Third degree AV block pathophysiology

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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]; Raviteja Guddeti, M.B.B.S. [4] Qasim Khurshid, M.B.B.S[5]


Normally SA node generates impulses that travel to the AV node and gets delayed there to assure that the contraction cycle in atria is complete before a contraction begins in the ventricles. From the AV node, the impulses pass through the His-Purkinje system to cause ventricular contraction. Pathological delay in the AV node 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 rate, 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 mostly be less than 45 to 50 beats/min, and most patients will be hemodynamically unstable.



The normal physiology of the electrical activity of the heart can be understood as follows:

  • Normal impulse is generated in the sinoatrial node (SAN).
  • Electrical pulse then travels through the atrium.
  • P wave is recorded in the ECG
  • Wave reaches the atrioventricular node (AVN).
  • Atrioventricular node later conducts the impulse to the His bundle.
  • The bundle of his again gets divided into the right and left bundles, ultimately conducting this impulse to the ventricles.
  • PR segment is recorded (atrial, AVN, and His-Purkinje conduction)
  • Complete heart block occurs when complete block of this conduction occurs.


    • In complete heart block, because the impulse is blocked, an accessory pacemaker below the level of the block will typically activate the ventricles; this is known as an escape rhythm.
    • When there is no electrical connection between atria and ventricles, two independent pacemakers will generate impulse independent of SA node. EKG will show two rhythms independent of each other
      • One independent pacemaker will activate the atria and create the P waves with typically with a regular P to P interval.
      • The second independent pacemaker in ventricles will activate the ventricles and produce the QRS complex with typically regular R to R interval.
      • The PR interval will be a variable that is a hallmark feature of complete heart block and with no apparent relationship between P waves and QRS complexes.
    • Morphology of the QRS complex helps in determining the location at which the escape rhythms are occurring.
    • If the site of complete heart block is at the level of the AV node, two-thirds of the escape rhythms have a narrow QRS complex.
    • If the site of block is the His bundle, typically a narrow QRS complex is seen.
    • Patients with trifascicular block have a wide QRS complex (seen in 80% of the cases).
    • In short, if escape rhythm has a narrow QRS complex, the level of the block can be either AV node or His bundle, and if the QRS duration is prolonged, the level of block is in the fascicles or bundle branches.
    • Block at the level of the AV node gives rise to an escape rhythm that generally arises from a junctional pacemaker with a heart rate of 45-60 beats per minute. Such patients are hemodynamically stable.
    • Escape rhythms arise from the His bundle or bundle branch Purkinje system at rates slower than 45 beats per minute when the block is below the AV node.
    • These patients are hemodynamically unstable, and their heart rate is unresponsive to exercise and atropine.


  • Third degree AV block is the result of ischemia in majority of the patients.
  • In certain disease like lyme disease also we might observe AV block.
  • Nevertheless, there are some rare cases of idiopathic AV block
  • In those cases, third degree AV block is transmitted in autosomal dominant pattern most of the time.
Genes involved in the pathogenesis of third degree AV block
  • NKX2.5
  • PRKAG2
  • SCN5A
  • KCNJ2 (type 1 Andersen-Tawil syndrome) [1]

Associated Conditions

Conditions associated with third degree AV block include:

AV dissociation

AV dissociation is defined as:

  • Independent atrial and ventricular activation either due to complete heart block or as a result of physiologic refractoriness of conduction tissue.
  • It also may develop when the atrial/sinus rate is slower than the ventricular rate (accelerated junctional tachycardia or VT).
  • This is called isorhythmic AV dissociation.
  • Acceleration of the atrial/sinus rate with either maneuvers or medications will result in restoration of normal conduction.


  1. Benson DW (2004). "Genetics of atrioventricular conduction disease in humans". Anat Rec A Discov Mol Cell Evol Biol. 280 (2): 934–9. doi:10.1002/ar.a.20099. PMID 15372490.

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