Atrioventricular Nodal Re-entrant Tachycardia (AVNRT)
cautionAtrioventricular Nodal Re-entrant Tachycardia (AVNRT)
Atrioventricular nodal re-entrant tachycardia (AVNRT) is the commonest regular supraventricular tachycardia in humans - a short circuit inside the AV node itself, rather than anywhere in the atrial or ventricular muscle. It switches on and off between one beat and the next, and typically runs at around 180 beats per minute.
No P waves are visible anywhere on the strip.
The circuit turns around inside the AV node, so the atria and the ventricles are activated at almost the same instant. The retrograde P lands inside the QRS and the far larger ventricular signal swallows it.
The QRS is narrow and identical from beat to beat.
Everything below the AV node is normal tissue conducting normally. The abnormality is entirely above the ventricles, which is what makes this a supraventricular tachycardia in the strict sense.
The rate is fast and unwavering.
A re-entrant circuit is a fixed loop of tissue, and every lap takes the same time. The rate does not drift with breathing, effort or anxiety the way a rhythm driven by the sinus node does.
- Rate150-250 bpmMost often around 180.
- RhythmRegularSmall beat-to-beat variation in the R-R can occur and does not argue against the diagnosis.
- P waveNot visibleThe retrograde P is buried in the QRS. When partly visible it appears as a small deflection at the end of the QRS that was not there in sinus rhythm - comparing with an old ECG is often what reveals it.
- RP relationshipShort - within the first half of the R-RThe retrograde P keeps a fixed position relative to every QRS, because atria and ventricles are driven by the same lap of the circuit.
- QRSNarrow (under 120 ms)Wide only with a pre-existing or rate-related bundle branch block, usually of right bundle type. The mechanism is unchanged; only conduction below the node differs.
- ST segmentDepression is commonRate-related, and not by itself evidence of ischaemia at these rates.
The AV node behaves as though it has two routes through it, one fast and one slow. AVNRT is an impulse going down one and straight back up the other, lap after lap. Because the loop is inside the node, each lap sends one impulse down to the ventricles and another back up to the atria at the same moment.
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The two routes differ in how fast they conduct and how quickly they recover, and the second difference is what starts the tachycardia. The fast route conducts quickly but takes longer to become ready again; the slow route is the opposite. A premature atrial beat arriving while the fast route is still refractory is turned back and travels down the slow route instead - which is why the PR interval of that one beat suddenly lengthens. By the time the impulse reaches the bottom, the fast route has recovered, so it runs back up it. If the slow route has also recovered by then, the impulse turns down again and the loop is established. That is why AVNRT begins abruptly on a single extra beat instead of warming up.
The atria are not actually part of the circuit - they are just along for the ride, activated backwards each lap. It follows that the tachycardia carries on even if the atria and ventricles occasionally fall out of step, and that squeezing the atria against closed valves is what produces the pounding in the neck patients describe. It also explains the cure: interrupting the slow route ends the arrhythmia permanently, because a loop with only one route left is not a loop.
AVNRT is not dangerous in a structurally normal heart and most episodes stop at the bedside. What it costs is quality of life - attacks arrive without warning and end as suddenly as they began - which is why ablation is offered early to anyone having them frequently.
Unstable - cardiovert
Hypotension, ischaemic chest pain, acute heart failure or altered mental state: synchronised cardioversion now. This is rare in AVNRT, but the rule does not change for it.
Vagal manoeuvres first
The modified Valsalva - strain, then lay the patient flat and raise their legs - converts substantially more patients than the standard version. Carotid sinus massage is an alternative in a patient with no bruit and no history of stroke.
Adenosine
6 mg by rapid IV push with an immediate flush, then 12 mg if needed. It blocks the AV node, which is part of the circuit, so it terminates most AVNRT outright. Warn the patient first: the few seconds of flushing, chest tightness and impending doom are expected and pass quickly.
Definitive - slow-pathway ablation
Curative in well over 90% of patients by interrupting the slow route, and offered early for frequent or poorly tolerated attacks. The main risk is damage to normal conduction requiring a pacemaker, which is uncommon. A beta-blocker or a non-dihydropyridine calcium-channel blocker is the alternative for prevention.
AVRT's circuit includes an accessory pathway outside the node, so the atria are reached later and the retrograde P is visible just after the QRS, in the ST segment, rather than buried in it.
A discrete P wave sits before every QRS, and it is abnormally shaped rather than absent. Adenosine typically exposes it without stopping the rhythm.
Flutter conducting 2:1 gives a regular narrow-complex rhythm near 150 that looks identical. Slowing AV conduction reveals a continuous sawtooth baseline where AVNRT has a flat one.
Also narrow, regular and fast with no clear P. It is automatic rather than re-entrant, so it warms up and slows down gradually and does not stop with adenosine.
- Atrioventricular Nodal Reentrant Tachycardia. Circulation, American Heart Association (2010).
- Atrioventricular nodal re-entrant tachycardia - classification and diagnosis. Europace, European Society of Cardiology (2006).
- Dual atrioventricular nodal pathway physiology and AVNRT. PubMed Central (PMC5013176).