Schematic tracings from the teaching model at 25 mm/s. P′ marks a retrograde P wave.
AV nodal re-entrant tachycardia, AVNRT, is a loop. An impulse circles between two inputs into the AV node, and every lap fires both the ventricles and the atria. It is the most common regular narrow-complex tachycardia in adults, and almost everything about it, from the hidden P wave to the pounding in the neck to where the ablation catheter goes, follows from that one small circuit.
If re-entry itself is new, the re-entry page builds the idea on a simple ring of tissue first. This page assumes the three ingredients: two paths, a block in one direction, and a lap long enough for the tissue to recover.
The AV node has two doors
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The compact AV node lies at the apex of Koch's triangle, a patch of the right atrial septum bounded by the tendon of Todaro above, the attachment of the tricuspid valve below, and the opening of the coronary sinus at its base. The impulse does not reach the node by one route. In many people it has two functional inputs:
A fast pathway that enters the node from above, near the tendon of Todaro. It conducts quickly but takes a long time to recover after each beat.
A slow pathway that runs up from below, along the tricuspid valve between the coronary sinus opening and the node. It conducts slowly but recovers quickly.
Those two opposite trade-offs, fast but slow to recover against slow but quick to recover, are the whole story. Picture each pathway as a door that shuts behind every impulse and reopens when the pathway has recovered.
| Fast pathway | Slow pathway | |
|---|---|---|
| Conduction | Fast. A normal PR. | Slow. A long PR when it carries the beat. |
| Refractory period | Long. Shut for a while after each beat. | Short. Open again soon. |
| Where | Enters from above, near the tendon of Todaro. | Runs up from below, between the coronary sinus opening and the tricuspid valve. |
A normal beat is a race
The diagram below is a ladder diagram, the way electrophysiologists draw conduction. Time runs left to right. The rows are the atria (A), the AV node and the ventricles (V). Each line is the impulse crossing a row: the steep blue line is the fast pathway, the shallow orange one the slow pathway. Under it, two bars show how recovered each pathway is. Scroll through one sinus beat.
Into the node
The sinus impulse crosses the atria and reaches the top of the AV node. Both doors are open: both pathways have had a whole cycle to recover.
Two waves at once
It enters both pathways at the same moment. The fast pathway is already far ahead.
The fast pathway wins
The fast wave reaches the bottom of the node first and runs on down the His bundle to the ventricles. The PR interval is normal. Notice the fast door has now shut behind it: its bar has emptied.
The slow wave dies
The slow wave arrives late and finds the bottom of the node already refractory, so it dies there. It never reaches the atria or the ventricles, so it never shows on the ECG. In a normal heart the slow pathway is silent.
An early beat finds one door shut
A premature atrial beat
A premature atrial beat (a PAC) reaches the AV node early, while the fast pathway is still recovering from the last sinus beat.
The fast door is shut
The fast pathway refuses it. Its long refractory period has not finished. A short stub with a bar marks the refusal.
The slow road
The slow pathway recovers quickly, so it is already open again. The impulse takes the long way down.
A long PR
It reaches the ventricles late. On the strip, the early P wave is followed by a long PR interval. In this heart the story ends here, because the fast pathway does not conduct backwards. Many people have two pathways and never have AVNRT.
The jump
Time the PAC yourself. Start late and move it earlier, a step at a time. Each PAC adds a point to the chart: how long it took to cross the AV node (the AH interval, which the PR follows). Watch what happens when you pass the fast pathway's refractory period.
Move the slider to try earlier PACs. Each one adds a point.
The loop closes
Down the slow pathway
The same premature beat, in a heart whose fast pathway can conduct backwards. It finds the fast door shut and goes down the slow pathway.
Time to recover
The slow trip takes so long that, behind it, the fast pathway is recovering. Watch its bar refill.
The turn
At the bottom the impulse goes on to the ventricles, and it also finds the fast pathway open, so it turns and runs back up it. This is the one-way block from the ring, made by timing: the fast pathway was shut on the way down and open on the way back.
An echo beat
It reaches the atria from below and activates them backwards: a retrograde P wave, called an echo beat.
Round again
By now the slow pathway has recovered too, so the impulse goes down it again. The loop is closed and needs no further trigger.
AVNRT
Every lap sends one wave down to the ventricles and one up to the atria at nearly the same moment. One lap is one beat, so the rate is set by the time it takes to go round.
Only some coupling intervals start the circuit. Too late, and the fast pathway carries the beat. Too early, and both pathways are shut. In between is the induction window: early enough to find the fast door shut, late enough to get through the slow one.
Why the P wave hides
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The circuit is tiny and electrically silent: no surface lead can see a few millimetres of nodal tissue. What the ECG records are its two exits. At the bottom of each lap one wave runs down the His bundle into the ventricles while the other turns up the fast pathway into the atria. Both leave from the same place at almost the same time, so the atria and ventricles depolarize together, and the small P wave lands inside the QRS or just at its end.
The same timing explains a sign you can see without an ECG. The atria contract while the ventricles are contracting, against closed tricuspid and mitral valves, so blood is pushed back up the neck veins with every beat. Patients feel it as pounding in the neck, and it can be seen as regular cannon waves in the jugular veins: the frog sign.
Pseudo r prime and pseudo S
The retrograde P wave starts low in the atrial septum and spreads upward and slightly forward. Upward means it points away from the inferior leads, so it is negative in II, III and aVF; forward means it points toward V1, so it is positive in V1. When it lands on the tail of the QRS it adds a small notch that looks like part of the QRS: a pseudo S in the inferior leads and a pseudo r prime in V1. Compare a sinus beat with the tachycardia. The notches vanish when the rhythm stops, which is how you know they were P waves.
Schematic tracings from the teaching model at 25 mm/s. P′ marks a retrograde P wave.
Typical and atypical
In about nine out of ten patients the circuit runs slow down, fast up: typical AVNRT, with the P wave hidden or right after the QRS. It can also run the other way, fast down, slow up. Then the trip up to the atria is the long one, so each P wave lands late, just before the next QRS, inverted in the inferior leads: atypical AVNRT, a long RP tachycardia.
A premature ventricular beat sends a wave back up into the AV node.
Schematic tracings from the teaching model at 25 mm/s. P′ marks a retrograde P wave.
| Rhythm | P wave position | Why |
|---|---|---|
| Typical AVNRT | Hidden in the QRS or at its end RP under 70 ms | Atria and ventricles are activated together from the bottom of the circuit. |
| Orthodromic AVRT | Clearly after the QRS, in the ST segment RP over 70 ms, less than half the RR | The impulse must cross the ventricles before it can climb the accessory pathway. |
| Atypical AVNRT | Late, just before the next QRS Long RP | The trip up to the atria uses the slow pathway. |
| Atrial tachycardia | Before the QRS, any shape Usually long RP | An atrial focus drives the ventricles through the node; the P comes first. |
| PJRT | Late, deeply inverted inferiorly Long RP | Re-entry back up a slowly conducting accessory pathway. |
Breaking the loop
The circuit is running
AVNRT at about 170 beats per minute. Every lap needs both pathways, so the circuit is only as strong as its weakest lap.
One lap blocked
Adenosine, or a vagal maneuver, briefly blocks conduction in AV nodal tissue. The wave coming over the top finds the slow pathway shut.
It ends on a P
Its last act was the retrograde P on the way up, so the run usually ends on a P wave with no QRS after it.
Sinus returns
After a pause the sinus node fires again and conducts normally down the fast pathway. Nothing has changed in the node, so another well-timed premature beat can start it again.
Because AVNRT needs the AV node for every lap, anything that blocks the node for a single beat stops it:
Vagal maneuvers first, in a stable patient. The modified Valsalva, a strain followed straight away by lying flat with the legs raised, converted about four in ten patients in a large trial, more than twice as many as the standard maneuver.
Adenosine next, as a rapid IV push followed by a flush, with a larger second dose if the first fails. It blocks the node for a few seconds, which is plenty.
If those fail, an IV calcium-channel blocker or beta-blocker, and synchronized cardioversion for an unstable patient.
Watching what adenosine does is also a diagnostic test. AVNRT and AVRT stop, because the node is part of their circuit. An atrial tachycardia often keeps going with the ventricles blocked for a moment, which unmasks the P waves, because its circuit lies in the atrium.
Cutting the slow pathway
The cure is catheter ablation of the slow pathway. It is targeted low in Koch's triangle, between the coronary sinus opening and the tricuspid valve, well below the compact node and the His bundle, which sit near the apex. Without the slow pathway there is no second route and no loop, at any timing, while the fast pathway keeps conducting normally. Success is about 95 percent or better. The main risk is damaging the compact node and causing complete heart block, under 1 in 100 when the ablation stays low, and this is why the catheter is kept away from the top of the triangle.
Predict what happens
A PAC arrives after the fast pathway has fully recovered.
What happens?
Key takeaways
Many people have two inputs into the AV node: a fast pathway that recovers slowly and a slow pathway that recovers quickly. Normally the fast one wins and the slow wave dies unseen.
A premature beat that finds the fast pathway shut goes down the slow one, with a jump in the PR. If the fast pathway has recovered by the time it reaches the bottom, the impulse turns back up it, and the loop can keep going.
Each lap fires the ventricles and the atria together, so the P wave hides in or at the end of the QRS: pseudo S in II, III and aVF, pseudo r prime in V1, RP under 70 ms. Atypical AVNRT runs the other way and has a long RP.
Blocking one lap in the AV node stops it, which is why vagal maneuvers and adenosine work. Ablating the slow pathway, low in Koch's triangle, removes the circuit.
Sources this page draws on
The teaching model on this page is tuned so its timings fall within the ranges these sources report.
- Denes P, Wu D, Dhingra RC, Chuquimia R, Rosen KM. Demonstration of dual A-V nodal pathways in patients with paroxysmal supraventricular tachycardia. Circulation. 1973;48(3):549-555.The discontinuous AH curve, the jump, that defines dual pathways.
- Inoue S, Becker AE. Posterior extensions of the human compact atrioventricular node: a neglected anatomic feature of potential clinical significance. Circulation. 1998;97(2):188-193.The anatomy behind the slow pathway.
- Katritsis DG, Josephson ME. Classification, electrophysiological features and therapy of atrioventricular nodal reentrant tachycardia. Arrhythmia & Electrophysiology Review. 2016;5(2):130-135.Typical and atypical forms and their frequency.
- Kalbfleisch SJ, el-Atassi R, Calkins H, Langberg JJ, Morady F. Differentiation of paroxysmal narrow QRS complex tachycardias using the 12-lead electrocardiogram. Journal of the American College of Cardiology. 1993;21(1):85-89.Pseudo r prime, pseudo S and the RP interval in telling AVNRT from AVRT.
- Gürsoy S, Steurer G, Brugada J, Andries E, Brugada P. The hemodynamic mechanism of pounding in the neck in atrioventricular nodal reentrant tachycardia. New England Journal of Medicine. 1992;327(11):772-774.Atrial contraction against closed AV valves.
- Appelboam A, Reuben A, Mann C, et al. Postural modification to the standard Valsalva manoeuvre for emergency treatment of supraventricular tachycardias (REVERT). Lancet. 2015;386(10005):1747-1753.The modified Valsalva against the standard maneuver.
- Page RL, Joglar JA, Caldwell MA, et al. 2015 ACC/AHA/HRS Guideline for the Management of Adult Patients With Supraventricular Tachycardia. Circulation. 2016;133(14):e506-e574.Acute treatment sequence and ablation outcomes.
- Brugada J, Katritsis DG, Arbelo E, et al. 2019 ESC Guidelines for the management of patients with supraventricular tachycardia. European Heart Journal. 2020;41(5):655-720.Acute and long-term management.
- Jackman WM, Beckman KJ, McClelland JH, et al. Treatment of supraventricular tachycardia due to atrioventricular nodal reentry by radiofrequency catheter ablation of slow-pathway conduction. New England Journal of Medicine. 1992;327(5):313-318.Slow pathway ablation.
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