AV Nodal Re-entrant Tachycardias
The fast and slow pathways are anatomically distinct within the AV node, but re-join to create the Bundle of His for onward transmission to the ventricles. As ventricular depolarisation is always via the Bundle of His, the QRS will be narrow unless pre-existing bundle branch block is present.
Fig 1: AV nodal re-entrant tachycardia
In nodal re-entry tachycardias, transmission is usually antegrade in the slow pathway and retrograde in the fast pathway, as described under re-entry circuits, and is known as slow-fast. Atrial & ventricular depolarisation are simultaneous, so retrograde P waves are often hidden within the QRS complex, or occur at the end of the QRS as a pseudo r’ or S wave. This is the ‘classical’ ECG of SVT, with a pattern of absent P waves and tachycardia.
Fig 2: AV nodal ‘slow-fast’ re-entry circuit
10% of patients have fast-slow circuits, with P waves clearly visible before the QRS and a long RP interval.