Pathophysiology of AVNRT

1. Substrate: dual AV nodal physiology

• Fast pathway: conducts quickly and has a longer refractory period.
• Slow pathway: conducts slowly and has a shorter refractory period (it repolarizes sooner).
• Fuster and Hurst's The Heart, 15th Ed, p. 1100

2. Three conditions needed for re-entry

1. Two conducting pathways that are separated longitudinally but connected at each end.
2. Different refractory periods in the two pathways, so a premature beat can be blocked in one but not the other.
3. A circuit time longer than the longest refractory period in the circuit.

• Braunwald's Heart Disease, Ch. 62 (Anatomic Reentry), Fig. 62.18

3. Initiation (typical slow-fast AVNRT)

1. A premature atrial beat arrives while the fast pathway is still refractory.
2. The impulse is blocked in the fast pathway and conducts down the slow pathway, which has already recovered. This causes a long PR interval on the premature beat.
3. The impulse reaches the His bundle and activates the ventricles through the His-Purkinje system.
4. By now the fast pathway has had time to recover. The impulse travels retrogradely up the fast pathway to the atrium.
5. If the circuit time is longer than the slow pathway's refractory period, the slow pathway is re-excited antegradely. The loop then repeats.

• Fuster and Hurst's The Heart, Table 35-1 legend, and Braunwald's Heart Disease, Ch. 62

4. Why the ECG looks the way it does

• Retrograde atrial activation via the fast pathway happens at the same time as antegrade ventricular activation. The atria and ventricles are activated almost simultaneously.
• In about 50% of cases the P wave is buried inside the QRS.
• In about 45% of cases the P wave falls at the end of the QRS. This gives a pseudo-R' in V1 and a pseudo-S wave in the inferior leads.
• Occasionally there is 2:1 conduction to the ventricles. The block is usually within or below the His bundle, which shows the atria and ventricles are not required parts of the circuit.
• Braunwald's Heart Disease, Ch. 65, Typical AVNRT

5. Types

• Typical (slow-fast): 85-90% of cases (Fuster and Hurst's The Heart, p. 1100).
• Atypical: the remaining 10-15% (same source). There are two subtypes:
  • Fast-slow: antegrade conduction down the fast pathway and retrograde conduction up the slow pathway.
  • Slow-slow: antegrade conduction down one slow pathway and retrograde conduction up a second slow pathway. Braunwald describes this as a rare variety.
  • In both, the retrograde P wave falls later, so the RP interval is longer than in typical AVNRT.

6. Clinical correlates of the mechanism

• The tachycardia is paroxysmal, with abrupt onset and termination. It is typically triggered by premature atrial beats, and sometimes premature ventricular beats (my addition).
• Because the AV node is part of the circuit, it ends with AV nodal blockade (vagal maneuvers, adenosine, and so on). This is general knowledge and not taken from the sections I read.
• Ablating the slow pathway cures the arrhythmia. The older approach of ablating the fast pathway carried a small but definite risk of AV block.

Recent literature check