Third degree AV block overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Cafer Zorkun, M.D., Ph.D. [2]
Overview
Third degree AV block, also known as complete heart block, is a defect of the electrical system of the heart, in which the impulse generated in the atria (typically the SA node on top of the right atrium) does not propagate to the ventricles.
Causes
Many conditions can cause third degree heart block, but the most common cause is coronary ischemia. Progressive degeneration of the electrical conduction system of the heart can lead to third degree heart block. This may be preceded by first degree AV block, second degree AV block, bundle branch block, or bifascicular block. In addition, acute myocardial infarction may present with third degree AV block.
Third degree heart block may also be congenital and has been linked to the presence of lupus in the mother. It is thought that maternal antibodies may cross the placenta and attack the heart tissue during gestation. The cause of congenital third degree heart block in many patients is unknown.
Epidemiology and Demographics
Prevalence of third degree heart block is 20 per 100,000 in the USA and 40 per 100,000 in the world. It is more commonly associated with advancing age.
Diagnosis
History and Symptoms
Patients with third degree AV block typically experience a lower overall measured heart rate (as low as 28 beats per minute during sleep), low blood pressure, and poor circulation. In some cases, exercising may be difficult, as the heart cannot react quickly enough to sudden changes in demand or sustain the higher heart rates required for sustained activity. Complete heart block associated with a slower pacemaker can result in dizziness, presyncope andsyncope.
Imaging
Echocardiography
Transthoracic echocardiography is used to diagnose cardiomyopathies and valvular heart diseases as causes of third degree heart block. Transesophageal echocardiogram aids in the diagnosis of valvular abnormalities like valve abscesses. Left ventricular function can be determined using an echo, before placement of a pacemaker or defibrillator.
Treatment
Medical Therapy
Correction of reversible causes of the block such as ischemia, medications, and vagotonic conditions should be considered. Treatment may also include medicines to control blood pressure and atrial fibrillation, as well as lifestyle and dietary changes to reduce risk factors associated with heart attack and stroke. Treatment in emergency situations are atropine and an external pacer.