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: Soroush Seifirad, M.D.[2] Cafer Zorkun, M.D., Ph.D. [3]
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.
Historical Perspective
In 1894, Dr. Engelman was the first to describe in detail the phenomenon of AV interval lengthening. In 1899, Karel Frederik published a paper on irregular pulses describing impairment of AV conduction and blockage. 1906 Einthiven was the first to present a presentation of normal and abnormal electrocardiograms recorded with string galvanometer. Dr. Ashmar in 1925 studied and described in detail this blocked impulses and their impact on the conduction in the muscle of the heart. In 1952 Dr. Paul Zoll developed first temporary trans-cutaneous pacing.
Classification
There is no established system for the classification of third degree AV block. But AV dissociation can be further classified into two subtypes as AV dissociation by default and AV dissociation by usurpation.
Pathophysiology
Physiologically AV node receives an impulse from the SA node. That impulse gets delayed in the AV node, assuring the contraction cycle in the atria is complete before a contraction begins in the ventricles. From the AV node, the electrical impulse passes through the His-Purkinje system to activate ventricular contraction. When there is a pathological delay in the AV node, it is visualized on an electrocardiogram as a change in the P-R interval. These delays are known as an AV block. No impulses from the SA node get conducted to the ventricles, and this leads to a complete atrioventricular dissociation. The SA node continues to activate at a set rhythm, but the ventricles will activate through an escape rhythm that can be mediated by either the AV node, one of the fascicles, or by ventricular myocytes themselves. The heart rate will typically be less than 45 to 50 beats/min, and most patients will be hemodynamically unstable. This rhythm is unresponsive to atropine and exercise.
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.