Atrioventricular Nodal Re-entrant Tachycardia (AVNRT)
Atrioventricular Nodal Re-entrant Tachycardia (AVNRT)
The commonest regular SVT: a re-entry circuit inside the AV node, fast, narrow, and with no P wave visible anywhere.
On the trace
Read the strip in this order.
- Rate and regularity.
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180 bpm, R-R 333 ms, and consecutive intervals differ by under a millisecond. Re-entry around a fixed circuit is as regular as a rhythm gets. - Hunt for a P wave.
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There is none. Not before the QRS, not after it, nowhere on the strip. - Look hard at the segment between the QRS and the T.
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Nothing negative lives there. The lowest point between 70 and 180 ms after the R is +0.02 mV, so the line simply climbs into the T. Orthodromic AVRT puts a clear negative notch in that same window; AVNRT does not. - Measure the QRS.
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Narrow. Everything below the node conducts normally, because the circuit never leaves the node. - Read the absence as the finding.
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The P is not missing, it is simultaneous: atria and ventricles are activated at nearly the same instant, so the retrograde P is drawn inside the QRS and cannot be separated from it.
How to recognise it
| Feature | Value | Notes |
|---|---|---|
| Rate | 140 to 280 bpm | 180 here. |
| Rhythm | Regular | Onset and offset are abrupt, unlike a sinus tachycardia. |
| P waves | Not visible | Or a small terminal deflection that was not there in sinus: a pseudo r' in V1, a pseudo S inferiorly. |
| RP relationship | Very short, RP/RR under 0.5 | In the typical slow-fast form. The atypical form is a long RP. |
| QRS | Narrow, under 120 ms | Unless there is aberrancy, which is usually right bundle branch type. |
| Other | ST depression, R-R alternans | Both common at speed and neither implies ischaemia. |
The strip
Mechanism
The AV node behaves as two pathways rather than one. A fast pathway conducts quickly but takes a long time to recover; a slow pathway conducts slowly but recovers quickly. In sinus rhythm the fast pathway wins and nothing is visible. An atrial ectopic arriving at the wrong moment finds the fast pathway still refractory, so it travels down the slow one instead, and by the time it reaches the bottom the fast pathway has recovered. It turns around, goes back up the fast pathway to the atria, and re-enters the slow pathway. The loop is now closed and turning inside the node.
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The jump
The dual pathways are visible in the lab as a discontinuity. Deliver atrial extrastimuli that are progressively earlier, and conduction time creeps up beat by beat until one extra 10 ms of prematurity produces a sudden jump of more than 50 ms. That is the moment the impulse stops using the fast pathway and switches to the slow one, and it is usually the beat that starts the tachycardia.
Typical, atypical and where the P goes
The slow-fast form is the great majority: activation goes up the fast pathway, so the atria are reached almost as the ventricles are, and the P hides in the QRS. In the fast-slow form, about 5 to 10% of cases, retrograde conduction takes the slow pathway instead, so the P appears well after the QRS and clearly before the next one, giving a long RP tachycardia with P waves negative in II, III, aVF and V6 but positive in V1. A slow-slow form exists too, with both limbs slow.
Why neither chamber is needed
The circuit is entirely within the node, so the atria and ventricles are passengers. That is why AVNRT can, unusually, continue with AV dissociation, and why blocking conduction below the node does not stop it.
When it is wide
Aberrancy is uncommon in AVNRT but does happen, usually as right bundle branch block. A wide regular tachycardia is ventricular tachycardia until proven otherwise; once VT is excluded, the candidates are AVNRT or an atrial tachycardia with aberrancy, AVNRT with a bystander accessory pathway, or antidromic AVRT.
Clinical impact
AVNRT is frightening rather than dangerous in most patients. What it costs is recurrent emergency visits, which is the argument for curing it rather than managing episodes.
Management
Vagal manoeuvres first
A modified Valsalva or carotid sinus massage. The circuit runs through the node, so raising vagal tone can break it outright.
Adenosine
6 mg by rapid IV push, then 12 mg. It blocks the node transiently and terminates most AVNRT. If it does not stop the rhythm but reveals atrial activity instead, the diagnosis was something else.
Unstable: synchronised cardioversion
Uncommon, since the rhythm is usually tolerated, but the rule is the same as for any tachycardia with haemodynamic compromise.
Definitive: slow pathway ablation
Ablating the slow pathway is curative with a high success rate and a small risk of AV block. It is the treatment of choice for recurrent symptomatic episodes.
Differential
The one to separate it from. Orthodromic AVRT is also fast, regular and narrow, but its circuit includes the atria and ventricles in sequence, so a retrograde P is visible on the ST segment. If you can see the P after the QRS, it is not typical AVNRT.
A discrete P before every QRS, with a flat baseline between. It may also persist through adenosine while AVNRT terminates.
Flutter at 2:1 is regular and narrow near 150. Its baseline is a continuous sawtooth; AVNRT has no atrial activity visible at all.
References
- 2019 ESC Guidelines for the management of patients with supraventricular tachycardia — European Heart Journal, 2020
- Classification and differential diagnosis of atrioventricular nodal re-entrant tachycardia — EP Europace, 2006
- 2015 ACC/AHA/HRS Guideline for the Management of Adult Patients With Supraventricular Tachycardia — Circulation, 2016