Orthodromic atrioventricular re-entrant tachycardia (AVRT) is a fast, regular, narrow-complex rhythm that turns in a loop through the AV node and an accessory pathway: down the node to the ventricles, back up the pathway to the atria, and down the node again. It needs a pathway, often one that also pre-excites sinus beats (the WPW pattern), and is started by a premature atrial beat that finds the pathway still recovering. The ECG shows a regular narrow QRS with no delta wave, because the node alone activates the ventricles, and a retrograde P wave on the ST segment, 70 ms or more after the QRS starts. It is stopped by slowing the node, and cured by ablating the pathway.
Remember
- Regular, narrow and fast: 150 to 250 per minute, QRS under 120 ms, and no delta wave.
- The P is after the QRS: a retrograde P on the ST segment, 70 ms or more after the QRS starts, and less than half the R-R.
- The loop uses both chambers: the atria and the ventricles are each part of it, so there is one P for every QRS.
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 retrograde P, and how far it sits from the QRS, is what tells orthodromic AVRT from the other narrow tachycardias.
Mechanism
An accessory pathway is a strand of ordinary muscle that crosses the insulating ring between an atrium and a ventricle, where the conduction system is not. With the AV node it forms a loop with two limbs, and the atria and ventricles complete it.
A premature atrial beat starts it. Early, it finds the pathway still recovering from the last beat, so it cannot go down it, but the AV node is ready, so the impulse goes down the node alone. By the time it has activated the ventricles the pathway has recovered, so it goes back up the pathway into the atria, and arrives at the node again ready to go round. Down the node, up the pathway, down the node: the loop is locked in.
The ventricles are activated by the node alone, down the normal system, so the QRS is narrow and the delta wave of the resting pattern is gone. The atria are activated last, from the pathway, so the P wave follows the QRS and lands on the ST segment.
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From the WPW pattern to the tachycardia
A person whose pathway conducts forwards has the WPW pattern in sinus rhythm: a short PR and a delta wave, because the pathway and the node both reach the ventricles. A premature atrial beat starts the loop, and the pattern becomes a tachycardia: from then on the node alone conducts, so the delta has gone. The pattern alone is a trait. With a tachycardia it is WPW syndrome. Switch this page to its Onset form, and turn on Study, to watch it happen on one recording.
Orthodromic and antidromic
Orthodromic means the impulse goes down the node and up the pathway, so the QRS is narrow. This page is that form, about 95% of AVRT in people with WPW. In antidromic AVRT the loop runs the other way, down the pathway and up the node, so the QRS is wide and fully pre-excited, and it can be mistaken for ventricular tachycardia.
Concealed pathways
Most pathways cannot conduct from the atria to the ventricles, only back. They leave the resting ECG normal, with no delta wave and a normal PR, and can still carry orthodromic AVRT. A normal resting ECG does not rule one out.
The 1:1 rule
Both chambers are part of the loop, so AVRT cannot continue with block either way: a P for every QRS, always. More P waves than QRS complexes excludes it.
Why the P is on the ST segment
In AVNRT the loop sits inside the node, so the atria and ventricles are activated together and the P lands on the QRS. In AVRT the atria are reached only after the ventricles, up the pathway, so the P falls clear of the QRS: 70 ms or more after it, and less than half the R-R, a short RP tachycardia. It is easy to miss on the ST segment and the start of the T.
Where the pathway is
A left lateral pathway activates the atria from the left, so the retrograde P is negative in I and aVL, as on this page. A posteroseptal pathway makes it inverted in the inferior leads.
How it ends
The node is the weak limb, so slowing it ends the loop: adenosine, a vagal manoeuvre, or a drug. It usually ends on the way down, so the run finishes with a retrograde P and no QRS after it.
ST depression and alternans
The ST is often depressed at these rates, and the QRS can alternate in height from beat to beat. Neither means ischaemia.
Clinical impact
- The rate: the ventricles are driven at 150 to 250 per minute and the atria contract against closed valves. It is usually well tolerated in a structurally normal heart.
- Symptoms: palpitations, neck pulsation, breathlessness, sometimes dizziness, with an abrupt start and stop.
- The pathway matters: one that also conducts forwards (the WPW pattern) carries a separate risk if atrial fibrillation starts, because it does not slow impulses as the node does.
Management
Vagal manoeuvres first
A modified Valsalva manoeuvre, or carotid sinus massage. The node is one limb of the loop, so slowing it can stop the rhythm outright.
Adenosine
6 mg by rapid intravenous push, then 12 mg. Blocking the node ends orthodromic AVRT, usually leaving a retrograde P as the last thing on the strip. Have a defibrillator close with a manifest pathway, because adenosine can start atrial fibrillation.
Unstable: synchronised cardioversion
As for any tachycardia that compromises the circulation.
Recurrent episodes: ablate the pathway
Catheter ablation cures it and removes the circuit rather than suppressing it. It is preferred to long-term drugs for recurrent symptoms, and it also removes the risk in atrial fibrillation.
Differential
The one to tell it from. Also fast, regular and narrow, but its loop sits inside the node, so the P lands on the QRS: a pseudo-S in the inferior leads, an RP under 70 ms. In AVRT the P is clear on the ST segment.
The resting pattern of the same pathway: a short PR and a delta wave in sinus rhythm. In the tachycardia the delta has gone.
A P wave before each QRS with a flat baseline between, and it can conduct with block, which AVRT never does.
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, and no retrograde P.
References
- Atrioventricular Reentrant Tachycardia — StatPearls
- 2015 ACC/AHA/HRS Guideline for the Management of Adult Patients With Supraventricular Tachycardia — Circulation, 2016
- 2019 ESC Guidelines for the management of patients with supraventricular tachycardia — European Heart Journal, 2020