How does third-degree atrioventricular (AV) block develop?

Updated: Jul 05, 2018
  • Author: Akanksha Agrawal, MBBS; Chief Editor: Jeffrey N Rottman, MD  more...
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In the heart, normal impulse initiation begins in the sinoatrial node (SAN). The excitation wave then travels through the atrium. During this time, surface electrocardiographic (ECG) recordings show the P wave. Following intra-atrial conduction to the area of the lower intra-atrial septum, this wavefront reaches the inputs to the atrioventricular node (AVN). The AVN then conducts the impulse to the His bundle. The His bundle divides into the right and left bundles, which distribute this impulse to the ventricles.

During atrial, AVN, and His-Purkinje conduction, the PR segment is observed. Heart block occurs when slowing or complete block of this conduction occurs. Traditionally, atrioventricular (AV) block can be classified into first-, second-, and third-degree block.

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