7.3 AV Nodal Decremental Conduction & Dual AV Nodal Physiology

7.3 AV Nodal Decremental Conduction & Dual AV Nodal Physiology

The atrioventricular (AV) node is the gatekeeper of cardiac electrical conduction. Positioned strategically between the atria and the ventricles, the AV node introduces an obligatory physiological delay that allows optimal ventricular filling during atrial systole while acting as a frequency filter that protects the ventricles against lethal rapid rates during atrial tachyarrhythmias. Uncovering the cellular mechanisms of AV nodal decremental conduction and the anatomical and electrophysiological features of dual AV nodal pathways is among the most heavily tested competencies on the RCES examination.

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Cellular Biophysics of Decremental Conduction

Decremental conduction is defined as a mode of electrical propagation in which the action potential upstroke velocity, amplitude, and conduction speed progressively decline as an impulse traverses tissue. In the heart, this behavior is uniquely characteristic of the AV node and sinoatrial node, contrasting sharply with the all-or-none conduction seen in atrial muscle, ventricular myocardium, and the His-Purkinje system.

Fast-Response vs. Slow-Response Tissues

The fundamental biophysical distinction between specialized conduction tissues lies in their ionic channel architecture:

Mechanisms of AV Nodal Conduction Delay & Filtering

1. Sodium Channel Inactivation: Because the resting potential of AV nodal compact cells is $-50\text{ to }-60\text{ mV}$, the voltage-dependent fast sodium channels remain permanently in an inactivated, non-conductive state. Phase 0 relies entirely on the slow, low-conductance L-type calcium channel ($I_{Ca,L}$).
2. High Intercellular Resistance: AV nodal myocytes are small, irregularly oriented cells coupled by sparse, low-conductance connexin 45 (Cx45) gap junctions. This architectural bottleneck generates high internal longitudinal resistance, slowing axial current flow.
3. Post-Repolarization Refractoriness: In ventricular or atrial cells, excitability is restored as soon as the resting potential returns to $-80\text{ mV}$. In AV nodal cells, however, L-type calcium channels require an extended period (tens to hundreds of milliseconds) after complete membrane repolarization to transition from their inactivated state back to their resting excitable state. When premature atrial impulses or rapid pacing trains arrive at the AV node, they encounter calcium channels that are still partially inactivated. Consequently, the resulting Phase 0 upstroke is blunted, current amplitude drops, and conduction velocity slows dramatically—the essence of decremental conduction.

Clinical Manifestations of Decremental Conduction

• During Incremental Atrial Pacing: As the pacing cycle length is progressively shortened (e.g., from 600 ms down to 300 ms), the AH interval on the His catheter progressively lengthens (e.g., from 80 ms to 140 ms to 220 ms) until an atrial beat fails to conduct to the His bundle, producing AV nodal Wenckebach block (Mobitz Type I block). The longest cycle length at which Wenckebach block occurs defines the AV nodal Wenckebach cycle length (normal: $\le 400\text{ to }500\text{ ms}$, corresponding to rates of 120-150 bpm).
• During Programmed Atrial Extrastimulation ($A_1-A_2$): As the $A_1-A_2$ coupling interval decreases, the $A_2-H_2$ interval progressively prolongs, reflecting greater decremental slowing through the AV node.

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Anatomical and Electrophysiological Substrates of Dual AV Nodal Pathways

In approximately 10% to 35% of the general population, the AV node exhibits dual AV nodal pathway physiology—the presence of two functionally and anatomically distinct electrical pathways that connect atrial myocardium to the compact AV node.

                                [Tendon of Todaro]
                             /                     \
            [Fast Pathway]  /                       \  [Compact AV Node & His]
                           /                         \--------(His Bundle)
   [Triangle of Koch]     /                           /
                         /                           /
                        /                           /
            [Slow Pathway]                         /
                          \                       /
                           \                     /
                            [Coronary Sinus Ostium] -- [Tricuspid Annulus]

Anatomy of Koch's Triangle

Dual AV nodal pathways reside entirely within the triangle of Koch, an anatomical region in the low right atrium delineated by three distinct anatomical landmarks:

1. Superior / Anterior Border: The tendon of Todaro, a fibrous subendocardial cord running from the central fibrous body to the Eustachian valve of the inferior vena cava.
2. Inferior Border: The attachment of the septal leaflet of the tricuspid valve.
3. Base (Posterior Border): The ostium of the coronary sinus (CS) and the sub-Eustachian sinus.
4. Apex: The compact AV node and the penetrating bundle of His, where the tendon of Todaro meets the tricuspid annulus.

Fast Pathway vs. Slow Pathway Properties

The Electrophysiological Paradox: The Fast Pathway conducts quickly, but has a long refractory period (it cannot handle premature beats). The Slow Pathway conducts slowly, but has a short refractory period (it recovers quickly and can accept premature beats).

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Programmed Electrical Stimulation: Uncovering the "Jump"

During routine sinus rhythm or baseline pacing, an atrial impulse enters both pathways simultaneously. Because the fast pathway conducts rapidly, the impulse traverses the fast pathway, depolarizes the compact AV node and His bundle (producing a normal, short AH interval of 60 to 120 ms), and simultaneously enters the slow pathway retrogradely from the bottom, where the two wavefronts collide and cancel each other out. Thus, slow pathway conduction is clinically silent during normal rhythm.

Defining the AV Nodal "Jump"

To uncover dual pathway physiology, programmed single atrial extrastimuli ($A_1-A_2$) are delivered following an 8-beat basal drive train ($A_1-A_1$) at BCL 600 or 500 ms. The $A_1-A_2$ coupling interval is decremented in 10-ms steps.

The Jump Criterion

An AV nodal jump is definitively diagnosed when:

$\Delta (A_2 - H_2) \ge 50\text{ ms} \quad \text{for a } 10\text{-ms decrement in } A_1-A_2$

A1-A2 = 350 ms  -->  A2-H2 = 90 ms   (Conduction via Fast Pathway)
A1-A2 = 340 ms  -->  A2-H2 = 100 ms  (Conduction via Fast Pathway: 10 ms physiological decrement)
A1-A2 = 330 ms  -->  A2-H2 = 185 ms  (*** JUMP: Delta A2-H2 = +85 ms! Fast blocked, conducted via Slow ***)
A1-A2 = 320 ms  -->  A2-H2 = 195 ms  (Conduction via Slow Pathway: decremental conduction)

Biophysical Mechanism of the Jump

1. At $A_1-A_2$ intervals down to 340 ms, the premature impulse conducts down the fast pathway with mild decremental slowing (10-ms increase in $A_2-H_2$).
2. At $A_1-A_2 = 330\text{ ms}$, the extrastimulus reaches the Effective Refractory Period of the Fast Pathway (Fast Pathway ERP). The impulse blocks completely in the fast pathway.
3. Because the Slow Pathway has a shorter ERP, it is not refractory and remains fully excitable.
4. The impulse is forced to travel exclusively down the slow pathway. Because conduction velocity down the slow pathway is sluggish, transit time to the His bundle increases dramatically, producing an abrupt, discontinuous lengthening of the $A_2-H_2$ interval by $\ge 50\text{ ms}$.
5. On the surface ECG, this manifest as an abrupt, visible prolongation of the PR interval (a PR jump).

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Reentrant Circuit Dynamics: Dual AV Nodal Echo Beats and AVNRT

The existence of dual pathways with unidirectional block and slowed conduction fulfills the classical requirements for reentry.

                    [Premature Extrastimulus A2]
                                |
               +----------------+----------------+
               |                                 |
      [Blocks in Fast Pathway]         [Conducts down Slow Pathway]
      (Fast Pathway is Refractory)     (Slow conduction: long AH delay)
               |                                 |
               |                                 v
               |                    [Depolarizes His & Ventricles]
               |                                 |
               |                                 v
               |                    [Turns around at Common Junction]
               |                                 |
               v                                 |
      [Retrograde Conduction up Recovered Fast Pathway] <--+
               |
               v
    [Atrial Echo Beat (A-Echo) / Typical AVNRT]

1. The Atrial Echo Beat

When the premature impulse conducts slowly down the slow pathway, the prolonged transit time through the AV node ($A_2-H_2 > 200\text{ ms}$) provides critical time for the previously refractory fast pathway to recover its excitability.

Upon reaching the lower turnaround point (the distal compact node or lower common pathway), the impulse bifurcates:

• It conducts antegradely into the His-Purkinje system, generating a normal His deflection ($H_2$) and narrow ventricular complex ($V_2$).
• It simultaneously enters the recovered fast pathway retrogradely, conducting back up Koch's triangle to depolarize the atria from bottom to top. This retrograde atrial depolarization produces a dual AV nodal echo beat ($A_{echo}$).

2. Induction of Typical (Slow-Fast) AVNRT

If the retrograde impulse emerging from the fast pathway finds the slow pathway recovered from its own refractory period, the impulse re-enters the slow pathway antegradely, initiating sustained circular reentry: Typical (Slow-Fast) Atrioventricular Nodal Reentrant Tachycardia (AVNRT).

Intracardiac & Electrocardiographic Hallmarks of Typical AVNRT

• Antegrade Limb: Slow pathway.
• Retrograde Limb: Fast pathway.
• Ventricular-to-Atrial (VA) Timing: Because the retrograde limb is the rapid fast pathway, the impulse reaches the atrium virtually simultaneously with the ventricles. The VA interval measured from the onset of ventricular activation to the earliest retrograde atrial deflection on the His or CS catheter is $\le 60\text{ ms}$ (typically 0 to 40 ms).
• Surface ECG Presentation: Because atrial and ventricular activation are simultaneous, the retrograde P wave is buried directly inside the QRS complex, or appears at the very end of the QRS as a pseudo $r'$ in lead V1 or a pseudo $S$ wave in inferior leads (II, III, aVF).
• Coronary Sinus Activation: Demonstrates concentric atrial activation with the earliest retrograde atrial electrogram recorded at the His bundle or proximal coronary sinus catheter (CS 9-10) near Koch's triangle apex.

3. Atypical AVNRT Variants

• Atypical (Fast-Slow) AVNRT: Antegrade conduction occurs down the fast pathway, while retrograde conduction travels up the slow pathway. Characterized by a long RP interval ($RP > PR$), with a negative P wave in inferior leads preceding the next QRS and earliest retrograde atrial activation recorded at the coronary sinus ostium.
• Slow-Slow AVNRT: Antegrade conduction proceeds down one slow pathway extension, and retrograde conduction travels up a second slow pathway extension. Also presents as a long RP tachycardia.

4. Radiofrequency Catheter Ablation of the Slow Pathway

Slow pathway modification is the definitive curative treatment for typical AVNRT:

• Target Anatomical Site: The inferior and posterior aspect of Koch's triangle, situated along the tricuspid valve annulus between the coronary sinus ostium and the mid-septal tricuspid leaflet.
• Electrogram Guidance: Ablation catheters search for characteristic slow pathway potentials—fractionated, sharp multicomponent potentials occurring between the local atrial and ventricular electrograms during sinus rhythm.
• Intraprocedural Monitoring: Application of RF energy elicits a characteristic accelerated junctional rhythm, confirming thermal engagement of nodal tissue. Pacing or continuous monitoring must verify intact 1:1 ventriculoatrial (VA) or antegrade conduction to avoid complete AV block.
• Procedural Endpoints: Complete elimination of slow pathway conduction (absence of jump and echo beats), or residual slow pathway conduction with at most a single non-reentrant echo beat and complete non-inducibility of AVNRT despite aggressive isoprenaline infusion.