Heart Block
Heart block is another term for atrioventricular block.
If the cardiac impulse is delayed or blocked intermittently in the area of the atrioventricular (AV) node, we refer to it as either first or second degree AV block.
Second degree AV block is further divided into Mobitz types I and II.
First and Mobitz type I second degree AV block are frequently caused by diseases which increase vagal tone, including neurologic, respiratory and gastrointestinal diseases, and various drugs (e.g. digitalis, xylazine, acetylpromazine). If related to drug toxicity, the dosage of the drug should be altered or the drug stopped. Otherwise, treatment is not generally required.
The more advanced Mobitz type II second degree AV block has the potential to progress to complete heart block. It is associated with various conditions including hypertrophic cardiomyopathy in dogs and cats, cancer, and electrolyte disorders. Sometimes, clinical signs such as weakness, collapse (due to poor circulation in the brain when the heart rate is too slow), or congestive heart failure (CHF) may develop. If type II AV block produces a slow heart rate with weakness or collapse, your veterinarian may recommend starting your animal on a drug such as atropine, theophylline, or propantheline. Sometimes, a pacemaker may be required.
In the most serious form of AV block, the electrical impulse is completely blocked in the region of the AV junction. This is often associated with the clinical signs mentioned above. There are a large number of diseases which can produce third degree of AV block, in addition to disturbances in the potassium levels in the blood. In those patients which demonstrate weakness, collapse, or CHF (coughing, difficulty breathing, rapid breathing, coughing), the most effective treatment is a permanent cardiac pacemaker. In dogs, a heart rate of less than 40 beats per minute frequently produces clinical signs. Alternatively, various drugs may be tried such as those used for type II AV block, in addition to corticosteroids (anti-inflammatory).
In order to consider pacemaker therapy, your veterinarian should establish a complete database, including a thorough physical exam, complete blood count, chemistry profile, electrocardiogram (ECG), thoracic radiographs, and echocardiogram. Cats should additionally be tested for Feline Leukemia Virus, Feline Immunodeficiency Virus, and hyperthyroidism (if over age 6). Holter monitoring (a 24 hour continuous ECG recording) or an event recorder(records ECG during collapse and can be worn for up to one week at a time) may be recommended when the major problem in your pet is collapse