Atrioventricular AV Dissociation

AV is a condition in which separate and independent pacemakers drive the atria and ventricles. It is not a primary rhythm disturbance but is secondary to another conduction or rhythm abnormality. There are two varieties of AV dissociation: passive (default or "escape") and active (usurpation).

Passive AV dissociation occurs when an impulse fails to reach the AV node because of sinus node failure or block. Usually an escape rhythm takes over and paces the ventricles. When the sinus node recovers, atrial activity resumes, but there may be a period during which the ventricles are still driven by the escape pacemaker and the P waves and QRS complexes occur independently of each other (Fig 24-25).

FIG. 24-25. Passive AV dissociation, secondary to third-degree AV block.

Active AV dissociation occurs when a lower pacemaker accelerates to usurp the sinus node and captures the ventricles but the atria are still paced as before ( Fig.

In both varieties of AV dissociation, fusion beats are common. It is also common for the two pacemakers to operate with nearly identical rates, possibly as a result of mechanical or electrical influences that tend to keep them in phase with each other—a condition termed isorhythmic dissociation.

CLINICAL SIGNIFICANCE Passive AV dissociation occurs when the sinus node discharge rate is slowed by sinus bradycardia, sinus dysrhythmia, SA block, or sinus pause. Common causes of this include (1) ischemic heart disease (especially acute inferior myocardial infarction), (2) myocarditis (especially acute rheumatic fever), (3) drug toxicity (especially digoxin), and (4) vagal reflexes. It may also be seen in well-conditioned athletes.

Active AV dissociation occurs when the automaticity of lower pacemakers is enhanced. Common causes include myocardial ischemia and drug toxicity (especially digoxin). This form of AV dissociation is seen only when the lower tachycardiac rhythm is not conducted to the sinus node with capture of the SA node. Ventricular tachycardia is the classic example of an active AV dissociation.

TREATMENT

1. Most occurrences of AV dissociation have an acceptable heart rate and are well tolerated.

2. Therapy, if any, is directed toward the underlying cause.

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