AV Dissociation

  Independent activation of the atria and the ventricles is seen.

  By definition, retrograde conduction from the ventricules to the atria is not seen.

  Atrial activation may be triggered by the sinus node or an ectopic atrial focus.

  Ventricular activation may be triggered by the AV node or a ventricular focus.

  All cases of complete AV block show AV dissociation.

  However, complete AV block is not the only cause of AV dissociation.

  AV dissociation can be continuous or intermittent.




AV dissociation developes when one of the below are present:

  Ventricular rate is slower than atrial rate (there is complete AV block).

  Slow atrial rate with a faster junctional or ventricular rate (
there is no AV block).



Isorhythmic AV dissociation

  Sometimes, the rate of the junctional or ventricular rhythm during AV dissociation is only slightly different from that of the atrial rhythm.

  Transient AV dissociation is caused by
competing atrial, and junctional or ventricular rhythms with similar rates (so-called isorhythmic AV dissociation).

  Temporary atrial pacing will accelerate atrial rate and overdrive the competing junctional or ventricular arrhythmia, demonstrating intact AV conduction.




AV dissociation may be

  Complete or incomplete

  Structural or functional

  Continuous or intermittent




Complete AV dissociation

  Atrial rate is constant.

  Ventricular rate is constant.

  The PR interval varies since none of the atrial beats are conducted to the ventricles.




Incomplete AV dissociation

  Some atrial impulses are conducted to the ventricles.

  This happens if an atrial impulse reaches the AV node when it is not refractory.

  The conducted beat is called
Capture beat.

  Since some atrial beats capture the ventricles, the ventricular rhythm is disturbed.




Causes of AV dissociation
  All cases of complete AV block (3rd degree)

 
Some cases of
- Ventricular tachycardia
- Ventricular pacing rhythm
- Atrioventricular junctional tachycardia
- Accelerated idioventricular rhythm





References:

  Clinical Arrhythmology and Electrophysiology: A Companion to Braunwald's Heart Disease 2nd Edition, Elsevier Saunders, 2012: pp 189-190. ISBN:1455737682, 9781455737680

  Clinical Cardiac Pacing, Defibrillation and Resynchronization Therapy. Ellenbogen, Kenneth A. 4th ed. Philadelphia, PA : Saunders. 2011. pp 323-360.





ECG 1. The ECG above is from a patient with coronary artery disease and severe left ventricular dysfunction.
Ventricular tachycardia (VT) is seen in leads II, C4, C5 and C6.
The P wave activity due to atrioventricular dissociation
supports the diagnosis of VT.
Ventricular rate during VT is higher than the atrial (P wave) rate.

Click here for a more detailed ECG





ECG 2. The Holter tracing above is from a 29 years-old woman with mild mitral stenosis.
The rhythm is isorhythmic AV dissociation.
The AV dissociation is intermittent (some P waves are conducted to the ventricles).
The cause of intermittent isorhythmic AV dissociation in this woman was increased vagal activity.
During the recording of the above tracing, she was experiencing near-syncope.
Her Holter recording also showed infrequent ventricular premature contractions (VPCs).

Click here for a more detailed ECG





ECG 3. Above is an ECG from a hypertensive woman. Four days ago, she had experienced syncope due to acute pulmonary embolism. This ECG shows accelerated junctional rhythm. Heart rate (ventricular rate) is about 83/minute.
Ectopic atrial tachycardia is also seen. Atrial rate (P wave rate) is about 165/minute. P waves are not related to QRS complexes: AV dissociation and complete AV block. Increased voltage suggests left ventricular hypertrophy due to chronic systemic arterial hypertension.

Click here for a more detailed ECG





ECG 4. Above is an ECG from a 25 years-old asymptomatic professional sporter.
It was recorded during her routine sports examination.
Atrioventricular dissociation is seen.
At first glance,
the coincidental P wave preceding the last QRS complex may seem as a delta wave.
However, she has no WPW syndrome.

Dr. Mounir Basalus has donated the above ECG to our website.