Junctional rhythm on ECG

Mechanism of origin and role in cardiac function

Junctional rhythm belongs to the group of heterotopic (ectopic) rhythms. Normally, a regular rhythm is set by the sinus node, which is confirmed by the presence of a P wave of normal polarity preceding each QRS complex. However, under pathological conditions, the automaticity of higher centers may be suppressed.

The pathophysiological consequence of decreased or cessation of SA node automaticity is the activation of escape ectopic rhythms (from the atria, the AV junction, as well as atrial fibrillation or flutter). In this case, impulses from the AV junction act as a protective mechanism.

Often, junctional rhythm manifests as "escape beats" (escape beats), which are subsidiary escape impulses from a lower pacemaker center that occur after a pause when the primary pacemaker is delayed.

Main ECG criteria

You can recognize an AV junctional rhythm on an electrocardiogram by three key features:

1. P-wave alteration. Since the impulse originates in the AV node, it can propagate to the atria retrogradely. On the ECG, this manifests as the absence of the P wave, its superimposition on the QRS complex, or the formation of a retrograde (inverted) P wave in leads II, III, and aVF.
2. Ventricular complex. Conduction to the ventricles proceeds via the standard pathway, so the QRS complex remains narrow and unchanged.
3. Heart rate (HR). The intrinsic automaticity of the AV junction is lower than that of the sinus node, so the rhythm rate is usually 40 to 60 impulses per minute (for comparison: normal isolated sinus bradycardia is diagnosed at an HR <60 bpm, whereas inappropriate sinus tachycardia is diagnosed at an HR >100 bpm).

Complex rhythm disorders involving the AV junction

Atrioventricular node function can be involved in more complex electrophysiological processes:

• Dissociation with interference: simultaneous independent activity of two pacemakers (e.g., sinus and AV nodal) without mutual suppression.
• Wandering supraventricular pacemaker: gradual shifting of the rhythm source between the SA node, various areas of the atria, and the AV junction. In this case, the shape, amplitude, and polarity of the P wave change from cycle to cycle on the ECG, accompanied by variability in the P-R and R-R intervals.

Differential diagnosis

Junctional rhythm must be differentiated from other automaticity disorders:

• Idioventricular (ventricular) rhythm: the source of impulses is located within the ventricles themselves. Unlike junctional rhythm, the QRS complexes are wide and deformed, the preceding P wave is absent, and the rate is critically low (typically 20–40 bpm). It serves as an escape rhythm when higher centers are severely suppressed.
• Slow atrial rhythm: the ectopic focus is typically localized in the left atrium. Infrequent excitation impulses are observed (less than 70–80 per minute).
• Sick sinus syndrome (SSS): persistent suppression of SA node function. It may manifest as tachycardia-bradycardia syndrome (alternating bradycardia and tachycardia), episodes of sinus arrest (sinus arrest), and slow recovery of sinus rhythm after termination of tachycardia.

Class and exam questions

What else to study on this topic

• Sinus node dysfunction (SND) and classification of sinoatrial blocks.
• Sick sinus syndrome (SSS) and tachycardia-bradycardia syndrome.
• ECG signs of a wandering supraventricular pacemaker.
• Differences between inappropriate sinus tachycardia and atrioventricular nodal reentrant tachycardia.
• Idioventricular rhythm: criteria for escape rhythm during suppression of automaticity centers.

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