Isorhythmic A-V dissociation

An atrioventricular dissociation characterized by independent and equally beating atrial and ventricular pacemakers, in the absence of a retrograde conduction from the ventricular depolarization to the atria is called as isorhythmic AV dissociation. Isorhythmic dissociation is not common in the general population and it is the most innocent type of 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 block. This allows an independent ventricular pacemaker response like either junctional rhythm (giving a normal or near normal QRS appearance and duration) or idioventricular rhythm (with a more bizarre, wide QRS) 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 not occur and a complete AV dissociation will ensue.
• Both fusion beats and capture beats may be present when either the 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 of 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.^[1]

Life-threatening causes include conditions which may result in death or permanent disability within 24 hours if left untreated.

• Diabetic ketoacidosis
• Digitalis toxicity
• Myocardial infarction
• Myocardial rupture
• Organophosphate poisoning

• Acetylcholine
• Acute coronary syndrome
• Acute rheumatic fever
• Amiodarone
• Antiarrhythmic agents
• Beta-blockers
• Calcium channel blockers
• Cardiomyopathy
• Congestive heart failure
• Digitalis toxicity
• Hypertensive heart disease
• Ischemic heart disease
• Myocarditis
• Sick sinus syndrome
• Sinus arrest
• Sinus bradycardia

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