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Second- and Third-Degree Heart Block Rebecca E. Gompf, DVM, MS, DACVIM (Cardiology) BASIC INFORMATION Description Second-degree heart block occurs when some of the impulses that arise in the small chambers of the heart (atria) are blocked from entering the lower chambers (ventricles). As a result, the heart rhythm pauses, and the ventricles do not contract. If these pauses are frequent, less blood is pumped by the heart, and the animal cannot exercise and may faint. With third-degree heart block, none of the impulses reach the ventricles; the blockage is complete. The ventricles may take over and produce a heartbeat by themselves, but the heart rate is very slow and the heart does respond when the animal becomes active (so it may faint). Causes The cause of these heart blocks may not be found. Second-degree block can arise from increased vagal tone due to irritation of the vagus nerve. Occasionally, high potassium levels (hyperkalemia) can cause heart block, and the block is reversible once the hyperkalemia is treated. Certain heart medications (beta-blockers, calcium channel blockers, digoxin) can slow conduction between the atria and ventricles, and second-degree block can result. If second-degree block is advanced or if complete block is present, then a major problem exists in the conduction system of the heart. Tumors, infections of the heart valves, and Lyme disease may all cause these advanced heart blocks. Most of the time, however, the cause is never found. Clinical Signs Animals with occasional second-degree block often have no signs. If another disease is causing hyperkalemia, signs of that disease are present. Drugs that interfere with conduction in the heart may also cause vomiting, diarrhea, and a lack of appetite. Animals with advanced second-degree block or complete block are not able to exercise and are lethargic. They may also faint when stressed. Diagnostic Tests An electrocardiogram (ECG) is needed to diagnosis these abnormal rhythms. If the ECG is normal and the animal is having fainting episodes, then a 24-hour continuous ECG (Holter monitor) or a 2-week intermittent ECG (event monitor) may be needed to document the cause of the fainting. Animals with second-degree block may undergo an atropine challenge test to determine whether the block will disappear. If it does disappear, then additional tests are usually performed to identify why the animal has increased vagal tone. Causes of increased vagal tone include diseases of the respiratory, digestive, and neurologic systems. If the second-degree block is advanced or does not disappear with atropine, or if third-degree block is present, then additional tests may be recommended, such as laboratory tests, chest and abdominal x-rays, and an echocardiogram (heart ultrasound). TREATMENT AND FOLLOW-UP Treatment Options If second-degree block disappears with the atropine challenge test, no further therapy may be required except for treatment of the underlying cause. If episodes of second-degree block are frequent and cause signs, then drugs such as propantheline bromide, albuterol, terbutaline, or theophylline may be tried. Response to these drugs is unpredictable, and side effects include anxiety, excessive panting, decreased appetite, vomiting, diarrhea, and constipation. If the drugs do not help the clinical signs, then insertion of a pacemaker may be necessary. Animals with third-degree block do not respond to atropine challenge tests or to any of the oral medications listed above. These animals always require insertion of a pacemaker. Follow-up Care If the animal responds to the atropine challenge test and the underlying disease resolves with treatment, no further follow-up may be needed. Periodic ECGs may be done to ensure that the seconddegree block has not recurred. Animals with second-degree block whose clinical signs respond to oral therapy usually have monthly recheck visits and ECGs to make sure the problem is not advancing to third-degree block. Animals that receive pacemakers usually have a follow-up visit with the cardiologist at 1 and 3 months. After the pacemaker has received its final adjustments, you will be asked to monitor the heart rate weekly and to have your veterinarian run ECGs every 3 months. Prognosis Animals whose second-degree block disappears after an atropine challenge test or following resolution of their underlying disease have an excellent prognosis. Animals with advanced seconddegree or complete heart block require pacemaker implantation. Placement of a pacemaker usually results in years of additional life for the patient. The pacemakers that are implanted in dogs and cats have a battery life of about 5 years and are usually replaced after that time. IF SPECIAL INSTRUCTIONS HAVE BEEN ADDED, THEY WILL APPEAR ON THE LAST PAGE OF THE PRINTOUT. Copyright © 2011 by Saunders, an imprint of Elsevier Inc. All rights reserved.