What is the physiology of atrioventricular nodal reentry tachycardia (AVNRT)?

Physiology of Atrioventricular Nodal Reentrant Tachycardia (AVNRT)

AVNRT is a reentrant tachycardia that requires two functionally and anatomically distinct pathways within the AV node region—a "fast" pathway located near the apex of Koch's triangle and a "slow" pathway extending inferoposterior to the compact AV node along the septal margin of the tricuspid annulus. 1

Anatomic Substrate and Dual Pathway Physiology

The reentrant circuit is confined to the AV node and a small amount of perinodal atrial tissue, though AVNRT can persist without participation of atrial tissue. 1, 2

Pathway Characteristics:

• Fast pathway: Located anteriorly near the apex of Koch's triangle, conducts rapidly but has a longer refractory period 1, 3
• Slow pathway: Extends inferoposterior to the compact AV node tissue, stretching along the septal margin of the tricuspid annulus at or slightly superior to the coronary sinus ostium; conducts slowly but has a shorter refractory period 1, 3

The unbalanced refractoriness between these two pathways creates the prerequisite for AVNRT—the fast pathway's longer refractory period allows a premature atrial beat to block in the fast pathway while conducting down the slow pathway, enabling retrograde activation through the now-recovered fast pathway. 3

Typical AVNRT Mechanism (90% of Cases)

In typical "slow-fast" AVNRT, the impulse travels anterogradely down the slow pathway and retrogradely up the fast pathway. 1, 4

Electrophysiologic sequence:

• A premature atrial beat finds the fast pathway refractory and conducts down the slow pathway, resulting in a sudden "jump" in AV conduction time 3
• After conduction through the slow pathway to the His bundle and ventricle, brisk retrograde conduction occurs back to the atrium over the fast pathway 1
• This produces near-simultaneous atrial and ventricular activation 5

ECG manifestations:

• P waves are buried within or immediately after the QRS complex (≤70 ms from QRS onset) 1, 5
• P wave duration is short (approximately 40 ms) 1
• Pseudo-r' pattern often appears in lead V1 due to retrograde atrial activation distorting the terminal QRS 1, 2
• Heart rate typically ranges between 140-250 bpm 1, 6

Atypical AVNRT Variants (5-10% of Cases)

Fast-slow AVNRT reverses the circuit direction—anterograde conduction proceeds over the fast pathway and retrograde conduction over the slow pathway, producing a long R-P tachycardia. 1

ECG characteristics:

• P wave appears before the QRS complex, creating a long RP interval 1
• P waves are negative in leads III and aVF 1

Slow-slow AVNRT uses slowly conducting tissue for both limbs of the circuit, with the P wave inscribed after the QRS (RP interval ≥70 ms). 1

Atrial Activation Patterns

The earliest site of atrial activation differs between AVNRT variants, reflecting distinct retrograde pathways:

• Common (typical) AVNRT: Earliest atrial activation occurs near the His-bundle recording site on the anterior interatrial septum 7
• Uncommon (atypical) AVNRT: Earliest atrial activation occurs at the coronary sinus ostium on the posterior interatrial septum 7

Clinical Implications

Patients with AVNRT frequently describe "neck pounding" or "shirt flapping" sensations, which result from pulsatile reversed flow when the right atrium contracts against a closed tricuspid valve during near-simultaneous atrioventricular activation. 5

AVNRT is the most common form of paroxysmal supraventricular tachycardia, more prevalent in females, and typically not associated with structural heart disease. 1

Common pitfall: Atypical AVNRT variants with long RP intervals can mimic atrial tachycardia, requiring careful electrophysiologic evaluation to distinguish the mechanism. 5