4.3 Junctional Rhythmias & AV Nodal Reentry

Junctional Rhythmias & AV Nodal Reentry

The AV junction, consisting of the AV node and the non-branching portion of the Bundle of His, serves as the heart's secondary pacemaker. When the SA node fails, or when an irritable focus in the AV junction becomes overly excitable, junctional dysrhythmias occur.

Because the impulse originates at the AV junction, it travels down the ventricles normally, resulting in a narrow QRS complex (unless there is a bundle branch block). However, the impulse must travel backward (retrograde) to depolarize the atria. This retrograde conduction dramatically alters the appearance of the P wave. The precise timing of this retrograde activity dictates exactly how the ECG looks.

The Junctional P Wave

The hallmark of any junctional rhythm or beat is the morphology and timing of the P wave. Depending on the exact location of the ectopic focus within the AV junction, the atria and ventricles may depolarize at different times:

1. High Junctional Focus: The impulse reaches the atria before the ventricles. The P wave is inverted in lead II and occurs before the QRS. The PR interval will be short (< 0.12 seconds).
2. Mid Junctional Focus: The atria and ventricles depolarize simultaneously. The P wave is hidden within the QRS complex and is invisible on the surface ECG.
3. Low Junctional Focus: The impulse reaches the ventricles before the atria. The QRS complex occurs first, followed by an inverted P wave (retrograde P wave) after the QRS.

Comparison Table: Junctional Foci

Premature Junctional Complexes (PJCs)

A PJC is a single ectopic beat that occurs earlier than the next expected sinus beat, originating from the AV junction. It frequently occurs due to enhanced automaticity of the AV node triggered by stress, caffeine, or certain medications.

ECG Criteria

• Regularity: Irregular due to the early beat.
• P Wave: Inverted before the QRS, hidden in the QRS, or inverted after the QRS.
• QRS Width: Normal (< 0.12 seconds).
• Pause: Typically followed by a non-compensatory pause. The SA node gets reset by the retrograde depolarization of the atria, causing the next normal cycle to shift.
• Clinical Significance: Often benign, but frequent PJCs can herald the onset of a sustained junctional dysrhythmia.

Categorizing Sustained Junctional Rhythms

Sustained junctional rhythms are categorized purely based on their rate. The morphology of the P wave and QRS remains consistent across all three types.

Junctional Escape Rhythm: Clinical Context

• This rhythm is a lifesaver. It is the heart's natural backup plan when higher-level pacemakers fail (e.g., severe sinus bradycardia, sinus arrest, or complete heart block). This essential fail-safe maintains at least minimal perfusion until higher functions can be restored.
• Treatment: You do not suppress an escape rhythm. Doing so could lead to asystole. If the patient is symptomatic due to the slow rate (40-60 bpm), the goal is to increase the heart rate using Atropine or pacing, addressing the failure of the primary pacemaker.

Accelerated Junctional Rhythm and Junctional Tachycardia

• These rhythms represent active usurpations of the SA node's authority due to irritability in the AV junction.
• Digitalis Toxicity: This is a classic and highly tested cause of junctional tachyarrhythmias. Digoxin increases vagal tone (slowing the SA node) while concurrently increasing the automaticity of the AV node. This combination is a perfect storm for accelerated junctional rhythms and junctional tachycardia. Recognizing this pattern is critical because administering more digoxin or certain other antiarrhythmics can be fatal.
• Common causes include digitalis toxicity (a classic board question), inferior myocardial infarction, acute rheumatic fever, and recent cardiac surgery. Inferior MIs often compromise the blood supply to the AV node (via the RCA), increasing irritability and triggering these rhythms.
• Treatment: Focuses on treating the underlying cause, stopping offending medications (like digoxin), and in the case of fast, symptomatic junctional tachycardia, potentially using amiodarone or beta-blockers under strict cardiological supervision. Antidotes like Digibind may be required in severe digitalis toxicity cases.

AV Nodal Reentry

In addition to enhanced automaticity, the AV node can be involved in reentrant tachycardia, specifically AV Nodal Reentrant Tachycardia (AVNRT). This happens when a premature beat enters the AV node, finding two distinct pathways (one fast, one slow). An electrical circuit loop forms within the AV node itself, spinning rapidly and sending impulses upwards to the atria and downwards to the ventricles simultaneously. This frequently manifests clinically as a narrow-complex Supraventricular Tachycardia (SVT) with hidden P waves. The rapid circuit rate often exceeds 150 bpm and necessitates immediate intervention, such as vagal maneuvers or adenosine, to break the cycle and restore normal sinus dominance.