not told to contract. Most animals will have secondary pacemakers in the conduction system of the ventricles trigger the heart to beat when the conduction through the AV node is stopped. This saves the animal. These secondary pacemakers do not discharge at the same rate as the primary natural pacemaker of the sinus node. Instead, they go slowly. Sometimes dangerously slow. This causes the animals to be weak and even collapse. Heart failure can actually develop. The majority of animals with this type arrhythmia must be treated with a pacemaker. Before a pacemaker is implanted the animals are evaluated thoroughly by echocardiography and other testing (e.g. troponin I concentrations) to determine other underlying problems.
Any breed but often medium to larger dogs
- Some dogs with ventricular arrhythmias or heart block may have underlying inflammation of the heart known as myocarditis. In these situations monitoring of the arrhythmia and other tests (e.g. echocardiography, troponin I, C-reactive protein) are required. Often the cause of the myocarditis cannot be identified, but this disease quite often must be handled carefully because often dogs die suddenly when affected. Treatment includes antiarrhythmic medications and anti-inflammatory drugs such as corticosteroids.