Atrioventricular dissociation

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Vendhan Ramanujam M.B.B.S [2]

Synonyms and keywords: AV dissociation

Overview

Atrioventricular (AV) dissociation is an electrocardiographic finding with features of independent functioning of atria and ventricles. It is to be remembered that it is only a descriptive term and not a diagnostic endpoint because the AV dissociation that appears in the electrocardiogram is secondary to some other underlying cardiac rhythm disturbance. To be accurate, AV dissociation means that the normal association between atrial and ventricular contraction no longer exists and they are independently driven by different pacemakers either for a single beat or forever.

Pathophysiology

The three key underlying pathophysiological mechanisms that would lead to AV dissociation are

• Slowing of the dominant atrial pacemaker (SA node) to an extent that would allow independent ventricular pacemaker (junctional or ventricular rhythm) responses.
• Acceleration of latent pacemakers at junctional or ventricular site (increased automaticity) that would take independent control of ventricles by exceeding the intrinsic normal atrial rate without a retrograde atrial capture.
• Although controversial yet functionally appealing, a complete heart block would feature with independent activities of atria and ventricle with either atria or ventricle rates rapid over one another.

Classification Based Upon Underlying Pathophysiological Mechanism

AV dissociation is never a primary disturbance of rhythm but rather a consequence of some other underlying rhythm disturbance due to different disorders. To understand this descriptive term better, it must be further classified by stating what pathophysiological mechanisms have produced it.

Isorhythmic AV dissociation

Isorhythmic AV dissociation is a AV dissociation initiated by slowing of SA node due to sinus arrhythmia, sinus bradycardia, sinus arrest, or sinoatrial exit block due to bradycardia. This allows an independent ventricular pacemaker response like either junctional (giving a normal or near normal QRS appearance and duration) or idioventricular (with a more bizarre, wide QRS) rhythm to take over the ventricles. In the presence of some degree of antegrade and retrograde atrioventricular block, there is a synchronization of independently beating sinus or atrial pacemaker with the junctional or ventricular pacemaker such that each discharges in the absolute refractory period of the other.

• Both the independent atrial and ventricular rates are bradycardic and nearly identical, in contrast to other types of AV dissociation.
• Both fusion beats (beats originating from both the ventricle and atrium simultaneously activate the ventricular myocardium, resulting in a QRS complex that has a morphology intermediate between both) and capture beats (beats where the normal conduction system has momentarily "captured" control of ventricular activation from the VT focus) may be present.
• Both P waves and the QRS complexes look related with the P wave moving closer to and then farther away from the QRS, maintaining an illusion of a normal atrioventricular conduction sequence. Occasionally, the P wave might move into and get buried within the QRS complex, only to move back out again in front of the QRS in the subsequent beats. The two pacemakers will remain independent as long as the SA node rate is bradycardic.

• When this rhythm occurs intermittently with normal sinus rhythm, it is called accrochage.
• When the isorhythmic dissociation is persistent, it is called as synchronization. Synchronization has two distinct patterns like, the pattern which is characterized by a rhythmic fluctuation of the interval between the P and QRS waves, most often the P wave oscillating gradually back and forth across the QRS; that is, with periodically varying P-R and R-P intervals. In the second pattern, the P-R or R-P interval do not undergo rhythmic fluctuations, but the P and R waves are in a relatively fixed position with respect to each other.

Interference AV dissociation

In interference AV dissociation, following the slowing of SA node and subsequent independent ventricular pacemaker response, there is an interference between both the impulses moving in opposite directions and toward each other, that is contradirectional. The interference may be direct, in which case the two waves of excitation meet head on and mutually obliterate each other. However, if one of the impulses precedes the other and is blocked, it may nevertheless set up a refractory period for subsequent passage of a contradirectional impulse. This phenomenon may be designated as delayed contradirectional interference. Contradireetional interference may be isolated or repetitive, if repetitive, contradirectional interference produces AV dissociation.

• The independent ventricular rate is faster than the atrial rate.
• Both fusion beats and capture beats are found.
• P wave appears to run up to the QRS complex, come abreast of it and finally pass it. There will be a progressive shortening of P to R time. When the P wave passes a QRS complex the following events may happen in subsequent beats.

• Ventricular capture
• Atrial capture
• Synchronization or accrochage