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 types of heart block, which are first degree, second degree (Mobitz type I), second degree (Mobitz type II), and third degree (complete heart block). This disorder happens due to the electrical impulses not being able to get from the ventricles to the atria and vice versa. A normal heart rate can fluctuate from 60 to 175 beats per minute, but with complete heart block, your dog’s heart rate can drop below 40 beats per minute, causing fainting, collapse, or sudden death. It is a life-threatening emergency, so if your dog’s heart rate is lower than 50 beats per minute you should get to the emergency animal hospital or see your veterinarian immediately.

Third degree heart block, or complete heart block, occurs when the upper (atria) and lower (ventricles) heart chambers are blocked so badly that the electrical impulses are not getting through at all. This does not mean that the blood is not flowing to the heart, it is the electrical impulses that are not getting through. When this happens, there is a pacemaker in the lower heart chambers that will take over until the heart block is repaired. However, the heart rate may drop extremely low (below 40 beats per minute), which can be fatal if not treated immediately.

Often, there may be no obvious signs of heart block besides tiredness, which you may not notice. However, some of the most often reported symptoms are:

Although first and second-degree heart block can be caused by nothing but a vitamin deficiency or intense exercise, a complete heart block almost always has an underlying condition that needs treatment. Some of the most common causes are:

Because a complete heart block is a life-threatening emergency, you should take your dog immediately to an animal hospital or clinic if you cannot get an immediate appointment with your veterinarian. However, let your veterinarian know where you are going and give the name and number of your veterinarian to the animal hospital. The veterinarian will first need to get your dog stabilized before performing an examination or tests. This may include draining any fluid built up in the abdomen by abdominocentesis, oxygen, and IV therapy.

Once your dog is stable, you need to provide the veterinarian with as much information as you can about what led up to the symptoms you have seen. They will also need information about previous illnesses and injuries, shot records, and any changes in behavior or appetite you have noticed.

The veterinarian will then examine your dog thoroughly and check vital signs, weight, height, heart rate, and respiration. He will pay special attention to the heart rate, and will probably hook your dog up to an ECG machine at this point to get an electrocardiogram. This is a recording of the heart’s electrical impulses which is performed by attaching electrodes to your dog’s knees and elbows. The test is not painful and your dog will not feel anything unusual during the procedure. Another test that can help determine the cause of the heart block is an echocardiogram, which is an ultrasound of the heart that gives the veterinarian the ability to see inside your dog’s chest and see the heart walls, valves, and chambers. Some blood tests will be done as well, such as a CBC, blood gas, and chemical panel.

No matter what the cause, surgery will probably need to be done to insert a pacemaker to keep your dog’s heart beating at the right pace. The veterinarian may prescribe medication, such as propranolol or atenolol.

If your veterinarian does not think your dog is well enough to make it through the surgery, he may not want to insert a pacemaker. The veterinarian may choose to give medication and see if your dog’s condition improves enough for surgery. However, without a pacemaker, your dog’s prognosis is not good. With a successful pacemaker implantation, your dog can live a relatively normal life, although you will have to see the veterinarian for regular check-ups.