Isorhythmic AV Dissociation: ECG Findings, Treatment, vs 3rd Degree AV Block, AIVR & Isorhythmic Dissociation

Content:

• What is Isorhythmic AV Dissociation?
• ECG Findings
• Treatment
• vs 3rd Degree AV Block
• AIVR
• Isorhythmic Dissociation

What is Isorhythmic AV Dissociation?

Isorhythmic AV dissociation is a cardiac rhythm disturbance in which the atria and ventricles beat independently, but at nearly the same rate. Unlike complete heart block, there is no permanent failure of conduction through the atrioventricular (AV) node. Instead, the atrial and ventricular pacemakers temporarily compete, resulting in a shifting relationship between P waves and QRS complexes on the ECG.

This condition often occurs when the ventricular pacemaker accelerates or the sinus node slows down, causing both rhythms to align closely. Isorhythmic AV dissociation is commonly seen in situations such as increased vagal tone, inferior myocardial infarction, digitalis toxicity, or during anesthesia. In many cases, it is a benign and transient finding rather than a dangerous arrhythmia.

ECG Findings

On ECG, isorhythmic AV dissociation is characterized by independent atrial and ventricular activity with similar heart rates. P waves may appear before, after, or within the QRS complexes, and the PR interval varies from beat to beat. Occasional capture beats or fusion beats may be seen when atrial impulses briefly conduct to the ventricles.

The QRS complexes are usually narrow unless there is an underlying bundle branch block or ventricular rhythm. The key ECG feature that distinguishes isorhythmic AV dissociation is the near-equal atrial and ventricular rates, which may periodically synchronize. Careful rhythm analysis is essential, as this pattern can be mistaken for complete heart block if not interpreted correctly.

Treatment

In most cases, isorhythmic AV dissociation does not require specific treatment, especially if the patient is asymptomatic and hemodynamically stable. Management focuses on identifying and correcting the underlying cause, such as electrolyte imbalance, medication effects, or myocardial ischemia.

If symptoms such as dizziness, hypotension, or syncope occur, treatment may include increasing the sinus rate with atropine or reducing ventricular automaticity. Temporary pacing is rarely needed and is reserved for unstable patients. Unlike high-grade AV block, permanent pacemaker implantation is generally not indicated for isolated isorhythmic AV dissociation.

vs 3rd Degree AV Block

Isorhythmic AV dissociation is often confused with third-degree (complete) AV block, but the two conditions are fundamentally different. In complete AV block, there is no conduction between the atria and ventricles, and the ventricular rate is usually significantly slower than the atrial rate.

In contrast, isorhythmic AV dissociation involves intact but temporarily overridden conduction, with atrial and ventricular rates being similar. The presence of intermittent capture beats and rate variability supports isorhythmic dissociation rather than complete block. This distinction is crucial, as complete AV block often requires permanent pacing, while isorhythmic AV dissociation usually does not.

AIVR

Accelerated idioventricular rhythm (AIVR) is closely related to isorhythmic AV dissociation. AIVR occurs when the ventricular rate exceeds the sinus rate slightly, leading to ventricular dominance and AV dissociation. It is commonly seen during reperfusion after myocardial infarction.

In AIVR, the ventricular rhythm is usually regular with wide QRS complexes, and P waves may appear unrelated. When the sinus and ventricular rates are nearly equal, the rhythm may appear isorhythmic. AIVR is typically benign and self-limiting, requiring observation rather than aggressive treatment.

Isorhythmic Dissociation

The term isorhythmic dissociation broadly refers to any situation where two pacemakers in the heart operate at nearly the same rate, leading to transient AV dissociation. This phenomenon highlights the dynamic interaction between atrial and ventricular pacemakers rather than fixed conduction failure.

Clinically, recognizing isorhythmic dissociation helps prevent misdiagnosis and unnecessary interventions. Careful ECG interpretation, correlation with symptoms, and awareness of clinical context are essential. In most patients, isorhythmic dissociation is a reversible and non-threatening rhythm disturbance.