Why the AV Node Delays
If the heart contracted all at once, it would be a terrible pump. The pause between the atria and ventricles is the AV node's greatest trick.
The electrical wave moves through the working atrial muscle at roughly 1 meter per second. It hits the His-Purkinje system and accelerates to 3 or 4 meters per second. But between these two high-speed highways sits the atrioventricular (AV) node, where the wave suddenly slams the brakes, crawling forward at a sluggish 0.05 meters per second.
This delay is not a defect; it is a mechanical necessity. The atria need time to squeeze blood into the ventricles before the ventricles contract and slam the AV valves shut. The electrical pause allows the "atrial kick" to complete. But how does the AV node achieve this extreme deceleration?
The Mechanism: Sluggish Sparks and Labyrinths
In Chapter 5 of Volume I, we learned that conduction velocity depends on two main factors: the push (the steepness of Phase 0) and the resistance (gap junctions and cellular architecture). The AV node manipulates both of these variables to slow things down.
1. The Calcium Upstroke
Just like the SA node, the core cells of the AV node have virtually no fast sodium channels. When the wavefront arrives from the atrium, the AV nodal cells depolarize via L-type calcium channels.
Because calcium channels open slowly, the Phase 0 upstroke is blunted and rounded. A slow upstroke injects very little local circuit current into the neighboring cell, so the next cell takes longer to reach threshold. The "push" is weak.
2. The Microscopic Labyrinth
Unlike the long, parallel, well-organized fibers of the working myocardium, the cells in the compact AV node are small, spindly, and arranged in a chaotic, interwoven mesh. Furthermore, they have significantly fewer gap junctions connecting them. High resistance combined with a winding path forces the electrical signal to navigate a microscopic labyrinth.
Decremental Conduction
The most remarkable property of the AV node is that its conduction velocity is not fixed. It possesses a property called decremental conduction.
If you stimulate working atrial or ventricular muscle at faster and faster rates, the conduction velocity stays basically the same (until it abruptly hits the absolute refractory period and blocks).
The AV node behaves differently. The faster you pace the atrium, the slower the AV node conducts. Because calcium channels recover from inactivation very slowly, a rapid heart rate means the next beat arrives while many calcium channels are still recovering. The subsequent Phase 0 upstroke is even slower, meaning the conduction delay becomes even longer. The harder you push the AV node, the more it pushes back.
As the atrial rate increases, calcium channels have less time to recover. The AV node conducts more slowly, lengthening the delay between atrium and ventricle.
Clinical Takeaway: The Ultimate Filter
Decremental conduction is not just an electrophysiological curiosity; it is a life-saving defense mechanism.
Consider a patient who goes into Atrial Fibrillation (AFib). The atria are firing chaotically at 400 to 600 impulses per minute. If the AV node behaved like a simple wire, all 600 impulses would conduct down to the ventricles. The ventricles would try to contract at 600 bpm, resulting in zero cardiac output. The patient would die of ventricular fibrillation within minutes.
But the AV node is decremental. As the storm of 600 impulses crashes into the top of the node, the calcium channels cannot recover fast enough. The impulses penetrate the node, conduct slower and slower, and ultimately block. Only a fraction of the impulses successfully emerge on the ventricular side.
This is why a patient in AFib might have a ventricular rate of 110 or 130 bpm, rather than 600 bpm. The AV node acts as an electrical filter, sacrificing its own conduction to protect the ventricles. When we give a patient in AFib a drug like diltiazem or metoprolol, we are simply assisting the filter — making it harder for calcium channels to recover, thereby increasing the block and slowing the ventricular rate.
On a surface ECG, the PR interval represents everything from the start of atrial depolarization to the start of ventricular depolarization. It lumps the atrial muscle, the AV node, and the His-Purkinje system together.
In the EP lab, we place a catheter directly across the tricuspid valve, right next to the Bundle of His. This gives us a highly detailed view called the His Bundle Electrogram. We can see three distinct deflections:
- A: Local Atrial activation (entering the AV node).
- H: His bundle activation (exiting the AV node).
- V: Ventricular activation.
The time between the A and the H — the A-H interval — represents conduction exclusively through the AV node. If we pace the atrium faster and faster during a study, we can watch the A-H interval progressively stretch out. We are observing decremental conduction in real-time. If the A-H interval stretches until an 'A' appears without a following 'H', we have just documented Wenckebach block inside the AV node.
Key Takeaways
- The AV node's delay allows the atria to complete their mechanical contraction ("atrial kick") before the ventricles fire.
- Conduction is slow because the AV node relies on a sluggish, calcium-driven Phase 0 upstroke, and because the cells are arranged in a chaotic, high-resistance labyrinth.
- Decremental conduction is the property where the AV node conducts more slowly at faster heart rates, due to the slow recovery of calcium channels.
- This decremental property allows the AV node to act as a filter during rapid atrial arrhythmias like AFib, protecting the ventricles from dangerous rates.
- In the EP lab, conduction through the AV node is isolated and measured precisely using the A-H interval on the His Bundle Electrogram.