Junctional (AV) Rhythm on ECG

What it looks like on ECG

Junctional rhythm is recognized by the combination of a usually narrow QRS and a 'lost' or displaced P wave. Three positions of P are possible:

Often the P wave is negative in the inferior leads II, III, aVF — this is explained by retrograde propagation of the depolarization wave from the AV junction upward to the atria, the direction being opposite to normal (from top to bottom originating from the sinus node). However, such polarity is not mandatory and depends on the specific pathway of retrograde conduction: P morphology in these leads, as well as in aVR and other leads, is variable and depends on the level of the pacemaker source in the AV junction and patient-specific features. For diagnosis, strict inversion in all inferior leads is less important than the temporal relationship between P and QRS and a short PR or RP interval, or the absence of visible retrograde atrial activation altogether.

QRS is usually narrow and identical in shape to the sinus complexes of the same patient because the impulse subsequently travels via the normal His-Purkinje system; with concomitant bundle branch block or aberrant conduction, QRS may be wide. The heart rate in a typical junctional rhythm is 40–60 bpm, which is the intrinsic automaticity rate of AV junction cells. The rhythm is regular, with equal RR intervals.

If independent sinus activity persists simultaneously at a rate close to the junctional rate, P waves may be registered independently of QRS — this is described as isorhythmic AV dissociation, while the ventricular rhythm remains junctional. In this situation, it is necessary to assess the atrial and ventricular rates and the direction of conduction, as well as to exclude complete AV block.

Mechanism

Normally, the pacemaker is the sinus node, which has the highest intrinsic automaticity. AV junction cells also possess automaticity, albeit slower (40–60 bpm), and are normally 'suppressed' by more frequent sinus impulses. Junctional rhythm appears when the sinus node slows down or ceases to generate impulses (including when impulses fail to exit it in sinoatrial block), or when sinus impulses are generated but not conducted to the ventricles in AV block — and the AV junction takes over the pacemaker role via an escape mechanism. The impulse from the junction spreads simultaneously downward to the ventricles (via the normal pathway, so QRS is narrow in most cases) and upward to the atria (retrogradely, so P may be inverted and appear before, within, or after the QRS depending on the speed of antegrade and retrograde conduction, or may not be conducted retrogradely at all).

Causes

• Suppression of sinus node automaticity: sick sinus syndrome, marked sinus bradycardia, sinoatrial block
• Increased parasympathetic nervous system tone (vagotonia, during sleep, in athletes)
• Administration of drugs that slow the sinus node and AV conduction: beta-blockers, digoxin, antiarrhythmics
• Ischemia or inferior wall myocardial infarction involving the AV node/AV junction (often in the right coronary artery territory); less commonly, sinus node dysfunction due to disease of its artery or atria
• Myocarditis, cardiac surgery, electrolyte disturbances
• As a transient phenomenon in healthy individuals during sinus arrhythmia or sinus pause

Differential diagnosis

• Junctional escape rhythm vs. sinus bradycardia — distinguished by the absence or inversion of P before QRS; in sinus bradycardia, P is positive in II and precedes QRS with a normal PR
• Junctional rhythm vs. accelerated junctional rhythm — the boundary is conventional and interpreted alongside the clinical context: AV junctional escape rhythm is typically ~40–60 bpm, accelerated junctional is ~60–100 bpm, above 100 bpm is junctional tachycardia
• Junctional rhythm vs. complete AV block — junctional rhythm is defined by the pacemaker source in the AV junction rather than obligatory synchronous activation of atria and ventricles: retrograde P waves before, within, or after QRS are possible, or the absence of visible retrograde atrial activation, including with independent (isorhythmic) atrial activity. Complete AV block is diagnosed by independent atrial and ventricular activity, absence of a fixed P-QRS relationship, and frequent presence of capture or fusion beats; meanwhile, the ventricular escape rhythm in complete AV block can be either a narrow junctional or a wide idioventricular rhythm
• Inverted P in junctional rhythm vs. ectopic atrial rhythm from the lower atria — the picture is similar, and reliably distinguishing them on ECG is often impossible; the diagnosis of 'junctional rhythm' is made based on the overall picture (short PR or RP, typical rate of 40–60 bpm)
• Junctional rhythm vs. wandering atrial pacemaker — in wandering pacemaker, the shape and polarity of P change from cycle to cycle, whereas in junctional rhythm, P is stably inverted or absent
• A narrow regular rhythm without a visible preceding P wave may be junctional, but this pattern is also possible in atrial fibrillation or flutter with subtle waves, as well as in ectopic atrial rhythm with P hidden within QRS; the diagnosis requires analyzing all leads, regularity, rate, and searching for retrograde P waves

Clinical significance and management

Asymptomatic stable junctional rhythm usually requires no immediate treatment — it is a sign that the sinus node is temporarily or permanently failing to function as the pacemaker, rather than an independent dangerous arrhythmia. However, in the presence of hypotension, syncope, ischemia, acute heart failure, or suspected high-grade AV block, urgent evaluation and treatment according to the symptomatic bradycardia algorithm are necessary, including atropine and/or temporary cardiac pacing as indicated.

The physician should: assess hemodynamics (symptoms of low cardiac output — fatigue, presyncope, hypotension), identify and correct reversible causes if possible (discontinue or adjust the dose of drugs that slow the sinus node, correct electrolytes, search for ischemia), and evaluate whether the junctional rhythm is a manifestation of sick sinus syndrome requiring future pacemaker placement. A short-lived asymptomatic episode during sleep or marked vagotonia may be a benign physiological phenomenon and requires no treatment; persistent, frequent, or symptomatic junctional rhythm requires a search for the underlying cause.

Common interpretation errors

• Mistaking a P wave hidden in QRS for its complete absence and erroneously diagnosing atrial arrest or fibrillation
• Failing to measure the exact heart rate and confusing junctional rhythm (40–60 bpm) with accelerated junctional rhythm (60–100 bpm) — which are findings of different clinical significance
• Forgetting to check leads II, III, aVF specifically for P-wave polarity and position — P changes are less noticeable in I and aVL
• Failing to search for the cause (medications, ischemia, electrolytes), limiting the diagnosis to a descriptive report based on the tracing
• Confusing isolated junctional rhythm with complete AV block or isorhythmic AV dissociation without assessing the relationship between P and QRS and the rates of both rhythms

Frequently asked questions

Related terms

More in the "Supraventricular Arrhythmias" section

Entire section

Sources

• Goldberger's Clinical Electrocardiography, 9th ed.
• Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice, 6th ed.
• Orlov V.N. Guide to Electrocardiography

Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.