AV Nodal Reentry Tachycardia

AV nodal reentry tachycardia (AVNRT) is one of the paroxysmal SVT. The tachycardia involves the AV node (at least 2 atrionodal connection) and all or a part of the atrial myocardium. So, the requirement for the AVNRT is the presence of the atrionodal connections or the existence of dual AV node physiology.

Dual AV Node Physiology

Some individual have 2 conduction pathways or tracts in the AV node. One pathway allows fast conduction of an impulse but with slow recovery or long refractory period (fast pathway – FP), and the other one has a slow conduction but fast recovery or short refractory period (slow pathway - SP). A sinus impulse will travel both pathways (fast pathway and slow pathway) but a normal PRI will manifest on the surface ECG because it traveled fast using the FP. The impulse traveling the SP is blocked because the final common pathway going to the ventricles is refractory (depolarized from the impulse coming from the FP). 

Impulse from the sinus node is conducted in the fast and slow pathways. Due to the fast conduction in the FP, the conduction will reach the His Bundle then the ventricles via the FP. Conduction in the slow pathway is blocked in the AV node because the AV node is rendered refractory because the FP conduction.

A properly-time premature atrial complex (PAC), will be conducted using the slow pathway because the fast pathway is still refractory (depolarized from the previous sinus beat). 

A premature atrial complex will conduct via the slow pathway because it has recovered but not the fast pathway.

On the surface ECG, the PAC is conducted with a long PRI. 

A PAC with long PRI initiating AVNRT.

As this impulse is going down, it find the SP fully recovered and will be conducted retrogradely (going-up the atria) using the slow pathway. This same impulse will be conducted antegrade because the FP has fully recovered. This completes the circuit for AVNRT.

Retrograde conduction will occur via the fast pathway because it has recovered. There will then be retrograde atrial depolarization. The impulse will then reenter the slow pathway and complete the AVNRT circuit.

ECG Recognition:

• A short RP tachycardia (common/typical form) with sudden onset and termination. Atypical forms can be long RP tachycardia.
• For typical AVNRT the RP interval is less than 70 ms.
• The P waves which are inverted in II, III and aVF can be seen just right after the QRS. This will alter the QRS complex and be seen as pseudo-r' in in V1 and pseuso-S in II, III and aVF.
• Sometimes, the P wave cannot be seen or are buried in the QRS because of simultaneous activation of the atria and ventricles.
• The rate can vary from 118-264. Rates can be similar with AV reentry tachycardia (AVRT) and other SVT. So, the rate is not useful to identify the kind of SVT.
• The QRS is narrow or wide because of aberrant ventricular conduction of preexisting intraventricular conduction defect.
• The episode is often initiated with a premature atrial complex (PAC) with a long PRI. A premature ventricular complex (PVC) can also inititate the arrhythmia. 

AV Nodal Reentry Tachycardia. This is a long RP tachycardia with ventricular rates around 180's bpm. Pseudo-r' can be seen in V1 and pseudo-S in lead II

AVNRT initiation. This is the initiation strip of the case above. It was initiated by a PAC with long PRI (red arrow).

Home