AVNRT
The classic loop. When the AV node's built-in dual pathways turn from a safety filter into an endless roundabout.
In Volume II, we explored the AV node and discovered its secret architecture: it doesn't have just one entrance. It has two. The Fast Pathway and the Slow Pathway.
Normally, this dual-pathway anatomy is completely silent. The Fast Pathway conducts the sinus impulse down to the His bundle, the Slow Pathway is a harmless bystander, and the heart beats at a steady 60 beats per minute. But this exact architecture is a loaded spring. It provides all three requirements for reentry: two distinct pathways connected at the top and bottom, with different conduction velocities, and different refractory periods.
All it takes is a single, perfectly timed premature beat to spark the most common regular narrow-complex tachycardia in adults: Atrioventricular Nodal Reentrant Tachycardia (AVNRT).
The Perfect Storm
To start AVNRT, we need a spark that arrives at the AV node when the Fast Pathway is asleep (refractory), but the Slow Pathway is awake (recovered).
Recall the rule from Volume II: The Fast Pathway conducts quickly, but recovers slowly. The Slow Pathway conducts slowly, but recovers quickly.
Imagine a Premature Atrial Contraction (PAC) fires. It races down toward the AV node. Because it is premature, it arrives earlier than a normal sinus beat would.
It hits the entrance of the Fast Pathway. Blocked. The Fast Pathway is still refractory from the previous beat.
It hits the entrance of the Slow Pathway. Open. The Slow Pathway has already recovered.
The impulse slowly trundles down the Slow Pathway. It takes a long time (resulting in a dramatically prolonged PR interval on the ECG for that specific beat). By the time the impulse finally reaches the bottom of the AV node (the bundle of His), a critical amount of time has passed.
Because of that delay, the Fast Pathway — which was refractory a moment ago — has now had time to wake up.
Down the Slow, Up the Fast
When the impulse reaches the bottom of the Slow Pathway, it has a choice. It goes down into the His bundle, causing a ventricular contraction (QRS). But it also looks "up" the Fast Pathway.
Because the Fast Pathway has recovered, the impulse turns around and blasts up the Fast Pathway in a retrograde direction. It travels very quickly back to the top of the AV node.
At the top, it does two things:
- It exits into the atrium, causing a retrograde atrial contraction (an inverted P wave).
- It looks "down" the Slow Pathway again.
If the Slow Pathway has recovered, the impulse plunges right back down. The circuit is closed. Down the Slow, Up the Fast. This is "Typical" or "Slow-Fast" AVNRT. It spins rapidly, usually around 150-200 bpm.
What the ECG Shows
Understanding the anatomy explains exactly why the ECG looks the way it does in Typical AVNRT.
Because the impulse travels UP the fast pathway extremely quickly, it hits the atrium at almost the exact same time it hits the ventricle (going down the His).
Therefore, the retrograde P wave occurs simultaneously with the QRS complex. It is completely buried inside the QRS, making it invisible.
Sometimes, the fast pathway is slightly slower than the His bundle, and the P wave emerges just a fraction of a second after the QRS. It looks like a little bump at the end of the QRS complex. In lead V1, this looks like a pseudo-R' wave. In inferior leads (II, III, aVF), it looks like a pseudo-S wave.
This is the hallmark of Typical AVNRT: a regular, narrow-complex tachycardia with no visible P waves, or P waves immediately buried at the terminal portion of the QRS (a short RP tachycardia).
In the EP lab, we can prove the existence of dual pathways by pacing the atrium progressively faster. Initially, conduction goes down the Fast Pathway. But as we pace faster, the Fast Pathway tires out and blocks.
Suddenly, conduction drops to the Slow Pathway. We see an abrupt "jump" in the conduction time (AH interval) of more than 50 milliseconds with just a 10 millisecond decrease in pacing cycle length. This jump proves the dual pathway physiology.
To cure AVNRT, we don't ablate the Fast Pathway (that's too risky and could cause complete heart block). Instead, we map the anatomical region of the Slow Pathway (inferior and posterior to the compact AV node, near the coronary sinus ostium) and deliver a small burn. By destroying the Slow Pathway, we break the circuit, and the patient is cured forever.
Key Takeaways
- Mechanism: Reentry within the AV node utilizing dual pathways.
- Typical AVNRT (Slow-Fast): The impulse travels anterograde down the Slow Pathway and retrograde up the Fast Pathway.
- Initiation: Usually triggered by a PAC that blocks in the Fast Pathway but conducts down the Slow Pathway.
- ECG: Regular, narrow QRS. P waves are usually buried inside the QRS or seen as terminal pseudo-R' in V1 or pseudo-S in inferior leads.
- Cure: Slow Pathway ablation.