Orthodromic Atrioventricular Re-entrant Tachycardia (AVRT)
Orthodromic Atrioventricular Re-entrant Tachycardia (AVRT)
A regular narrow-complex tachycardia circling between the AV node and an accessory pathway, with a retrograde P visible after the QRS.
On the trace
Read the strip in this order.
- Rate and regularity.
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180 bpm, R-R 333 ms, varying by under a millisecond. A fixed circuit gives a fixed cycle length. - Look at the ST segment, not in front of the QRS.
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There is a distinct negative deflection of about 0.10 mV, 108 ms after the R peak, sitting on the ST segment before the T wave rises. That is the retrograde P. - Measure how far it sits from the QRS.
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Short, well under half the R-R, but plainly separate from it. That separation is the entire difference from typical AVNRT, where the P is inside the QRS and cannot be seen at all. - Count P waves against QRS complexes.
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One for one, and it must be: the atrium is part of the circuit, so a beat that fails to reach it stops the tachycardia. - Measure the QRS.
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Narrow. The ventricles were reached down the AV node and the His-Purkinje system, exactly as in sinus rhythm.
How to recognise it
| Feature | Value | Notes |
|---|---|---|
| Rate | 150 to 250 bpm | 180 here. |
| Rhythm | Regular | Abrupt onset and offset. |
| P waves | Retrograde, after the QRS | On the ST segment or the early T. Inverted in the inferior leads. |
| RP relationship | Short, but measurable | Longer than AVNRT's, because the chambers activate in sequence rather than together. |
| AV relationship | Always 1:1 | More P waves than QRS complexes excludes AVRT outright. |
| QRS | Narrow in orthodromic AVRT | Wide in the uncommon antidromic form, which goes down the pathway instead. |
| Other | QRS alternans | A beat-to-beat change in QRS height at speed, not a sign of effusion here. |
The strip
Mechanism
An accessory pathway is a strand of muscle that crosses the insulating ring between atrium and ventricle, somewhere the conduction system is not. It and the AV node form two limbs of a loop, and the atrium and ventricle complete it. In orthodromic AVRT the impulse goes down the node, activates the ventricles normally, finds the pathway and travels back up it to the atrium, then arrives at the node again ready to repeat. Every structure is used in turn, so every structure has to keep working for the rhythm to continue.
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Concealed pathways, and why the resting ECG is usually normal
Most accessory pathways cannot conduct from atrium to ventricle at all. They are concealed: invisible in sinus rhythm, with no delta wave and no short PR, because nothing goes down them. They conduct only backwards, which is all AVRT needs. A pathway that does conduct forwards produces the pre-excitation pattern in sinus rhythm and is called manifest.
Why the P is visible here and not in AVNRT
In AVNRT the circuit is inside the node, so the atria and ventricles are activated in parallel and the retrograde P lands inside the QRS. In AVRT the atria are only reached after the ventricles have been depolarised and the impulse has climbed back up the pathway, so the P falls clear of the QRS onto the ST segment. Sequence, not parallel, is the whole reason it can be seen.
The 1:1 rule
Both chambers are obligatory parts of the loop, so AVRT cannot continue with AV block in either direction. This gives one of the cleanest exclusions in rhythm reading: if there are more P waves than QRS complexes, the rhythm is not AVRT. An atrial tachycardia can conduct 1:1 or with block, and most AVNRT is 1:1 too, so the rule excludes rather than confirms.
How it starts and how it stops
A ventricular ectopic arriving near the ventricular end of a concealed pathway can block in the His-Purkinje system while travelling back up the pathway, which closes the loop. A regular narrow-complex tachycardia that begins with a PVC is most likely AVRT. Termination usually happens in the node on the way down, so the run ends with a retrograde P and no QRS after it.
Clinical impact
The tachycardia itself is usually well tolerated in a structurally normal heart. What changes the stakes is the direction the pathway can conduct, which the resting ECG answers.
Management
Vagal manoeuvres first
A modified Valsalva or carotid sinus massage. The node is one limb of the circuit, so slowing it can break the loop.
Adenosine
6 mg by rapid IV push, then 12 mg. Blocking the node terminates orthodromic AVRT, usually on the downstroke, leaving a retrograde P as the last thing on the strip.
Unstable: synchronised cardioversion
As for any tachycardia causing haemodynamic compromise.
Definitive: pathway ablation
Ablating the accessory pathway is curative and removes the circuit rather than suppressing it. It is preferred over long-term drugs for recurrent symptomatic episodes.
Differential
The main one. AVNRT is also fast, regular and narrow, but its circuit sits inside the node, so the atria and ventricles fire together and no P wave is visible anywhere.
The resting pattern produced by a pathway that conducts forwards as well as backwards. Its presence changes what is safe to give if the patient ever fibrillates.
Has a P before every QRS rather than after it, and can conduct with block, which AVRT never does.
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