Heart Block (Mobitz Type 2) in Dogs

Your dog’s heart is similar to your own, having four chambers, which are the two atria on top and two ventricles on the bottom. When the upper heart chambers are not communicating well with the lower chambers, the body (left ventricle) and lungs (right ventricle) are not getting enough blood, this is a condition called heart block (AV block). There are four kinds of heart block, which are first degree, second degree (Mobitz type I), second degree (Mobitz type II), and third degree (complete heart block). A normal heart rate can fluctuate from 60 to 175 beats per minute, but in second degree Mobitz type II, your dog’s heart can skip beats and become slower than 50 beats per minute at times. Unlike type I, this disorder is almost exclusively caused by an underlying disease or disorder, and affects older dogs (over eight years) most often. The causes of type II heart block vary, but heart infection or inflammation are the most commonly found conditions and certain breeds (Airedale Terriers, American Cocker Spaniels, Dachshunds, Doberman Pinschers, and Pugs) seem to be more susceptible than others.

Heart block is a disorder in which the heart’s electrical impulses are not performing the way they should. Second degree heart block has two types, Mobitz type I and Mobitz type II, which are both serious conditions, although type II is the more serious kind that can quickly become a life-threatening emergency if not treated right away. In second degree heart block, some of the electrical signals from the atria (upper chambers of the heart) are not able to get through to the ventricles (lower chambers of the heart), which causes the heart to skip some beats and slow down the heart rate. This type is not as common as second degree Mobitz type I, but should always be carefully ruled out since it is more dangerous and can lead to third degree (complete) heart block. It is most often reported in certain breeds and older dogs, but can occur in any breed of dog at any age.

Some dogs may have no symptoms at all until the heart block becomes constant enough to cause the heart rate to slow considerably. The most often seen symptoms at that point are:

• Extreme sleepiness or tiredness
• Disoriented
• Eating less than normal
• Weight loss
• Diarrhea
• Vomiting
• Fainting
• Collapse

Type II heart block is more often seen in dogs over the age of eight and certain breeds, which are Airedale Terriers, American Cocker Spaniels, Dachshunds, Doberman Pinschers, and Pugs. This can also be a side effect of some types of medications, such as digoxin, or from accidental ingestion of human drugs like calcium channel blockers. There are various other causes of type II heart block, which include:

• Lyme disease
• Myocarditis (inflammatory heart condition)
• Hypertrophic cardiomyopathy
• Metabolic disease
• Cardiomyopathy
• Idiopathic (unknown reason)
• Cancer
• Infections (bacterial, parasitic, or viral)
• Injury or trauma

The veterinarian will ask you to describe the symptoms you have noticed that brought you there as well as background information about your dog’s recent activities, such as visiting the dog park or doggy day care where he can be exposed to other dogs that may have a contagious illness. Be sure to also tell the veterinarian about any unusual behavior or changes in appetite or energy level. A comprehensive physical examination will be done, which will include your dog’s weight, height, body temperature, blood pressure, and they will pay special attention to heart rate and respirations.

To rule out any underlying diseases or illness, the veterinarian will need to perform some laboratory tests. A complete blood count (CBC) can show the amount of cells of each type in the bloodstream and the amount of hemoglobin in the red blood cells. This is important because with type II heart block there will be less oxygen in the blood, which is revealed in the hemoglobin measurement. A blood chemical panel will also be done as well as a urinalysis and blood clotting test. These tests can give the veterinarian a general view of your dog’s health and specific details he may need for diagnosis.

The most important test the veterinarian will do is an electrocardiogram (ECG), which is a recording of the electrical impulses in your dog’s heart. Another important test is an echocardiogram (cardiac ultrasound) which shows the veterinarian the inside of your dog’s chest and heart so he can see how the valves, walls, and chambers are behaving. If the veterinarian still feels he needs more information he may keep your dog overnight for 24-hour observation, or send your dog home with a halter monitor attached. This is like a portable ECG machine inside of a small vest that takes readings of your dog’s heart rate for 24 hours, which you bring back the next day so the veterinarian can read the results.

The most common form of treatment for any dog with type II heart block is surgery to install a pacemaker. The pacemaker is a tiny device placed in your dog’s neck that will give the heart a little stimulation through an electrical lead when it skips a beat or slows down. The veterinarian will also prescribe medication or another treatment to treat the underlying cause.

Your dog’s recovery depends on the underlying cause of the heart block, but chances are usually good when it is caught before progressing to complete heart block. If your dog has a pacemaker, you will be making frequent regular trips to see the veterinarian to be monitor and maintain the pacemaker and its battery.