Atrioventricular nodal re-entrant tachycardia (AVNRT) is a regular, fast, narrow-complex rhythm that turns in a circuit inside the AV node. The node has two pathways: a fast one and a slow one. A premature atrial beat finds the fast pathway still recovering and goes down the slow one, then comes back up the fast one and round again. Each lap sends one impulse down to the ventricles and one back up to the atria, so the ventricles beat about 180 times a minute, and the atria are activated at the same moment as them. The ECG shows a regular narrow QRS tachycardia with no P waves before it: the P is hidden in the QRS or shows as a small terminal wave, a pseudo-S in the inferior leads. It is usually harmless to the heart's structure, can be frightening, and is stopped by slowing the node.
Remember
- Regular, narrow and fast: 150 to 250 per minute, QRS under 120 ms.
- No P wave before the QRS: the atria are activated at the same moment as the ventricles, so the P falls inside the QRS.
- The pseudo-S: a small terminal S in the inferior leads that sinus rhythm did not have. It is the P wave.
How to recognise it
The rate, the regularity and the narrow QRS are the criteria of the 2015 ACC/AHA/HRS guideline on supraventricular tachycardia. The pseudo-S is how the typical form is told from the other narrow tachycardias, and it is found by comparing with an ECG from sinus rhythm.
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 and recovers quickly. In sinus rhythm the fast pathway wins and nothing shows.
A premature atrial beat can arrive while the fast pathway is still recovering, so it goes down the slow one. By the time it reaches the bottom of the node the fast pathway has recovered, and the impulse turns round and goes back up it to the atria, then down the slow pathway again. The circuit is closed and turns inside the node: down the slow pathway for about 280 ms, up the fast one in about 50 ms, a lap of about 330 ms.
At the bottom of the node the impulse leaves in two directions at once: down the bundles to the ventricles, and up the fast pathway to the atria. So the atria are activated just as the ventricles are, and the P wave lands on the QRS instead of before it. The atria are activated from below, so the P points upwards and is negative in the inferior leads. Added to the end of the QRS it makes a small S in II, III and aVF that is not there in sinus rhythm: the pseudo-S.
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The jump
The two pathways show in the laboratory as a jump. Give the atria earlier and earlier extra beats and the conduction time creeps up until one beat 10 ms earlier takes more than 50 ms longer: that is the beat that switched to the slow pathway, and it is usually the one that starts the tachycardia.
Typical and atypical
The slow-fast form on this page is the great majority of AVNRT. In the fast-slow form, about 5 to 10%, the circuit runs the other way, down the fast pathway and up the slow one, so the P comes well after the QRS and close before the next one: a long RP tachycardia with a P that is negative in II, III and aVF.
Why the atria and ventricles are not needed
The circuit is inside the node, so the atria and ventricles are passengers. AVNRT can carry on with a ventricle that is not conducting, and a bundle branch block does not stop it.
When it is wide
Aberrancy is uncommon in AVNRT but happens, usually as a right bundle branch block at the fast rate. 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, and antidromic AVRT.
A pseudo-r′ in V1
The same retrograde P can add a small terminal r′ in V1 (and in aVR), which can pass for an incomplete right bundle branch block. If an ECG from sinus rhythm shows no r′ in V1, the r′ in tachycardia is the P wave, and it goes when the rhythm stops. This page draws the pseudo-S; the pseudo-r′ varies from person to person and is not always there.
ST depression and alternans
The ST is often depressed at these rates, and the R waves can alternate in size from beat to beat. Neither means ischaemia, and neither should be treated as an infarct.
Clinical impact
- The rate: the ventricles are driven at 150 to 250 per minute and the atria contract against closed valves.
- Symptoms: palpitations, neck pulsation, breathlessness, sometimes dizziness, with an abrupt start and stop.
- Rarely dangerous: the heart is usually structurally normal, so the rhythm is tolerated.
AVNRT is frightening rather than dangerous in most people. It costs repeated emergency visits, which is the argument for curing it.
Management
Vagal manoeuvres first
A modified Valsalva manoeuvre, or carotid sinus massage. The circuit runs through the node, so raising vagal tone can stop it outright.
Adenosine
6 mg by rapid intravenous push, then 12 mg. It blocks the node for a few seconds and ends most AVNRT. If it only reveals atrial activity, the rhythm was something else.
Unstable: synchronised cardioversion
Uncommon, since it is usually tolerated, but the rule is the same as for any tachycardia that compromises the circulation.
Recurrent episodes: ablate the slow pathway
Catheter ablation of the slow pathway cures most people, with a small risk of AV block. It is the treatment of choice for recurrent symptoms.
Differential
The one to tell it from. It is also fast, regular and narrow, but its circuit runs through the atria and ventricles in turn, so the retrograde P is clear on the ST segment, 70 ms or more after the QRS. In typical AVNRT it is on the QRS.
A P wave before each QRS, with a flat baseline between, and it can carry on through adenosine while AVNRT stops.
Flutter at 2:1 is regular and narrow near 150, but it has a sawtooth in II, III and aVF and a flutter wave in V1. AVNRT has no atrial activity beyond the pseudo-S.
A pathway carrying a re-entry can look the same in tachycardia. In sinus rhythm the short PR and the delta wave tell them apart, which AVNRT does not have.
References
- 2019 ESC Guidelines for the management of patients with supraventricular tachycardia — European Heart Journal, 2020
- 2015 ACC/AHA/HRS Guideline for the Management of Adult Patients With Supraventricular Tachycardia — Circulation, 2016
- Classification and differential diagnosis of atrioventricular nodal re-entrant tachycardia — EP Europace, 2006