AV dissociation

Atrioventricular Dissociation (AVD) is dissociated or independent beating of the atria and ventricles. This is a symptom of an underlying rhythm disturbance.

It can either be due to the three causes or a combination:

• When the dominant pacemaker (usually the SA node) slows down, the subsidiary (or back-up pacemaker) “kicks-in”. 

• When a back-up (latent) pacemaker fires at a fast rate and usurps (takes control) of the ventricle. This is what happens in non-paroxysmal AV junctional tachycardia or in ventricular tachycardia without retrograde atrial capture.

• When there is a block at the AV junction that prevents the impulse from dominant pacemaker from reaching the ventricles (as seen in complete heart block). The ventricle will be under the control of either junctional escape beats or ventricular escape beats. Complete heart block is not synonymous with complete AVD.

• Combination of causes, as seen in non-paroxysmal AV junctional tachycardia associated with SA or AV block.

Isorhythmic AV dissociation. The rhythm is sinus with some P waves merging with the QRS (R1 – R6 highlighted with red oval). As we move from the left to the right, you will notice a gradual increase in the P to P rate. A distinct P wave is seen before R7 with a change in QRS morphology (R7-R10 and highlighted by a blue oval). What does it mean? This mean that from R7 to R10, the ventricle is depolarized by the SAN and R1-R6 is depolarized by the AV junction. 

Intermittent AV dissociation. The dominant rhythm is sinus at about 88 bpm but there is an intervening junctional rhythm at a rate of about 45 bpm that intermittently creates AV dissociation. This manifest on the surface ECG as QRS alternans or alternating QRS contour/morphology and distortion of the initial part of the QRS morphology as the P wave merges with the QRS. 

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