Accelerated Junctional Rhythm
Accelerated Junctional Rhythm
The same junctional beat at 60 to 100 bpm, fast enough to outrun the sinus node and take over from it.
On the trace
Read the strip in this order.
- Take the rate.
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75 bpm, R-R 800 ms on every beat. Unremarkable on its own, and that is exactly why this rhythm gets read as sinus. - Look for a sinus P.
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There is none in front of any QRS. A normal rate with no P in front of it is the finding, and it is easy to skim past. - Measure the QRS.
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48 ms, narrow and identical throughout. Supraventricular. - Look in the ST segment.
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The P is there, 98 ms after the R, inverted and 0.14 mV deep. Retrograde atrial activation, from a focus in the junction. - Compare it with the escape rhythm.
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Identical in every respect except the rate. The beat carries no information about which of the two you are looking at. Only the number does.
How to recognise it
| Feature | Value | Notes |
|---|---|---|
| Rate | 60 to 100 bpm | 75 here. Below 60 it is an escape rhythm, above 100 a junctional tachycardia. |
| Rhythm | Regular | Marked variability suggests an automatic tachycardia rather than this. |
| QRS | Narrow, 48 ms here | Identical to the escape rhythm's. |
| P wave | Inverted, after the QRS | 98 ms past the R here. May also precede it with a PR under 120 ms, or hide inside it. |
| Relationship to sinus | Faster than the sinus node | The definition. If dissociation is present, the ventricles are the faster chamber. |
| First thought | Digoxin | The classic association, and the one to exclude before anything else. |
The strip
Mechanism
The junction normally fires at 40 to 60 bpm and is normally beaten to it by the sinus node. Enhanced automaticity raises the junctional rate above its intrinsic range, and once it passes the sinus rate the junction becomes the dominant pacemaker by simply getting there first every time.
Nothing is blocked. The sinus node is healthy and still firing, and its impulses arrive at a junction that has already depolarised and is refractory. The result looks like a takeover because it is one.
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Why digoxin comes first
Digitalis increases automaticity in the junction and slows conduction through the AV node at the same time, which is a combination almost designed to produce this rhythm. Accelerated junctional rhythm is one of the more specific ECG signs of digoxin toxicity, and in a patient taking digoxin it should be treated as that until a level and a clinical assessment say otherwise. The other causes worth naming are myocardial ischaemia, especially inferior, the first days after cardiac surgery, myocarditis, beta-agonists and theophylline.
Digoxin effect against digoxin toxicity
The sagging ST segment and shortened QT of digitalis effect appear at therapeutic levels and mean only that the patient is taking the drug. An arrhythmia means something different. Effect is expected, toxicity is a diagnosis, and this rhythm belongs to the second category.
Isorhythmic dissociation, and the trap in it
When the junctional and sinus rates happen to be close, the two chambers run independently at almost the same rate. P waves then drift slowly in and out of the QRS over many beats rather than staying put, which reads as a varying PR interval. It is not a conduction disturbance and needs a longer strip to see properly. The giveaway is that the P waves never settle into a fixed relationship with the QRS.
What the loss of atrial kick costs
Atrial contraction contributes roughly a fifth of ventricular filling at rest. When the atria are depolarised backwards and out of sequence, that contribution is lost, and they may contract against closed valves. Most people tolerate it at these rates. Someone with a stiff, poorly compliant ventricle may not, and the fall in output can be what brought them in.
Clinical impact
The rate is normal and the rhythm is well tolerated, so nothing about the strip is urgent. The reason to recognise it is that it is a window onto something else, and that something else is often treatable and occasionally dangerous.
Management
Check the digoxin first
Ask whether the patient takes it, then check the level and the potassium alongside it. Hypokalaemia makes a therapeutic level toxic.
Treat the cause, not the rhythm
Hold the offending drug, correct electrolytes, and manage ischaemia. The rhythm resolves when the trigger does.
Digoxin immune Fab for genuine toxicity
Reserved for significant arrhythmia, hyperkalaemia or haemodynamic compromise, not for a raised level alone.
Leave stable patients alone
At 60 to 100 bpm with a preserved blood pressure there is nothing to treat. Antiarrhythmics have no role here.
Differential
The same beat at 40 to 60 bpm. Identical morphology, opposite meaning: a rescue rather than a takeover. The rate is the only thing that separates them.
The same focus above 100 bpm. Usually dissociated rather than retrograde, and it raises a different set of causes.
The rhythm this is most often mistaken for, because the rate is normal. The difference is entirely in whether a P precedes the QRS.
Accelerated too, and also a usurping focus, but ventricular. The QRS is wide, and fusion or capture beats confirm it.
References
- Accelerated Junctional Rhythm (AJR) — Life in the Fast Lane, ECG Library, 2024
- Junctional Rhythm: Background, Pathophysiology and Treatment — Medscape Drugs and Diseases, 2024
- Atrioventricular Dissociation — StatPearls, NCBI Bookshelf NBK563205, 2023