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