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Atrioventricular Dissociation

Background

Atrioventricular (AV) dissociation is a condition whereby the atria and the ventricles activate independently of each other. The normal activation—sinus node followed by the atria, AV node, and then the His-Purkinje system causing ventricular activation—is no longer observed. AV dissociation may occur when a subsidiary pacemaker in the AV node or the ventricle overtakes the sinus node for impulse initiation due to slowing of the sinus node, or it may occur when a subsidiary site (ie, the ventricle) beats faster than the sinus node, such as in ventricular tachycardia. The causes of AV dissociation are important to understand as they impact the treatment plan.

The escape of a subsidiary (latent) pacemaker in cardiac tissue may occur if the dominant pacemaker (the sinus node) slows considerably. A subsidiary pacemaker in the AV junction or below may activate at a faster rate compared to the sinus node and thereby cause AV dissociation without retrograde atrial capture.
 For example, sinus bradycardia, with a very slow sinus rate, may allow the AV junction to become a subsidiary pacemaker and thus activate independently from the sinus node (see the image below). AV dissociation does not imply AV block, but both AV block and AV dissociation can occur concurrently.

Significant slowing of the sinus node allows for a

Significant slowing of the sinus node allows for a subsidiary pacemaker (atrioventricular [AV] junction) to activate, causing AV dissociation.

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In general, AV block is associated with a faster atrial rate than ventricular rate. The P waves, representing atrial conduction, cannot activate the ventricles in complete heart block. In AV dissociation, a block is not necessarily present. If a P wave is properly timed, it may conduct to the ventricle in AV dissociation; this is termed a capture beat. Heart block, which may occur with AV dissociation, is discussed in detail in the Medscape Drugs & Diseases article Atrioventricular Block.

The prevalence of AV dissociation is unknown. No racial preponderance or age predilection exists, and men and women are equally affected.

Types of AV dissociation

There are two types of AV dissociation, complete and incomplete.

Complete AV dissociation

Complete AV dissociation occurs when the atria and the ventricles activate independently from one another and the atrial rate is slower or equal to the ventricular rate. The fact that none of the P waves conduct has more to do with the timing of the P waves in relation to the QRS complex rather than the presence of AV block.

Incomplete AV dissociation

Incomplete AV dissociation occurs when there is either intermittent atrial capture from  the ventricles or intermittent ventricular capture from the atria. During incomplete AV dissociation, some of the P waves conduct and capture the ventricles (ie, interference AV dissociation, see the image below). 

Interference AV dissociation occurs when a well-timed P wave conducts via a nonrefractory AV conduction system. Interference AV dissociation is initiated by slowing of the sinus node due to sinus bradycardia or sinus arrest, thereby allowing an independent subsidiary pacemaker in the junction (narrow QRS complex) or the ventricle (wide QRS complex) to take over ventricular activation.

This rhythm strip reveals interference atrioventri

This rhythm strip reveals interference atrioventricular dissociation, as there is a P wave conducting to the ventricle in the third and seventh beats, whereas the P wave fails to conduct to the ventricle in the other beats.

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In contrast, during isorhythmic AV dissociation, a synchronized dissociation occurs when the atrial and ventricular rates are similar, demonstrating an apparent association of the two cardiac chambers.
 Either slowing of the sinus node discharge rate or the emergence of a slightly faster subsidiary pacemaker controlling the ventricles is the common initiating event. Junctional rhythms may show isorhythmic AV dissociation as the P waves and QRS complexes appear to have a close relationship to one another; however, they are actually activating independently from one another. With close observation of the rhythm strip, the P wave may be either just prior, inscribed within, or seen in the terminal portion of the QRS complex (see the image below). 

Isorhythmic AV dissociation. There is AV dissociat

Isorhythmic AV dissociation. There is AV dissociation with independent atrial and ventricular conduction at similar rates.

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Ventricular tachycardia may occur without retrograde atrial activation (due to complete retrograde block) causing AV dissociation, because the atria and ventricles beat independently and the atrial rate is slower than the ventricular rate. An accelerated junctional rhythm with a slower sinus node rate may also be associated with AV dissociation and retrograde block. 

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