Atrioventricular dissociation
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 atrial rate rapid than the ventricular rate.
The AV dissociation is said to be complete when captures, either anterograde or retrograde, do not occur, and incomplete, when captures do occur.
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 based on their underlying pathophysiological mechanism.
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. When they both are bradycardic and synchronized, captures will usually not occur and a complete AV dissociation will ensue.
- Both fusion beats and capture beats may be present when either atrial or ventricular rate becomes faster than the other with antegrade or retrograde conduction.
- 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
Interference AV dissociation is a dissociation where either following the slowing of SA node and subsequent independent ventricular pacemaker response or following the acceleration of subsidiary pacemakers (eg, AV junctional or ventricular tachycardia and independent atrial activity), an interference happens between both the impulses moving in opposite direction and towards 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 the subsequent passage of a contradirectional impulse. This phenomenon may be designated as delayed contradirectional interference. Contradirectional interference may be isolated or repetitive, if repetitive, contradirectional interference produces AV dissociation.
- Usually the independent ventricular rate is faster than the atrial rate.
- Both fusion beats and capture beats are found following antegrade and retrograde conduction.
- 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
Complete AV Dissociation following Complete AV Block
It is important to emphasize that complete AV dissociation is not synonymous with complete AV block. Complete AV dissociation indicates that atria and ventricles are completely dissociated, ie, separate pacemakers control the atria and ventricles without anterograde or retrograde captures. But complete AV block is a cause of complete AV dissociation. The diagnosis of complete AV block should be restricted to instances in which the escape ventricular rate is less than 40 beats per minute.
- As a general rule, when complete AV block occasions AV dissociation, the actual atrial rate exceeds the discharge rate of the ventricular escape pacemaker.
- Fusion and capture beats do not occur due to the complete degree of AV block and absence of antegrade and retrograde conduction.
- P waves bear no relation to the QRS complexes. PP and RR intervals are regular but the PR interval varies.