page_title: Third-degree atrioventricular block
section_title: Cause
hierarchical_section_title: Third-degree atrioventricular block / Cause
caption_attribution_description: English: Rhythm strip showing third degree heart block.
context_page_description: Third-degree atrioventricular block is a medical condition in which the nerve impulse generated in the sinoatrial node in the atrium of the heart can not propagate to the ventricles.
  Because the impulse is blocked, an accessory pacemaker in the lower chambers will typically activate the ventricles. This is known as an escape rhythm. Since this accessory pacemaker also activates independently of the impulse generated at the SA node, two independent rhythms can be noted on the electrocardiogram.
  The P waves with a regular P-to-P interval represent the first rhythm.
  The QRS complexes with a regular R-to-R interval represent the second rhythm. The PR interval will be variable, as the hallmark of complete heart block is lack of any apparent relationship between P waves and QRS complexes.
context_section_description: 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.
  An inferior wall myocardial infarction may cause damage to the AV node, causing third-degree heart block.  In this case, the damage is usually transitory.  Studies have shown that third-degree heart block in the setting of an inferior wall myocardial infarction typically resolves within 2 weeks. The escape rhythm typically originates in the AV junction, producing a narrow complex escape rhythm.
  An anterior wall myocardial infarction may damage the distal conduction system of the heart, causing third-degree heart block.  This is typically extensive, permanent damage to the conduction system, necessitating a permanent pacemaker to be placed. The escape rhythm typically originates in the ventricles, producing a wide complex escape rhythm.
  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. Studies suggest that the prevalence of congenital third-degree heart block is between 1 in 15,000 and 1 in 22,000 live births.
  Hyperkalemia in those with previous cardiac disease and Lyme disease can also result in third-degree heart block.
caption: Leads I and II demonstrating complete AV block. Note that the P waves are not related to the QRS complexes (PP interval and QRS interval both constant), demonstrating that the atria are electrically disconnected from the ventricles. The QRS complexes represent an escape rhythm arising from the ventricle.
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