AV dissociation

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This is part of: Ventricular Arrhythmias
Third degree AV block with AV dissociation: there is no relation between p-waves and the (nodal) QRS complexes.

During AV dissociation the relationship between P-waves and QRS complexes is lost. AV dissociations is present in:

  • Third degree AV block: the electrical connection between atria and ventricles is blocked
  • Ventricular Tachycardia: the ventricles contract independent of the atria. Sometimes VT with retrograde conduction can result in AV association during VT (especially in slow VT's or when an accessory connection between ventricles and atria is present)

A rare presentation of AV dissociation is called isorhythmic A-V dissociation. The P waves and QRS complexes in this type of arrhythmia have a 1:1 relation. However the intervals are such short that this 1:1 relationship cannot be the result of normal AV conduction. Two forms of isorhythmic A-V dissociation can be distinguished[1]:

  • Pattern I: the P wave fluctuates cyclically back and forth across the QRS complex. Sinus node discharge in this form is influenced by a biological feedback mechanism through the baroreceptor reflex, based on stroke volume and blood pressure.
  • Pattern II, the P wave is in a fairly constant position relative to the QRS complex. It is usually coincides with the QRS complex or appears on the ST segment or first half of the T wave. The mechanism producing synchronization in pattern II type of isorhythmic dissociation is unclear.


  1. Levy MN and Edflstein J. The mechanism of synchronization in isorhythmic A-V dissociation. II. Clinical studies. Circulation. 1970 Oct;42(4):689-99. DOI:10.1161/01.cir.42.4.689 | PubMed ID:11993309 | HubMed [AV]