Junctional Tachycardia
Junctional Tachycardia
An automatic junctional focus above 100 bpm, usually dissociated from the atria and usually not stopped by adenosine.
On the trace
Read the strip in this order.
- Take the rate and the width.
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130 bpm, R-R 462 ms, narrow QRS. A regular narrow-complex tachycardia, which at this point could be any of half a dozen rhythms. - Look for P waves in the gaps.
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They are there, upright, about 0.15 mV, but they do not come with the QRS complexes. Some sit in front of one, some land on a T wave, some disappear into a QRS and come back two beats later. - Time the P waves against each other.
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706 ms apart, every time. That is 85 bpm, and it does not change. - Time the QRS complexes against each other.
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462 ms apart, every time. That is 130 bpm, and it does not change either. - Note that neither divides into the other.
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Two independent pacemakers, each perfectly regular, each ignoring the other. The PR interval is different on every beat because there is no PR interval to measure. - Ask which chamber is faster.
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The ventricles, 130 against 85. Remember that, because it is the finding that rules out complete heart block.
How to recognise it
| Feature | Value | Notes |
|---|---|---|
| Ventricular rate | Above 100 bpm | 130 here. Commonly 100 to 140 in adults, and far faster in postoperative children. |
| QRS | Narrow | The focus is above the ventricles and uses the normal conduction system. |
| Atrial activity | Independent | 85 bpm here, unrelated to the QRS. May instead be retrograde, or absent. |
| Which is faster | The ventricles | The discriminator from complete heart block, where the atria always lead. |
| Onset and offset | Gradual | It warms up and cools down. Re-entrant tachycardias switch on and off between beats. |
| Adenosine | Usually no conversion | It slows the atria and exposes the rhythm without stopping it. |
The strip
Mechanism
A focus in the AV junction develops enhanced automaticity and fires faster than anything else in the heart. It drives the ventricles through the His-Purkinje system, which keeps the QRS narrow, and it would ordinarily also drive the atria backwards. Often it cannot, because retrograde conduction through the node is blocked or too slow, and the atria are left to the sinus node.
So two pacemakers run at once, neither aware of the other. The junction is faster, so it owns the ventricles. The sinus node keeps the atria at its own rate, and its P waves scatter across the strip wherever the junctional beats leave room.
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Automatic, and why that changes everything
AVNRT and AVRT are circuits. A circuit can be interrupted, which is why adenosine terminates them and why they start and stop abruptly. An automatic focus is not a circuit but a piece of tissue depolarising on its own, and there is nothing to interrupt. It speeds up as it gets going and slows as it settles, and adenosine blocks the AV node without touching the focus.
That has a practical consequence worth holding onto. A narrow regular tachycardia that does not respond to adenosine is not a failed diagnosis. It is a finding, and it points here or at atrial tachycardia.
Dissociation against complete heart block
Both show P waves and QRS complexes at unrelated rates, and they are opposite diagnoses. In complete heart block the atria are faster and the ventricles are slow, because the atrial impulses cannot get through and something below has taken over out of necessity. Here the ventricles are faster, because a junctional focus has outrun a sinus node that is working normally and there is no block at all. Compare the two rates before anything else.
Capture beats
Occasionally a sinus impulse arrives when the junction happens to be recovered, conducts normally, and produces a beat with a proper P and PR that interrupts the sequence early. A capture beat confirms that conduction is intact and that this is dissociation rather than block.
The paediatric form
Junctional ectopic tachycardia appears in the first day or two after repair of congenital heart disease, most often after operations near the AV node such as tetralogy of Fallot repair. It is fast, it abolishes atrial kick at exactly the moment a fresh postoperative ventricle needs it, and it is a recognised cause of low cardiac output. Management is unusual: cooling, careful reduction of catecholamines, magnesium, amiodarone or dexmedetomidine, and atrial pacing faster than the focus to restore synchrony. A congenital form exists in infants, is rarer, and is notably resistant to treatment.
Clinical impact
In an adult the rhythm is usually tolerated and its value is diagnostic, pointing at a cause. After paediatric cardiac surgery it is the opposite: the rhythm itself is the problem, and it is a recognised cause of low cardiac output.
Management
Look for digoxin toxicity first in an adult
Check the level and the potassium together. This rhythm is one of the more specific digitalis arrhythmias, and immune Fab is the treatment when toxicity is genuine.
Use adenosine to see, not to cure
It slows the atria briefly and exposes the dissociation. Failure to convert supports the diagnosis rather than refuting it.
Correct what is driving the focus
Ischaemia, electrolyte disturbance, theophylline and beta-agonists all raise junctional automaticity, and the rhythm follows them.
Postoperative JET is managed differently
Cool the patient, reduce catecholamines where possible, give magnesium, and consider amiodarone or dexmedetomidine. Atrial pacing above the junctional rate restores synchrony.
Differential
The one it is usually called. Re-entrant, so it starts and stops abruptly and adenosine terminates it. The P is buried in the QRS rather than marching independently.
Also re-entrant, also adenosine-sensitive, with a retrograde P just after each QRS at a fixed interval. Fixed is the word: nothing here is fixed.
The other automatic narrow tachycardia. It warms up and resists adenosine in the same way, but its P comes before each QRS and conducts.
The mirror image. Dissociated in the same way, but the atria are faster than the ventricles, and the reason is block rather than usurpation.
References
- 2019 ESC Guidelines for the management of patients with supraventricular tachycardia — European Heart Journal, 2020
- Atrioventricular Dissociation — StatPearls, NCBI Bookshelf NBK563205, 2023
- Junctional Rhythm: Background, Pathophysiology and Treatment — Medscape Drugs and Diseases, 2024