Orthodromic Atrioventricular Re-entrant Tachycardia (AVRT)
cautionOrthodromic Atrioventricular Re-entrant Tachycardia (AVRT)
Atrioventricular re-entrant tachycardia (AVRT) depends on an accessory pathway - an extra electrical connection between atrium and ventricle that bypasses the AV node. It is the arrhythmia of Wolff-Parkinson-White syndrome, though most accessory pathways leave no trace on a resting ECG and only declare themselves when the tachycardia starts.
An inverted P sits just after each QRS, on the ST segment.
The circuit reaches the ventricles first and the atria afterwards, one then the other, so the P has time to emerge clear of the QRS instead of hiding inside it. It is inverted because the atria are being activated from below.
The QRS is narrow.
The impulse descends through the AV node and the normal conduction system, exactly as a sinus beat does. The accessory pathway is used only on the way back up, so it never distorts ventricular depolarisation.
There is exactly one P for every QRS, without exception.
Both chambers are part of the circuit, so neither can be skipped. This obligatory 1:1 relationship is diagnostically powerful in reverse: more P waves than QRS complexes rules AVRT out entirely.
- Rate150-250 bpm
- RhythmRegular
- P waveRetrograde, just after the QRSSuperimposed on the ST-T wave, and separate from the QRS rather than hidden in it.
- AV relationshipAlways 1:1Both chambers are obligatory parts of the circuit. If P waves outnumber QRS complexes, it is not AVRT.
- QRSNarrow (orthodromic)Wide if the loop runs the other way (antidromic), which is uncommon and mimics ventricular tachycardia.
- QRS alternansMay be presentThe complex changes shape slightly every other beat at fast rates. Not the same as the electrical alternans of a large pericardial effusion.
- Resting ECG between attacksUsually normalOnly a pathway that also conducts forwards shows a delta wave and short PR. Most do not, so a normal baseline ECG does not exclude AVRT.
There is an extra strand of muscle joining atrium to ventricle somewhere outside the AV node. AVRT is an impulse going down the node and back up that strand, lap after lap - a loop that has to travel through both chambers to complete itself.
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Most accessory pathways conduct only backwards, from ventricle to atrium, and are therefore completely invisible between attacks - the resting ECG is normal and the pathway is described as concealed. A minority conduct forwards as well, and those announce themselves in sinus rhythm with the Wolff-Parkinson-White signature: a delta wave, a short PR interval and a broadened QRS, because part of the ventricle is being activated early through the pathway rather than waiting for the node. Which direction the loop runs decides the width of the complex during tachycardia. Down the node and up the pathway - orthodromic, and much the commoner - gives the narrow QRS seen here; down the pathway and up the node gives a wide one, and is uncommon.
A ventricular ectopic beat is the classic trigger. Arising near the ventricular end of the pathway, it finds the normal conduction system refractory but the pathway ready, travels up to the atria, and comes back down the node to start the loop. A regular narrow-complex tachycardia that began with a ventricular ectopic is AVRT until shown otherwise.
Orthodromic AVRT itself rarely causes harm and usually stops at the bedside. The reason accessory pathways are worth curing is the other scenario: the AV node normally protects the ventricles by refusing most impulses, and an accessory pathway offers no such gatekeeping, so pre-excited atrial fibrillation can reach the ventricles fast enough to degenerate into ventricular fibrillation.
Unstable - cardiovert
Synchronised cardioversion, without waiting on a rhythm diagnosis.
Vagal manoeuvres first
Slowing the AV node breaks the loop, because the node forms one of its two limbs. The modified Valsalva - strain, then supine with the legs raised - converts substantially more patients than the standard version.
Adenosine - but check the rhythm first
6 mg by rapid IV push, then 12 mg, terminates most orthodromic AVRT. NEVER give adenosine or any other AV-nodal blocker for an IRREGULAR wide-complex tachycardia: that is pre-excited atrial fibrillation, and blocking the node forces everything down the accessory pathway. Use procainamide or cardioversion instead.
Definitive - pathway ablation
Ablating the accessory pathway is curative and removes the risk of pre-excited atrial fibrillation as well as the tachycardia. It is offered early, and particularly to anyone whose pathway also conducts forwards.
AVNRT's loop turns around inside the AV node and drives both chambers at once, so the retrograde P is buried in the QRS. AVRT drives them in sequence, so the P clears the QRS and lands in the ST segment.
The P comes before the QRS, not after it, and the baseline between P waves is flat. An atrial focus also does not need the ventricles, so it can continue when conduction drops a beat.
A continuous sawtooth baseline, and more atrial waves than QRS complexes. That ratio alone excludes AVRT, which is obligatorily 1:1.
Also narrow and regular with a retrograde P near the QRS, but it is an automatic focus rather than a circuit - it warms up gradually and does not stop with adenosine.